Kurzgesagt – In a Nutshell 

We are thankful to the experts for their critical reading and input to the script:


PhD in Sport Physiology and training an nutrition expert

https://www.youtube.com/@RenaissancePeriodization





Proteins produced by fat cells, like sLR11, stop other fat cells from burning fat.


#University of Cambridge Research News. Stored fat fights against the body’s attempts to lose weight. 2015

https://www.cam.ac.uk/research/news/stored-fat-fights-against-the-bodys-attempts-to-lose-weight

Quote: “Dr Andrew Whittle, joint first author, said: “Our discovery may help explain why overweight individuals find it incredibly hard to lose weight. Their stored fat is actively fighting against their efforts to burn it off at the molecular level."




#Arvind Kumar Patel, et al. Thermodynamic life cycle assessment of humans with considering food habits and energy intake. 2020

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7785455

Quote: “In the thermodynamic system, the human body is considered an open system. Human metabolism is the conversion of food into heat, work, and stored in the form of ATP.”


Calorie is a unit of energy, it refers to the potential energy contained in the chemical bonds of food. Through digestion, absorption and metabolism, that energy is captured in our bodies. Calorie is used for simplification, but generally we are talking about kilocalories (or kcal).


#Eva V. Osilla, et al. Calories. 2022

https://www.ncbi.nlm.nih.gov/books/NBK499909/#:~:text=Calories%20are%20a,calories%20(1%20kilocalorie)

Quote: “Calories are a measure of energy. "Small" calories (cal) estimate the amount of energy required to raise the temperature of exactly one gram of water by one degree Celsius at one atmospheric pressure, and “big” calories, also known as kilogram calories (Cal), are more commonly known and refer to the calories in food. The big calorie is named because it is equivalent to 1000 of the small calories (1 kilocalorie).”



You can calculate an approximate value of how many calories you burn with different exercises with this formula: METs x 3.5 x Body weight (kg) / 200 = Kcal/min


MET (metabolic equivalent) is a widely used term and represents the energy cost of physical activities as multiples of resting metabolic rate (RMR). 1 MET (3.5 ml O2·kg−1·min−1 or 1 kcal·kg−1·h−1) is the resting O2 consumption (V̇o2) of one person, a 70-kg, 40-yr-old man, and therefore has some limitations.


# ND. Estimating Energy Expenditure. 2004 reviewed 2009

https://www.ucdenver.edu/docs/librariesprovider65/clinical-services/sports-medicine/estimating-energy-expenditure.pdf?sfvrsn=eb335bb9_2

Quote “To estimate how many calories you’ll burn during exercise, you need to know your weight and the METlevel of an activity. (You can get the MET level of an activity from the table below). MET means metabolic equivalent and is the amount of energy you expend and oxygen you consume. One MET is the amount of energy you expend while sitting. When you exercise, your MET level increases because your working muscles need more energy and oxygen. Use the following formula to calculate how many calories per minute you will burn during the activity. Then multiply that number by your total exercise minutes. Energy expenditure (calories/minute) = .0175 x MET (from chart) x weight (in kilograms) (To find out your weight in kilograms, simply divide your weight in pounds by 2.2.) To get a more accurate idea of how fast you’ll burn calories to lose weight:   Figure out what you would have burned if you were just sitting around, instead of exercising, for the same time period. (The MET for sitting is 1.0.) Subtract these calories from your exercise calories.”


You can find an extensive list of METs for different exercises here: https://pacompendium.com/


#Stephen D. Herrmann, et al. 2024 Adult Compendium of Physical Activities: A third update of the energy costs of human activities. 2024

https://www.sciencedirect.com/science/article/pii/S2095254623001084.

Quote: “Collectively, the revisions in the 2024 Adult Compendium provide standard MET values for adults to assist researchers, practitioners, and public health specialists in assigning and describing intensity to a wide range of PAs commonly performed in a variety of settings, including inactivity, leisure, transportation, sports and conditioning, and in-home and occupational settings. The new website with supporting materials is located at https://pacompendium.com.”


The values we cite in our script are based on calculations by Harvard Health that take into account several types and intensities of exercises, based on a 155-pound (70 kg) person.


#ND. Calories burned in 30 minutes for people of three different weights. 2021

https://www.health.harvard.edu/diet-and-weight-loss/calories-burned-in-30-minutes-for-people-of-three-different-weights.

Quote: “The table below lists the calories burned by doing dozens of activities listed by category (such as gym activities, training and sports activities, home repair etc.) for 30 minutes. Activities and exercises include walking (casual, race, and everything in between), swimming, jogging, yoga, and many more.”




Actually, depending on your body composition, 2 pounds of body weight will be between 7000 and 7500 calories. Two pounds of pure body fat equal 7500 calories, two pounds of body weight that also include muscle mass, water, etc. equal around 7000 calories. 


#Diana M. Thomas, et al. Time to Correctly Predict the Amount of Weight Loss with Dieting. 2014

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4035446/

Quote: “He first drew upon the 1911 chemical analysis of Bozenrad showing that 87% of human adipose tissue is "fat", the remainder water and non-fat solids. We now recognize that most of adipose tissue fat is triglyceride and Wishnofsky correctly assigned this lipid fraction a bomb-calorimetry energy density of 9.5 kcal/g. Wishnofsky then reasoned one pound (454 g) of adipose tissue has an energy content of 3750 kcal.”




All major health organizations recommend physical activity for a myriad of reasons. 


#WHO. Physical activity. 2022

https://www.who.int/news-room/fact-sheets/detail/physical-activity

Quote: “Regular physical activity is proven to help prevent and manage noncommunicable diseases such as heart disease, stroke, diabetes and several cancers. It also helps prevent hypertension, maintain healthy body weight and can improve mental health, quality of life and well-being.”




Studies have shown that lots of people get frustrated and stop exercising when they don’t lose weight. 


A systematic review has shown that lots of people get frustrated and stop doing exercise, particularly when performing strength training (ST). When people do not see the expected result, (which for most overweight people is usually losing weight) they are less motivated to keep on exercising.


#Vasudevan A, Ford E. Motivational Factors and Barriers Towards Initiating and Maintaining Strength Training in Women: a Systematic Review and Meta-synthesis. 2022

https://link.springer.com/article/10.1007/s11121-021-01328-2

Quote: “The results of the training also influenced maintenance of ST. Where women saw progress, this was motivating, but when progression was slow or unexpected, this was a barrier. Other factors associated with poor adherence included boredom and repetition of ST exercises, poor knowledge of ST, poor accessibility in gyms, lack of supervision or routine, and difficulty in balancing work, time, family life, and other commitments with ST.”

“Women who were overweight or obese generally expected weight loss after ST and felt particularly demotivated to continue ST if their weight was increasing despite knowing this may be a result of increasing muscle mass.”



Before 1982, human basal metabolism in an active person could not be measured. The doubly labeled water method (discussed in this paper) was standardized for its application in humans and since 1988 it is the reference method for measuring energy expenditure.


#Klaas R. Westerterp. Doubly labeled water assessment of energy expenditure: principle, practice, and promise. 2017

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5486561/

Quote: “The doubly labelled water method has become the gold standard for the assessment of energy requirement (FAO/WHO/UNU 1985, 2004). Assessment of energy expenditure with doubly labelled water has demonstrated that self-report measures of food intake and physical activity are not accurate (Dhurandhar et al. 2015). There is not yet a method for the accurate determination of habitual dietary intake, and thus, energy requirement is derived from measured energy expenditure (Schoeller et al. 1990; Trabulsi and Schoeller 2001). Most studies show a lower value for reported energy intake compared with measured total energy expenditure.”



The Myth of the Workout



The Hadza people, in Tanzania, have a similar lifestyle to what was common during humanity's evolution.


