Energy stocks

Carbohydrates. One gram contains around 4 kilocalories (around 17 kilojoules) of energy. When carbs are consumed, they are split into monosaccharides. Starch is split into molecules of glucose, sugar – into glucose and fructose, lactose – ingo glucose and galactose. And we absorb them in these forms. 

Fats. One gram contains around 9 kilocalories (around 38 kilojoules) of energy. This is the highest density of energy available to our organism.

Proteins. It is thought that one gram of proteins contains around 4 kilocalories (around 17 kilojoules) of energy, just like carbohydrates. Proteins are split into separate amino acids, which we absorb.  But we know that not all amino acids are used as a source of energy. 

Of course, this is about "dry" substances, without moisture content.

For a comparison: ATP contains 7.3 kilocalories per mole (the mass of one mole of ATP is 507 grams). Thus, one gram contains about 0.014 kilocalories (0.059 kilojoules) – a very low density of energy. But the advantage is that this energy is readily available almost without losses and exactly where it is needed.

We already know that stocks of ATP are tiny – for around 100 seconds of calm and 10 to 20 seconds of very intensive physical activity. But they are in permanent readiness and are located right in the cells, where life processes occur. This was selected by evolution – in case of danger, the organism may start to react almost immediately, and then we have a few seconds for speeding up the processes of ATP charging. And these several seconds often saved lives of our ancestors, and even now save lives sometimes. 

Glucose is stored mostly in muscles and liver in the form of glycogen. Glycogen is a biopolymer that forms around a molecule of protein glycogenin, which is an enzyme initiating formation of chains of glucose molecules. These chains easily attach to themselves lots of glucose molecules and easily give them away when necessary and in required quantities. "Easily" means that the energy spent on attachment and detachment of these molecules is rather low. 

But there is a nuance: together with molecules of glucose, glycogen attaches also molecules of water. As a result, the energy density of glycogen is not high – approximately 1.5 kilocalories per gram, almost three times as low as that of carbohydrons that we eat. 

If storages of glucose (liver and muscles) are full with glycogen, this energy stock will be enough for about one and a half days (about 36 hours) of life at very moderate physical activity. 

People who tried to lose weight by refraining from food (only-water fasting), noticed that in the first day of fasting they had lost a kilogram or more of weight. This is because of that nuance – storage of water together with glucose. Water is removed, and the weight decreases. And then, the stocks of glucose in glycogen reduce, and weight loss substantially slows down. 

Fats are stored in the form of fats. That is, the energy density of the stock of fat is the same as of the fats we eat – around 9 kilocalories per gram. Fats are stored in the fat tissue – special cells for storing fats. These cells can substantially increase their size to accept more fats, or reduce in size when fats are spent. 

With this energy density, 200-220 grams of fats are enough to satisfy a 24-hour