#Herman Pontzer, et al. Hunter-Gatherer Energetics and Human Obesity. 2012

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0040503

Quote: “The Hadza are a population of hunter-gatherers living in a savannah-woodland environment in Northern Tanzania; their traditional foraging lifestyle has been documented extensively in previous work [17]. While no living population is a perfect model of our species’ past, the Hadza lifestyle is similar in critical ways to those of our Pleistocene ancestors. The Hadza hunt and gather on foot with bows, small axes, and digging sticks, without the aid of modern tools or equipment (e.g., no vehicles or guns). As in many other forager societies [10], there is a sexual division of foraging effort; Hadza men hunt game and gather honey, while Hadza women gather plant foods. Men’s forays are typically longer than women’s, as reflected in their mean daily travel distances (see below). Women typically forage in groups, while men tend to hunt alone [17]. As is typical among traditional-living Hadza, over 95% of their calories during this study came from wild foods, including tubers, berries, small- and large-game, baobab fruit, and honey.”

“Hadza were highly active and lean, with body fat percentages on the low end of the normal healthy range for Western populations.“


In this study, the Hadza walked a mean of 11.4 km /day (man) and 5.8 (women), which is a mean of 8.6 km/day. We rounded those numbers up to 9 km/day for the Hadza people in general.


#Herman Pontzer, et al. Hunter-Gatherer Energetics and Human Obesity. 2012

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0040503

Quote: “Daily walking distances for Hadza women (mean 5.8, std. dev. ±1.7 km/day) and men (11.4±2.1 km/day) were significantly different (p<0.001, t-test), consistent with previous measurements in hunting and gathering societies.”




According to Pontzer’s paper, adults living in industrialized countries spend a mean of 2.347 (women) and 3.053 kcal/day (men).

The values of 1900 and 2600 mentioned in the script are the averages for Hadza men and women. These are not the averages for Westerners listed in the table, but they become similar once you recalculate Western values "at a typical Hadza body mass".


#Herman Pontzer, et al. Hunter-Gatherer Energetics and Human Obesity. 2012

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0040503).

Quote: “Contrary to expectations, measures of TEE among Hadza adults were similar to those in Western (U.S. and Europe) populations. In multivariate comparisons of TEE controlling for FFM and age, Hadza women’s energy expenditure was similar to that of Western women (n = 186) and Hadza men’s TEE was similar to Western men (n = 53); lifestyle had no effect on TEE (women: F(139) = 0.18, p = 0.67; men: F(49) = 0.17, p = 0.68) (Fig. 1, Table S1).”

Table 1. Population characteristics, energy expenditure, and body composition.

To make this easier to understand, here is an infographic from Scientific American of the same statement, based on data from the same paper:

#Herman Pontzer, The Exercise Paradox, American Scientific, 2017

https://www.scientificamerican.com/article/the-exercise-paradox/ 

As an average, a healthy adult requires around 2000 (woman) and 2500 (men) calories per day. The requirements are influenced by various factors, including gender, height, weight, activity level, and age and vary from 1.600 to 2.400 for adult women and 2.000 to 3.000 for men.


#Eva V. Osilla, et al. Calories. 2022

https://www.ncbi.nlm.nih.gov/books/NBK499909/

Quote: “On average, a woman should eat 2000 calories per day to maintain her weight, and she should limit her caloric intake to 1500 or less in order to lose one pound per week. For the average male to maintain his body weight, he should eat 2500 calories per day, or 2000 a day if he wants to lose one pound per week.”


The table A2-2, in page 140 from the following guide gives a detail calorie intake requirement for men and women.


# U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020-2025. 2020. 

https://www.dietaryguidelines.gov/resources/2020-2025-dietary-guidelines-online-materials

Quote: “Table A2-2 Estimated Calorie Needs per Day, by Age, Sex, and Physical Activity Level, Ages 2 and Older."

There are various studies from the past decades that also found that people living in industrialized countries have similar total energy expenditures compared with people living in developing countries, but those studies did not get a lot of attention at the time they were published.


#Dugas Lara R., et al. Energy expenditure in adults living in developing compared with industrialized countries: a meta-analysis of doubly labeled water studies. 2011

https://www.sciencedirect.com/science/article/pii/S0002916523020750

Quote: “TEE, unadjusted for body size or weight, was ≈1 MJ/d lower for both men and women in the low- or middle-HDI group than in the high HDI group."

Other studies also showed no difference in energy expenditure between rural and industrialized populations.


#Amy Luke, et al. Energy expenditure does not predict weight change in either Nigerian or African American women. 2009

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2647711/

Quote: “These data suggest that interindividual levels of energy expended during activity do not have a large influence on age-related trends in adiposity. In addition, contrary to expectations, these data suggest that mean activity energy expenditure does not vary substantially between contemporary social groups with low and high prevalences of obesity.”


The Aymara, a rural agro pastoral community living in the mountains of Bolivia also have similar total energy expenditure compared to people living at lower altitudes.


# H Kashiwazaki, et al. Energy expenditure determined by the doubly labeled water method in Bolivian Aymara living in a high altitude agropastoral community. 1995

https://pubmed.ncbi.nlm.nih.gov/7572734/#:~:text=When%20compared%20with,the%20adult%20males.

Quote: “When compared with other DLW studies for free-living nonobese adults, the RMR of adult Aymara subjects normalized by the ratio method (RMR:FFM) and by the regression-based method (RMR adjusted with FFM as the covariate) was not significantly different from that observed in subjects living in low altitudes. As compared to FAO/WHO/UNU (1985) recommendations, activity levels were classified as heavy for the adult females and moderate-heavy for the adult males.”




The amount of energy that active people burn on a daily basis is not correlated to the amount of physical activity they do. Instead of a linear relationship, like in “more activity, more energy burn”, the total amount of energy they spend reaches a plateau in the most active people, and their metabolism adapts to burn the same energy as in less active people. 


#Herman Pontzer, et al. Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans. 2016

https://www.cell.com/fulltext/S0960-9822(15)01577-8.

Quote: “Here we tested a Constrained total energy expenditure model, in which total energy expenditure increases with physical activity at low activity levels but plateaus at higher activity levels as the body adapts to maintain total energy expenditure within a narrow range. We compared total energy expenditure, measured using doubly labeled water, against physical activity, measured using accelerometry, for a large (n = 332) sample of adults living in five populations [9]. After adjusting for body size and composition, total energy expenditure was positively correlated with physical activity, but the relationship was markedly stronger over the lower range of physical activity. For subjects in the upper range of physical activity, total energy expenditure plateaued, supporting a Constrained total energy expenditure model. Body fat percentage and activity intensity appear to modulate the metabolic response to physical activity. Models of energy balance employed in public health [1, 2, 3] should be revised to better reflect the constrained nature of total energy expenditure and the complex effects of physical activity on metabolic physiology."

The graph below shows that the adjusted total energy expenditure increases with more physical activity (measured as CPM/d), but the slope (yellow line) decreases at around 230 CPM/d (marked with the blue vertical line) and reaches a plateau. People in the group that exercised most (around 400 CPM/d), on average burned around 100 kcal more than the group that exercised less (around 50 CPM/d).


Quote: “The effect of physical activity on total energy expenditureADJ was non-linear, with a plateau in daily energy expenditure over the upper four deciles (60th–100th percentile) of CPM/d (Figure 2A). This plateau was evident in the lowess regression and in the change in median total energy expenditureADJ over the range of CPM/d deciles (Figure 2A). The slope of the lowess regression decreases markedly above 200 CPM/d, such that above 219 CPM/d, each additional increment of 100 CPM/d is associated with less than 50 kcal/day increase in total energy expenditureADJ. We used two approaches to determine the activity level above which the effect of physical activity on total energy expenditureADJ was negligible."

One apple of medium size provides around 95 kcal.


#Harvard T.H. Chan School of Public Health. The Nutrition Source: Apples. ND

https://www.hsph.harvard.edu/nutritionsource/food-features/apples/

Quote: “One serving, or one medium apple, provides about 95 calories, 0 gram fat, 1 gram protein, 25 grams carbohydrate, 19 grams sugar (naturally occurring), and 3 grams fiber.”




The main predictor of TEE (total energy expenditure) is body mass (which is another way of saying "per kg of body mass"), and not physical activity, or lifestyle. 


Different populations (e.g. the Hadza people and people living in western countries) have different average body mass, so to draw comparisons between them, studies normalize the measurements to body mass, or weight.


#Herman Pontzer. Energy Constraint as a Novel Mechanism Linking Exercise and Health. 2018

https://journals.physiology.org/doi/full/10.1152/physiol.00027.2018?rfr_dat=cr_pub++0pubmed&url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org

Quote: “TEE measured during normal daily life reflects years or even decades of habituation to some customary level of physical activity. It follows that physical activity is a poor predictor of TEE among populations (19, 20, 58–64). The duration effect should also inform future tests of the Constrained TEE model, since compensation to increased physical activity is inconsistent before ~6 mo (FIGURE 2). Notably, metabolic compensation appears to be much faster in rodents and birds, apparent in weeks rather than months (56, 63), suggesting that the rate of compensation might be related to body size or mass-specific metabolic rate.”


The paper below explains that people with different levels of activity still have similar total energy expenditures. The body distributes the energy consumed among all “energy consuming buckets” — physical activity, NEAT (non-exercise activity thermogenesis), basal metabolism, food-induced thermogenesis — and keeps an overall stable total energy consumption. 


#Herman Pontzer. Energy Constraint as a Novel Mechanism Linking Exercise and Health. 2018

https://pubmed.ncbi.nlm.nih.gov/30303776/.

Quote: “Humans and other species adapt dynamically to changes in daily physical activity, maintaining total energy expenditure within a narrow range. Chronic exercise thus suppresses other physiological activity, including immunity, reproduction, and stress response. This exercise-induced downregulation improves health at moderate levels of physical activity but can be detrimental at extreme workloads."

The total energy expenditure stays constant and the increase in activity gets compensated by basal metabolic processes that keep our internal organs working. Some people compensate more than others, and during a person's life, the degree of compensation can vary.


#Vincent Careau, et al. Energy compensation and adiposity in humans. 2021

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8551017/

Quote: “Thus, humans living typical modern lives – not undertaking exceptional levels of activity or experiencing chronic food shortages – exhibit a fairly strong compensation between the energy they expend on activity and that expended on basal metabolic processes; over the long term more than a quarter of the extra calories burned by people during activity do not translate into extra calories expended that day.”


This graph shows in C. the three energy management models. The authors proved the compensation model, where total energy expenditure stays the same when physical activity increases, and gets compensated by decrease in basal energy expenditure.


All types of resistance training increase muscle volume compared to no exercise.


#Brad S Currier, et al. Resistance training prescription for muscle strength and hypertrophy in healthy adults: a systematic review and Bayesian network meta-analysis. 2023

https://bjsm.bmj.com/content/57/18/1211

Quote: “Twelve distinct RT prescriptions and non-exercising control groups were compared using network meta-analysis to determine their effect on gains in muscle strength, hypertrophy and improvements in physical function in healthy adults. Compared with no exercise, most load, sets and frequency combinations increased muscle strength and hypertrophy, indicating that several RTx resulted in beneficial skeletal muscle adaptations.”


Exercise increases muscle volume either by adding more cytoplasmic volume or more nuclei to the muscular fiber.


#Michael Attwaters, et al. Cellular and molecular pathways controlling muscle size in response to exercise. 2021

https://febs.onlinelibrary.wiley.com/doi/full/10.1111/febs.15820

Quote: “Skeletal muscle can grow in three ways, by generation of new syncytial fibres, addition of nuclei from muscle stem cells to existing fibres or increase in cytoplasmic volume/nucleus. Evidence suggests the latter two processes contribute to exercise-induced growth. Fibre growth requires increase in sarcolemmal surface area and cytoplasmic volume at different rates.”


Maintaining a bigger body requires more energy. More body mass means more cells and bigger muscle fibers which in turn need more energy.


# K R Westerterp. Control of energy expenditure in humans. 2017

https://www.nature.com/articles/ejcn2016237#:~:text=Body%20size%20mainly,at%20adult%20age.

Quote: “Body size mainly affects the maintenance component of TEE, through the relation between body size and fat-free mass. Larger body size implies a larger fat-free mass (Figure 1). Taller subjects have a larger fat-free mass than subjects with a short stature and fat-free mass is larger in overweight and obese subjects than in lean subjects with the same stature. Thus, REE is generally higher in men than in women of similar weight and height and increases with weight gain at adult age.”

“In conclusion, energy expenditure is higher in larger subjects due to higher energy expenditure for maintenance.”


The size of your muscles (what is considered fat-free mass) is directly related to your total energy expenditure – more so than physical activity. 


#Herman Pontzer, et al. Energy expenditure and activity among Hadza hunter-gatherers. 2015

https://onlinelibrary.wiley.com/doi/10.1002/ajhb.22711 (Paywall)

Quote: “Fat-free mass was the single strongest predictor of TEE among Hadza adults (r2 = 0.66, P < 0.001). Hadza men used greater daily walking distances and faster walking speeds compared with that of Hadza women, but neither sex nor any measure of physical activity or work load were correlated with TEE in analyses controlling for fat-free mass. Compared with developed, industrial populations, Hadza adults had similar TEE but elevated levels of metabolic stress as measured by 8-hydroxydeoxyguanosine.”


But studies show that even during a long period where people gain or lose weight, the total energy they spent remained constant.


#Rebecca Rimbach, et al. Total energy expenditure is repeatable in adults but not associated with short-term changes in body composition. 2022

https://www.nature.com/articles/s41467-021-27246-z

Quote: “Our findings show that TEE measurements are repeatable in adults, also in adults older than 50 y, and over extended periods of time. The stability in adjusted TEE among adults is remarkable given the degree to which body weight and composition changed among subjects in our sample.”


Another study of 383 men and 387 women aged 4–91 years, recruited between 2008 and 2015 in the UK showed that total energy expenditure (TEE) increases with age but remain stable in groups of adults with similar body composition. The study found no effect of diet or geographical location on TEE.


#Soren Brage, et al. Descriptive epidemiology of energy expenditure in the UK: findings from the National Diet and Nutrition Survey 2008–15. 2020 

https://academic.oup.com/ije/article/49/3/1007/5810163?login=false.

Quote: “We found little evidence that energy expenditure varied by geographical region, over time, or by dietary macronutrient composition.”

“Here, we report gold-standard-measured EE from a nationally representative cross-sectional UK survey. Our results show how TEE and PAEE vary according to age, sex and body composition but no differences were observed by geographical region of the UK or over time in the period between 2008 and 2015.


Our results demonstrate that males accumulate higher overall levels of TEE and PAEE than females across all ages—a finding that is consistent with other British cohort studies investigating energy expenditure by objective methods.31–36 Age was an important correlate of PAEE and TEE in both sexes, with similar patterns across the lifespan for all EE measures; absolute TEE peaks in the early adult years, before dropping off around retirement age, whereas relative TEE and PAEE are highest in the earliest years of life before gradually declining steeply at first and reflecting in part natural growth and development, and then more shallowly after the age at which adult height is typically attained.”




There are different mechanisms at play when it comes to regulating total energy expenditure. One of these mechanisms happens after a brief period where more physical exercise results in using more energy – but then energy expenditure reaches a plateau, even if you are more physically active. One of its effects is adapting your behavior, so for example you might fidget less. 


#Herman Pontzer, et al. Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans. 2016

https://www.cell.com/fulltext/S0960-9822(15)01577-8

Quote: “Rather than increasing total energy expenditure linearly in response to physical activity, individuals tend to adapt metabolically to increased physical activity, muting the expected increase in daily energy throughput [5, 10, 11, 12]. These metabolic changes can be behavioral, such as sitting instead of standing, or fidgeting less, but they may also include reductions in other, non-muscular metabolic activity. For example, men and women enrolled in a long-term exercise study exhibited reduced basal metabolic rate at week 40 [11], and studies in healthy adult women have shown suppressed ovarian activity and lower estrogen production in response to moderate exercise [13]. Other species have also been shown to keep total energy expenditure remarkably constant in response to increased physical activity, reducing energy expenditure on growth [14], somatic repair [15, 16], and basal metabolic rate [17, 18] and even reducing lactation and cannibalizing nursing offspring [19], even when food is available ad libitum and total energy expenditure is well within maximum sustained levels.”


Most people are unaware that they were compensating for their increase in physical activity.


#Phillip Gray, et al. A qualitative investigation of physical activity compensation among older adults. 2017

https://bpspsychub.onlinelibrary.wiley.com/doi/10.1111/bjhp.12282.

Quote: “The findings suggest that the majority of participants were unaware that they had compensated in their PA, suggesting that this may be a non-volitional process. Most participants perceived PA compensation to hold negative implications for health and well-being. Physiological processes of fatigue and delayed onset of muscle soreness were cited as the principal cause of PA compensation, whereas psychological processes including a drive to be inactive, fear of overexertion, deficient motivation, and perceived time constraints were cited to a lesser extent.”




Several types of exercise do induce modest weight loss, at least during the first 6 or 12 months.


# Alice Bellicha, et al. Effect of exercise training on weight loss, body composition changes, and weight maintenance in adults with overweight or obesity: An overview of 12 systematic reviews and 149 studies. 2021

Quote: “Exercise led to a significant weight loss (4 SR-MAs, MDs ranging from −1.5 to −3.5 kg), fat loss (4 SR-MAs, MDs ranging from −1.3 to −2.6 kg) and visceral fat loss (3 SR-MAs, SMDs ranging from −0.33 to −0.56)."

This study looked at how total energy expenditure changes over time after people start exercising: 

at the beginning of a physical activity practice, some people may have an increased amount of energy expenditure than others with strong differences at 10 weeks, which decrease with time and stabilize at 80 weeks of physical exercise. The degree of energy compensation reached 84% at 80 weeks.


#Marie-Ève Riou, et al. Predictors of Energy Compensation during Exercise Interventions: A Systematic Review. 2015

Quote: “In conclusion, results from this systematic review show that initial FM, age and the duration of the intervention are the most significant predictors of energy compensation. The current findings demonstrate that when negative energy compensation is achieved with ExEE, it can only be maintained over a relatively short time span. In contrast, longer term exercise interventions are accompanied by levels of energy compensation that hover around 84%, which could be related to the more potent expression of compensatory mechanisms that oppose the decrease of body energy stores over longer periods of time.”


This graph shows that in the long run, almost all the amount of energy burned by physical activity gets compensated by the body.

There is one exception to all this – professional athletes. A Tour de France cyclist weighing less than 70 kg can burn an astonishing 8,000 calories a day. But professional athletes are rare cases, who push the limits of human capacity, and who could hardly be compared to the vast majority of human beings.


#Braun H. et al. Position of the Working Group Sports Nutrition of the German Nutrition Society (DGE): Energy Needs in Sports. 2020

https://www.germanjournalsportsmedicine.com/archive/archive-2020/issue-7-8-9/energy-needs-in-sports-position-of-the-working-group-sports-nutrition-of-the-german-nutrition-society-dge/

Quote: “In extreme situations such as the Tour de France or Race Across America, energy expenditure may be five times as much as when the athletes are at rest (18, 43). For individual long-distance races, this can result in an energy expenditure of >10,000 kcal per day.”


Generally, endurance athletes have increased total energy expenditure.


#Eimear Dolan, et al. Energy constraint and compensation: Insights from endurance athletes. 2023

https://www.sciencedirect.com/science/article/abs/pii/S1095643323001332.

Quote: “Endurance athletes have high energy expenditure, indicating that compensation to increased physical activity is partial.”


Professional cyclists for example had a mean total daily energy expenditure of 7576,482 kcal/day (31.7 MJ/day).


# Bas Van Hooren, et al. Determination of energy expenditure in professional cyclists using power data: Validation against doubly labeled water. 2022

https://onlinelibrary.wiley.com/doi/10.1111/sms.14271

Quote: “Mean TDEE over the 3-week Vuelta was 31.7 MJ/day (range 27.7–35.2 MJ/day), corresponding to a mean PAL-value of 4.0 (range 3.5–4.3). These findings are very similar to those obtained by DLW during the Tour de France (i.e., mean TDEE 33.7 MJ/day, PAL 4.3–5.3) and Giro d'Italia (i.e., mean TDEE of 32.3 MJ/day, PAL 4.47, 8) and can be explained by the strong comparability of all three race events.”




When a tissue is damaged, the immune system acts rapidly to repair it and keep the infection under control.


# Amro M. Soliman, et al. Acute Inflammation in Tissue Healing. 2023

https://www.mdpi.com/1422-0067/24/1/641

Quote: “Following an injury, danger/damage-associated molecular patterns (DAMPs) [30], generated by necrotic cells, as well as pathogen-associated molecular patterns (PAMPs) [31], including conserved motifs of invading pathogens, are recognized by innate receptors on tissue-resident cells to trigger an acute inflammatory reaction [17] (Figure 1). As a result, various inflammatory mediators are released to promote leukocyte recruitment and regulate immune responses at the injury site."


The acute inflammation triggered by an injury or infection can last a few days and it is the body’s first line of defense. If it lasts for longer periods, like months or years it can cause more harm than good. 


# Roma Pahwa, et al. Chronic Inflammation. 2023

https://www.ncbi.nlm.nih.gov/books/NBK493173/

Quote: “Inflammation is part of the body's defense mechanism. It is the process by which the immune system recognizes and removes harmful and foreign stimuli and begins the healing process. Inflammation can be either acute or chronic.[1][2][3]” 

“Tissue damage due to trauma, microbial invasion, or noxious compounds can induce acute inflammation. It starts rapidly, becomes severe in a short time and symptoms may last for a few days for example cellulitis or acute pneumonia. Subacute inflammation is the period between acute and chronic inflammation and may last 2 to 6 weeks.”

“Chronic inflammation is also referred to as slow, long-term inflammation lasting for prolonged periods of several months to years. Generally, the extent and effects of chronic inflammation vary with the cause of the injury and the ability of the body to repair and overcome the damage.” 




Physical inactivity and obesity is one of the causes of chronic inflammation, and chronic inflammation is related to cardiovascular, autoimmune and neurodegenerative diseases as well as cancer and other health problems.


# David Furman, et al. Chronic inflammation in the etiology of disease across the life span. 2019

https://www.nature.com/articles/s41591-019-0675-0#

Quote: “physical inactivity can increase individuals’ risk for various non-communicable diseases because it is linked to obesity100 and, in particular, excessive visceral adipose tissue (VAT), which is a significant trigger of inflammation104,105,106. VAT is an active endocrine, immunological and metabolic organ composed of various cells (including immune cells, such as resident macrophages) that expands mostly through adipocyte hypertrophy, which can lead to areas of hypoxia and even cell death, resulting in activation of hypoxia-inducible factor-1α, increased production of reactive oxygen species, and release of DAMPs (for example, cell-free DNA). These events can induce the secretion of numerous pro-inflammatory molecules, including adipokines, cytokines (for example, IL-1β, IL-6, TNF-α), and chemokines (especially monocyte chemoattractant protein-1) by adipocytes, endothelial cells and resident adipose tissue immune cells (for example, macrophages)105,106,107,108. This in turn leads to the infiltration of various immune cells in the VAT, including monocytes, neutrophils, dendritic cells, B cells, T cells and NK lymphocytes, and a reduction in T regulatory cells, thereby amplifying inflammation, which can eventually become prolonged and systemic in some individuals."

The chronic inflammation also debilitates the immune system and makes the body less able to defend against infections.


# David Furman, et al. Chronic inflammation in the etiology of disease across the life span. 2019

https://www.nature.com/articles/s41591-019-0675-0

Quote: “Despite the observation that SCI generally increases with age, a majority of older adults experience a down-regulation of components of the immune response that leads to an increased susceptibility to viral infections and weakened responses to vaccines.”




When we don´t exercise, our body is in a state of chronic inflammation and stress, in part because calories coming from the diet have to be burned or stored somewhere (that is why we called them “free calories”) but also because of the lack of all the benefits that exercise has on our body. 


As we start exercising, this excessive inflammation is reduced, the nervous system produces less cortisol (the stress hormone) and our body has less reproductive hormones like estrogen, progesterone and testosterone, reducing the risk of cardiovascular diseases, cancer and infections associated with low levels of exercise. Exessive levels of exercise can backfire and actually damage our health.


#Herman Pontzer. Energy Constraint as a Novel Mechanism Linking Exercise and Health. 2018

https://journals.physiology.org/doi/full/10.1152/physiol.00027.2018?rfr_dat=cr_pub++0pubmed&url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org.


Quote: “The Constrained TEE model posits that TEE is maintained within a narrow range (63). As daily physical activity increases, other components of daily energy expenditure are reduced to keep TEE in check. Non-essential expenditure is expected to decrease first; essential activity should be spared unless physical activity workload becomes too great. Moving from a sedentary to an active lifestyle leads to downregulation of non-essential expenditures including reduced inflammation (WBC, white blood cell), reduced HPA and SNS reactivity (C, cortisol; NE, norepinephrine), and reduced reproductive hormone levels (E, estrogen; Prog., progesterone; T, testosterone). These reductions lower the risk for a broad range of chronic diseases (CVD, cardiovascular disease; T2D, Type 2 diabetes). At extreme levels of physical activity (e.g., elite athletes), essential function is compromised, negatively affecting health. Consequently, all-cause morbidity and mortality may exhibit a U-shaped curve with physical activity, with increased chronic disease among sedentary people and increased infection and other maladies among the extremely active."


Cortisol has several functions on our body: it regulates the stress response, metabolism and circadian cycle. It follows a cycle in our body, increasing and decreasing daily (to regulate sleep) and in response to stressors. What we do not need are disregulated cortisol levels. 

Regular exercise keeps levels of cortisol regulated, increasing and decreasing when we need it. Several studies have shown that people that exercise normally have a more regulated pattern of cortisol, meaning they have less exaggerated increases in cortisol after a stressful event, compared to people that are more inactive.


#Herman Pontzer. Energy Constraint as a Novel Mechanism Linking Exercise and Health. 2018

https://journals.physiology.org/doi/full/10.1152/physiol.00027.2018

Quote: “In an early meta-analysis, Crews and Land (17) demonstrated that the stress response was reduced among physically fit subjects. In a recent and comprehensive review, Silverman and Deuster (76) discuss the body of evidence consistently showing that individuals with greater physical fitness (i.e., with greater long-term levels of daily physical activity) exhibit less HPA and SNS reactivity to acute stressors. For example, Rimmele and colleagues (69) reported that trained, physically fit men showed a smaller increase in cortisol and heart rate (a measure of SNS activity) in response to a psychological stressor (Trier Social Stress Test) than untrained, less fit counterparts. Notably, trained and untrained men had similar baseline cortisol levels and heart rates (69). Intense physical training can also blunt the cortisol response to submaximal exercise (82). Studies are mixed as to whether chronic exercise also blunts the cortisol awakening response (CAR); the effect may only be evident with large exercise workloads (2). A recent within-subjects study design in recreational runners found a decrease in cortisol awakening response with greater exercise workload.”


Because cortisol increases with stress and acute stressful events (like running from a tiger), cortisol is also known as the stress hormone. In a situation where our body needs to react quickly to stay alive, it helps to provide energy by increasing blood sugar, heart rate and alertness. 


#Lauren Thau, et al. Physiology, Cortisol. 2023

https://www.ncbi.nlm.nih.gov/books/NBK538239/#:~:text=In%20times%20of,to%20the%20body.

Quote: “In times of stress, the SNS gets activated. The SNS is responsible for the fight or flight response, which causes a cascade of hormonal and physiological responses. The amygdala is responsible for processing fear, arousal, and emotional stimuli to determine the appropriate response. If necessary, the amygdala sends a stress signal to the hypothalamus.[5] The hypothalamus subsequently activates the SNS, and the adrenal glands release a surge of catecholamines, such as epinephrine. This results in effects such as increased heart rate and respiratory rate. As the body continues to perceive the stimuli as a threat, the hypothalamus activates the HPA axis. Cortisol is released from the adrenal cortex and allows the body to continue to stay on high alert. Acutely, cortisol’s catabolic mechanisms provide energy to the body.”


Stress can lead to increased hormone levels including glucocorticoids, catecholamines, growth hormone and prolactin, by the pituitary-adrenal axis, gonads, hypothalamus and thyroid glands. Some of these changes are necessary for the fight or flight response to protect oneself.


# Salam Ranabir and K. Reetu. Stress and hormones. 2011

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3079864/.

Quote: “Reactions to stress are associated with enhanced secretion of a number of hormones including glucocorticoids, catecholamines, growth hormone and prolactin, the effect of which is to increase mobilization of energy sources and adapt the individual to its new circumstance.”




Apart from mediating the stress response, cortisol also regulates metabolism, sleep, inflammatory response, and immune function and is directly related to metabolism. But sustained levels of cortisol can be detrimental to the body.

When we are under stress for a long period, the sustained cortisol production affects almost all parts of the body, from the brain, to the immune system and muscles.


# Agnese Mariotti. The effects of chronic stress on health: new insights into the molecular mechanisms of brain–body communication. 2015

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5137920/#:~:text=In%20situations%20in,organs%20and%20tissues

Quote: “In situations in which the stressor is overwhelming and cannot be resolved, stress becomes chronic. In this case, the GC-dependent negative feedback mechanism that controls the stress response does not work, GC receptor resistance develops, and the systemic levels of the molecular mediators of stress remain high, compromising the immune system and damaging in the long-term multiple organs and tissues."

People that suffer chronic stress are more prone to depression and other mental problems.


# Agnese Mariotti. The effects of chronic stress on health: new insights into the molecular mechanisms of brain–body communication. 2015

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5137920/.

Quote: “It has been shown that chronic stress is linked to macroscopic changes in certain brain areas, consisting of volume variations and physical modifications of neuronal networks. For example, several studies in animals have described stress-related effects in the prefrontal cortex (PFC) and limbic system, characterized by volume reductions of some structures, and changes in neuronal plasticity due to dendritic atrophy and decreased spine density [4]. These morphological alterations are similar to those found in the brains of depressed patients examined postmortem, suggesting that they could also be at the basis of the depressive disorders that are often associated with chronic stress in humans. This hypothesis is supported by imaging studies that evidenced structural changes in the brain of individuals suffering from various types of stress-related disorders, such as those linked to severe traumas, major negative life events or chronic psychosocial strain.”




Since a long time, it is well known that exercise has an anti-inflammatory effect on our body, reducing chronic inflammation.


# Michael Gleeson, et al. The anti-inflammatory effects of exercise: mechanisms and implications for the prevention and treatment of disease. 2011

https://www.nature.com/articles/nri3041#:~:text=The%20protective%20effect,bout%20of%20exercise.

Quote: “The protective effect of exercise against chronic inflammation-associated diseases may, to some extent, be ascribed to an anti-inflammatory effect of regular exercise. The anti-inflammatory effect of regular exercise may be mediated by a reduction in visceral fat mass (with a subsequent decreased release of adipokines from adipose tissue) and/or by the induction of an anti-inflammatory environment with each bout of exercise.”


This graph shows that in normal adipose tissue, where fat cells are normal sized, and anti-inflammatory M2 macrophages can be seen. In fat tissue  related to obesity, fat cells are bigger, and pro-inflammatory macrophages (M1) predominate.

This graph shows the antiinflammatory effect of exercise on several organs.

Moderate exercise has a positive effect on the immune system, while high intensity can have harmful effects, as this systematic review found.


# Érica Cerqueira, et al. Inflammatory Effects of High and Moderate Intensity Exercise—A Systematic Review. 2020

https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2019.01550/full.

Quote: “In summary, intense long exercise can lead, in general, to higher levels of inflammatory mediators, and thus might increase the risk of injury and chronic inflammation. In contrast, moderate exercise or vigorous exercise with appropriate resting periods can achieve maximum benefit.”


People that exercise regularly have less cortisol levels, better mood and less anxiety.


# Manuel Mücke, et al. Influence of Regular Physical Activity and Fitness on Stress Reactivity as Measured with the Trier Social Stress Test Protocol: A Systematic Review. 2018

https://pubmed.ncbi.nlm.nih.gov/30159718/#:~:text=Cortisol%20and%20heart,to%20the%20TSST.

Quote: “Cortisol and heart rate reactivity were attenuated by higher PA or better fitness in seven of twelve studies and four of nine studies, respectively. Two of four studies reported smaller increases in anxiety and smaller decreases in calmness in physically active/fitter participants. Three of four studies found that higher PA/fitness was associated with more favorable mood in response to the TSST.”


Physical exercise is an effective intervention for depression. It also could be a viable adjunct treatment in combination with antidepressants.


# Siri Kvam, et al. Exercise as a treatment for depression: A meta-analysis. 2016

https://www.sciencedirect.com/science/article/abs/pii/S0165032715314221#:~:text=A%20total%20of,trended%20toward%20significance.

Quote: “Physical exercise had a moderate to large significant effect on depression compared to control conditions (g=−0.68), but the effect was small and not significant at follow-up (g=−0.22). Exercise compared to no intervention yielded a large and significant effect size (g=−1.24), and exercise had a moderate and significant effect compared to usual care (g=−0.48). The effects of exercise when compared to psychological treatments or antidepressant medication were small and not significant (g=−0.22 and g=−0.08, respectively). Exercise as an adjunct to antidepressant medication yielded a moderate effect (g=−0.50) that trended toward significance.”


Those beneficial health effects are also especially visible in more active populations. Different hunter-gatherers and other small-scale population have remarkable metabolic and cardiovascular health, that is more related to their environment and lifestyle, than their genes.


# H. Pontzer. Hunter-gatherers as models in public health. 2018

https://onlinelibrary.wiley.com/doi/full/10.1111/obr.12785

Quote: “One common element of traditional lifestyles that clearly seems to be protective against non-communicable disease is a high level of daily physical activity. Hunter-gatherers and subsistence farmers accumulate several times as much physical activity each day as people in developed countries (Fig. 2). The health benefits of exercise are well established, and increasing daily activity is already a common focus in public health worldwide.”


This graph shows that traditional hunter-gatherer populations have more physical activity than people living in industrialized countries.


This is the core message of the constrained energy expenditure hypothesis: increase in daily exercise does not increase the total energy expenditure. Instead, the amount of calories burnt basically stays the same, because our bodies compensate for the additional energy needs elsewhere. 


# Herman Pontzer. Energy Constraint as a Novel Mechanism Linking Exercise and Health. 2018

https://pubmed.ncbi.nlm.nih.gov/30303776/

Quote: “Consider these changes in energy allocation in the context of a Constrained TEE model, in which TEE is limited and increased physical activity expenditure incurs corresponding reductions in other expenditures. We should expect most (perhaps all) species to exhibit an evolved energy compensation strategy that reduces non- essential expenditures first, sparing essential functions as physical activity increases. Once non-essential reductions are exhausted, further increases in physical activity would lead to reductions in essential functions, compromising survival and reproduction."

When the activity level is very high, like in endurance athletes, this compensation could have harmful effects on the overall health.


# Eimear Dolan, et al. Energy constraint and compensation: Insights from endurance athletes. 2023

https://www.sciencedirect.com/science/article/abs/pii/S1095643323001332.

Quote: “That energetic compensation that occurs in the face of high activity expenditure may be primarily driven by low energy availability i.e., the amount of energy available for all biological processes after the demands of exercise have been met, and not by activity expenditure per se."

Why Humans Are So Hungry


 

We evolved in an environment where we mostly had to work hard to get our food and that also followed seasonal changes. Our bodies evolved to limit the amount of energy we spend on a daily basis, to keep other bodily functions working, like reproduction or fighting diseases. 

Moving was not optional, was part of life and as vital as other mechanisms, so our bodies have a mechanism to adapt to increased levels of activity and biological needs.


#Pontzer, Herman. Constrained Total Energy Expenditure and the Evolutionary Biology of Energy Balance. 2015

https://journals.lww.com/acsm-essr/fulltext/2015/07000/constrained_total_energy_expenditure_and_the.3.aspx.

 Quote: “A Constrained TEE strategy keeps energy requirements in check while allowing the organism to prioritize and allocate energy among various organ systems in a dynamic manner that is responsive to current conditions and maximizes lifetime reproductive fitness.”




From an evolutionary perspective, the Constrained Total Energy Expenditure model has the advantage of promoting survival during periods of scarcity or disease and to prioritize reproduction.


#Pontzer, Herman. Constrained Total Energy Expenditure and the Evolutionary Biology of Energy Balance. 2015

https://journals.lww.com/acsm-essr/fulltext/2015/07000/constrained_total_energy_expenditure_and_the.3.aspx.

Quote: “However, in nearly every conceivable ecological scenario relevant to humans, a Constrained TEE strategy would be favored by natural selection. For organisms with high PA during food-poor periods, an Additive TEE strategy would maximize energy requirements precisely when the risk of starvation was most severe, whereas a Constrained TEE strategy would reduce energy requirements and mortality risk. For organisms that work hardest when stockpiling food energy during food-rich periods, a Constrained TEE strategy would lower energy requirements and thereby maximize surplus energy gain. A Constrained TEE also could reduce the risk of predation while foraging by reducing the amount of time needed to obtain food during high-PA periods.”




#Pontzer, Herman. Constrained Total Energy Expenditure and the Evolutionary Biology of Energy Balance. 2015

https://journals.lww.com/acsm-essr/fulltext/2015/07000/constrained_total_energy_expenditure_and_the.3.aspx.

Quote: “For organisms with high PA during food-poor periods, an Additive TEE strategy would maximize energy requirements precisely when the risk of starvation was most severe, whereas a Constrained TEE strategy would reduce energy requirements and mortality risk. For organisms that work hardest when stockpiling food energy during food-rich periods, a Constrained TEE strategy would lower energy requirements and thereby maximize surplus energy gain. A Constrained TEE also could reduce the risk of predation while foraging by reducing the amount of time needed to obtain food during high-PA periods.”


Also primates in captivity have similar total energy expenditure compared to primates living in the wild.


# Herman Pontzer. The crown joules: energetics, ecology, and evolution in humans and other primates. 2017

https://onlinelibrary.wiley.com/doi/10.1002/evan.21513.

Quote: “TEE was measured in 19 populations of primates (17 species) using DLW (Pontzer et al., 2014a). After controlling for body mass, populations housed in captivity (zoos, labs, and sanctuaries) had similar energy expenditures to those in the wild. Residual log10TEE, calculated from the primate mass: TEE regression in Pontzer et al. (2014a) do not differ between wild and captive populations (p = 0.42 t-test). Similar results have been reported for wild and captive kangaroos (Pontzer et al., 2014a and pandas (Nie et al., 2015). Variation in habitual activity levels is not correlated with TEE among human populations (Box 2)."


Obesity has increased dramatically in the last decades, and it is considered an epidemic by the WHO since 1990.


# WHO. Controlling the global obesity epidemic. ND

https://www.who.int/activities/controlling-the-global-obesity-epidemic.

Quote: “At the other end of the malnutrition scale, obesity is one of today’s most blatantly visible – yet most neglected – public health problems. Paradoxically coexisting with undernutrition, an escalating global epidemic of overweight and obesity – “globesity” – is taking over many parts of the world. If immediate action is not taken, millions will suffer from an array of serious health disorders.”


Obesity is a worldwide problem that is higher in industrialized countries like the USA.


# Hannah Ritchie and Max Roser. Obesity. 2017, revised 2024.

https://ourworldindata.org/obesity

Quote: “Globally, it’s estimated that around two-fifths of adults were overweight or obese in 2016.”

“As you can see, the share of people who are overweight tends to be higher in richer countries and lower in poorer countries. In many high-income countries such as the United States, it’s estimated that over 60% of adults are overweight or obese."

Recent research finds that our environment may also be influencing the obesity epidemic and maybe is changing our biology to make us eat more.


# Jerrold J. Heinde, et al. Obesogens: a unifying theory for the global rise in obesity. 2024

https://www.nature.com/articles/s41366-024-01460-3

Quote: “Here we propose a combined model of obesity, a unifying paradigm that links four general models: the energy balance model (EBM), based on calories as the driver of weight gain; the carbohydrate-insulin model (CIM), based on insulin as a driver of energy storage; the oxidation-reduction model (REDOX), based on reactive oxygen species (ROS) as a driver of altered metabolic signaling; and the obesogens model (OBS), which proposes that environmental chemicals interfere with hormonal signaling leading to adiposity. We propose a combined OBS/REDOX model in which environmental chemicals (in air, food, food packaging, and household products) generate false autocrine and endocrine metabolic signals, including ROS, that subvert standard regulatory energy mechanisms, increase basal and stimulated insulin secretion, disrupt energy efficiency, and influence appetite and energy expenditure leading to weight gain. This combined model incorporates the data supporting the EBM and CIM models, thus creating one integrated model that covers significant aspects of all the mechanisms potentially contributing to the obesity pandemic.”




A larger brain size — such as in the Homo lineage — has a high evolutionary price that could be met by increasing energy turnover or allocation to other body functions. Because we learned to cooperate, mothers were freed from exclusive childcare, so that development of cooperative childcare (which other primates don´t have) also freed time and energy to let our brains evolve to unprecedented levels in nature.


# Karin Isler and Carel P. van Schaik. How Our Ancestors Broke through the Gray Ceiling

Comparative Evidence for Cooperative Breeding in Early Homo. 2012

https://www.journals.uchicago.edu/doi/10.1086/667623#_i23:~:text=In%20conclusion%2C%20if,with%20extant%20apes

Quote: “In conclusion, if we rely on estimating the effect of evolutionary processes known to operate in primates or in vertebrates in general, there is evidence for several factors that allowed for brain-size expansion throughout the evolutionary history of the human lineage. A more seasonal environment, a change in diet toward higher-quality food sources, and more efficient locomotion all may have played a role (Potts 2011). Instead of a comprehensive but unique “adaptive suite” of human traits (Lovejoy 2009), however, we find broad comparative support for a decisive role of cooperative breeding as the initial trigger of many subsequent changes in human biology (Burkart, Hrdy, and van Schaik 2009; Burkart and van Schaik 2010). As such a redistribution of energy toward mothers and infants is possible without changing the overall energy budget, it may have facilitated subsequent changes that led to the relatively high energetic throughput of modern humans as compared with extant apes.”


Brain size of early humans increased over time, becoming more complex and providing adaptation advantages to new environments.


#James Di Loreto, & Donald H. Hurlbert, Smithsonian Institution. Bigger Brains: Complex Brains for a Complex World. ND

https://humanorigins.si.edu/human-characteristics/brains

Quote: “As early humans faced new environmental challenges and evolved bigger bodies, they evolved larger and more complex brains. Large, complex brains can process and store a lot of information. That was a big advantage to early humans in their social interactions and encounters with unfamiliar habitats."

Pontzer, H. et al. (2016): Metabolic acceleration and the evolution of human brain size and life history. Nature 533

https://www.nature.com/articles/nature17654 

Quote: “Humans are distinguished from the other living apes in having larger brains and an unusual life history that combines high reproductive output with slow childhood growth and exceptional longevity1 . This suite of derived traits suggests major changes in energy expenditure and allocation in the human lineage, but direct measures of human and ape metabolism are needed to compare evolved energy strategies among hominoids. Here we used doubly labelled water measurements of total energy expenditure (TEE; kcal day−1 ) in humans, chimpanzees, bonobos, gorillas and orangutans to test the hypothesis that the human lineage has experienced an acceleration in metabolic rate, providing energy for larger brains and faster reproduction without sacrificing maintenance and longevity. In multivariate regressions including body size and physical activity, human TEE exceeded that of chimpanzees and bonobos, gorillas and orangutans by approximately 400, 635 and 820 kcal day−1 , respectively, readily accommodating the cost of humans’ greater brain size and reproductive output. Much of the increase in TEE is attributable to humans’ greater basal metabolic rate (kcal day−1 ), indicating increased organ metabolic activity. Humans also had the greatest body fat percentage. An increased metabolic rate, along with changes in energy allocation, was crucial in the evolution of human brain size and life history.




Children require a lot of energy from the mother to raise alone. They say, “it takes a village to raise a child.” 


# Rebecca Sear and Ruth Mace. Who keeps children alive? A review of the effects of kin on child survival. 2008

https://www.sciencedirect.com/science/article/abs/pii/S1090513807001055.

Quote: “Children pose a problem. The extended period of childhood dependency and short interbirth intervals mean that human mothers have to care for several dependent children simultaneously. Most evolutionary anthropologists now agree that this is too much of an energetic burden for mothers to manage alone and that they must enlist help from other relatives to share the costs of raising children.


# Marcus E. Raichle* and Debra A. Gusnard. Appraising the brain's energy budget. 2002

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC124895/

Quote: “In the average adult human, the brain represents about 2% of the body weight. Remarkably, despite its relatively small size, the brain accounts for about 20% of the oxygen and, hence, calories consumed by the body (1). This high rate of metabolism is remarkably constant despite widely varying mental and motoric activity.”


Brains of other primates are smaller in size compared to ours, and consume less calories.


# Karina Fonseca-Azevedo and Suzana Herculano-Houzel. Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution. 2012

https://www.pnas.org/doi/full/10.1073/pnas.1206390109

Quote: “The human brain, in particular, has come to cost ∼20% of the total body resting metabolic rate, even though it represents only 2% of total body mass (MBD), whereas, in other primates, the brain consumes a lower percentage of the body resting metabolic rate of approximately 9%.”


Non-primate brains also consume less energy compared to humans.


# Trisha M Zintel, et al. Astrocytes Drive Divergent Metabolic Gene Expression in Humans and Chimpanzees. 2024

https://academic.oup.com/gbe/article/16/1/evad239/7504579

Quote: “The human brain is more energetically costly than that of other primates, utilizing ∼20% of all of the body's metabolic resources, in comparison with nonhuman primate brains that use <10%.”


Children’s brains require around 66% of the body’s resting metabolism and 43% of the body’s daily energy requirement that decreases as their body grows into puberty. This could mean that when the brain grows, the body waits, and when the brain is done with growing, then the body catches up, the price we pay for our brains is a long childhood.


# Christopher W. Kuzawa, et al. Metabolic costs and evolutionary implications of human brain development. 2014

https://www.pnas.org/doi/full/10.1073/pnas.1323099111

Quote: “We find that glucosermr% and glucoseder% do not peak at birth (52.5% and 59.8% of RMR, or 35.4% and 38.7% of DER, for males and females, respectively), when relative brain size is largest, but rather in childhood (66.3% and 65.0% of RMR and 43.3% and 43.8% of DER). Body-weight growth (dw/dt) and both glucosermr% and glucoseder% are strongly, inversely related: soon after birth, increases in brain glucose demand are accompanied by proportionate decreases in dw/dt. Ages of peak brain glucose demand and lowest dw/dt co-occur and subsequent developmental declines in brain metabolism are matched by proportionate increases in dw/dt until puberty. The finding that human brain glucose demands peak during childhood, and evidence that brain metabolism and body growth rate covary inversely across development, support the hypothesis that the high costs of human brain development require compensatory slowing of body growth rate.”


# Ethan Remmel. The Benefits of a Long Childhood. ND

https://www.americanscientist.org/article/the-benefits-of-a-long-childhood#:~:text=A%20big%20and,creating%20complex%20cultures.

Quote: “A big and complex brain takes a lot of time to develop, and in humans much of that development must occur after birth, because bipedalism limits birth-canal width, which has in turn constrained the head size of newborns. More specifically, social intelligence has often been postulated as the driving factor. In this view, as humans achieved ecological dominance, they became one another's principal competition for resources. Consequently, the ability to manage social relations and alliances was selected for, in what evolutionary biologist Richard Alexander has characterized as a cognitive "arms race" within the species. The result is that we are much smarter than we would need to be simply to succeed at hunting and gathering, and we are thus capable of creating complex cultures.”


Another quite unique characteristic of Homo sapiens is the high (potential) fertility rate given its prolonged lifespan. Across species, normally the ones that live longer, have fewer offspring. ​​


Mice for example live for about two years but after just a few weeks can give birth to three to eight babies for up to eight times a year. Elephants, on the other hand, may live for up to 80 years, but over the course of their lives, elephant cows are only able give birth to ten calves at most. 


# Stefano Giaimo and Arne Traulsen. Generation Time Measures the Trade-Off between Survival and Reproduction in a Life Cycle. 2019

https://www.journals.uchicago.edu/doi/10.1086/704155#:~:text=Survival%20and%20fertility%20are,in%20a%20life%20cycle.

Quote: “Survival and fertility are the two most basic components of fitness, and they drive the evolution of a life cycle. A trade-off between them is usually present: when survival increases, fertility decreases—and vice versa. Here we show that at an evolutionary optimum, the generation time is a measure of the strength of the trade-off between overall survival and overall fertility in a life cycle.”


Humans live longer and reproduce faster than other non-human apes. We do not only evolved an unexpected brain-capacity but also our longevity is linked to an unexpected fertility.


# Robert S Walker, et al. The trade-off between number and size of offspring in humans and other primates. 2008

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2596903/.

Quote: “Humans reproduce on average at rates of approximately 1.7-fold faster than non-human apes and only 19% slower than non-ape primates.”



In graph A we see that great apes can produce about 1,500 kcal per day, male hunter-gatherers at least 3,000. Humans have to invest around 5 hours for this (see graph B), the great apes around 7.5 hours. 

Great apes thus have to invest more effort for less compared to humans. This is summarized in graph C. There you can see that great apes produce around 200 kcal per hour, humans almost 1,000.  


#Thomas S. Kraft, et al. The energetics of uniquely human subsistence strategies. 2021

https://www.science.org/doi/full/10.1126/science.abf0130.

Quote: “Our cross-cultural database of traditional hunter-gatherers and horticulturalists (nhunter-gatherer = 14, nhorticulturalist = 22) confirms our findings from detailed study of the Hadza and Tsimane (table S2). Specifically, we found further evidence that humans of both subsistence modes produce more calories per day, spend less time on subsistence, and have higher return rates than other great apes (Fig. 8; see fig. S4 for results using observational acquisition data for nonhuman great apes). The few available published estimates of efficiency in subsistence horticulturalists also support the observation that human efficiencies are not elevated above those of other great apes (fig. S2)."

Humans cooperate with each other more than any species and producing lots of calories is deeply engrained in our species, so we keep doing it today. But contrary to our ancestors, today we don't need to do all the physical work that back then required harvesting those same calories. So this has created a "calorie imbalance": we keep harvesting as many calories as before, but we don't use them in physical activity anymore. And this imbalance creates health problems like obesity and and chronic inflammation.


# Robert Boyd and Peter J. Richerson. Culture and the evolution of human cooperation. 2009

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2781880/.

Quote: “Humans cooperate on a larger scale than most other mammals. Among social mammals, cooperation is mainly limited to relatives. There is little division of labour, no trade and no large scale conflict. Communication is limited to a small repertoire of self-verifying signals.”


# Brian Hare & Vanessa Woods. Humans Evolved to Be Friendly. Cooperation made Homo sapiens the last human species standing. 2020

https://www.scientificamerican.com/article/humans-evolved-to-be-friendly/#:~:text=The%20idea%20that,to%20cooperative%20communication.

Quote: “The idea that friendliness led to our success is not new. Neither is the idea that as a species, we became more intelligent. Our discovery lies in the relation between the two ideas: it was an increase in social tolerance that led to cognitive changes, especially those related to cooperative communication.”


# Thomas S. Kraft, et al. The energetics of uniquely human subsistence strategies. 2021

https://www.science.org/doi/full/10.1126/science.abf0130#:~:text=Our%20findings%20thus,care%2C%20and%20prosociality.

Quote: “Our findings thus help to unify and shed light on two dominant theories in human evolutionary studies: (i) the embodied capital model, which attributes large brain growth and a long developmental period in humans to the need to learn and develop difficult skills associated with extractive food acquisition (2), and (ii) the cooperative breeding hypothesis or pooled energy budget model, which posits that many derived features of human behavior, intelligence, and cognition are linked to a cooperative provisioning system that arose after the evolutionary split with the Pan lineage (38, 75, 76). Our findings suggest that increased daily production associated with extractive foraging in humans (2) was enabled by increased foraging intensity that reduced the time cost, but not the energy cost, of food acquisition. Such a strategy is unlikely to be tenable in the absence of a cooperative production and provisioning system with widespread sharing, divisions of labor, alloparental care, and prosociality.”


# Martin A. Nowak. Five rules for the evolution of cooperation. 2006

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3279745/#:~:text=Evolution%20is%20based,cooperation%20and%20defection.

Quote: “Evolution is based on a fierce competition between individuals and should therefore only reward selfish behavior. Every gene, every cell and every organism should be designed to promote its own evolutionary success at the expense of its competitors. Yet we observe cooperation on many levels of biological organization. Genes cooperate in genomes. Chromosomes cooperate in eukaryotic cells. Cells cooperate in multi-cellular organisms. There are many examples for cooperation among animals. Humans are the champions of cooperation: from hunter gatherer societies to nation states, cooperation is the decisive organizing principle of human society. No other life form on earth is engaged in the same complex games of cooperation and defection.”




As explained above, early humans became super efficient calorie harvesters. But that trait didn’t stay in the past. Rather, it fueled an evolutionary trend towards further increasing gathering and harvesting efficiency with time:

Kraft, T. et al. (2021): The energetics of uniquely human subsistence strategies. Science, 374
https://www.science.org/doi/full/10.1126/science.abf0130
 Quote: “Relative to other great apes, human subsistence strategies are characterized by high-intensity, high-cost extractive activities and expanded day ranges that provide more calories in less time. These results suggest that energy gained from improvements in efficiency throughout human evolution were primarily channeled toward further increasing foraging intensity rather than reducing the energetic costs of subsistence.” 


Moreover, this evolutionary increase in food gathering exploded in the last years due to the unprecedented rates of food production in industrial times:


Kraft, T. et al. (2021): The energetics of uniquely human subsistence strategies. Science, 374

https://www.science.org/doi/full/10.1126/science.abf0130  

Quote: “Our results also provide deeper evolutionary context for understanding modern trends in human time and energy budgets. [...] This has allowed for an unprecedented increase in the energy return on investment of labor (ratio of food energy produced to endosomatic energy invested in labor) for modern agriculture since the 1950s (103–106). [...] With the subsequent decoupling of industrial production from human and animal labor, industrialized populations have continued to experience reductions in the time costs of “subsistence.”


And this changed suddenly (in evolutionary terms) our relation to the energy contained in food.

Kraft, T. et al. (2021): The energetics of uniquely human subsistence strategies. Science, 374

https://www.science.org/doi/full/10.1126/science.abf0130   

Quote: “[...] humans have experienced a fundamental shift in our relationship with energy, setting up one of the major health challenges of our time: the rise of chronic noncommunicable “diseases of civilization” such as obesity, metabolic syndrome, and cardiovascular disease. Unburdened by the high physiological costs of food production, a human body that evolved to expend large quantities of energy to acquire food has now found itself in a potentially deadly mismatch.”


If to this modern increase in food production we add the fact that the total energy expenditure (TEE) in humans is only very weakly dependent on physical activity, a natural conclusion is that modern obesity stems from our evolutionary tendency to overproducing food, rather than to changes in physical activity:


Pontzer, H. et al. (2018): Hunter-gatherers as models in public health. Obesity Reviews, 19.
https://onlinelibrary.wiley.com/doi/10.1111/obr.12785 

Quote: “The constancy of TEE among a diverse range of lifestyles, including living hunter-gatherers and other small-scale societies, strongly suggests that the modern obesity pandemic stems from increased energy intake rather than decreased energy expenditure.”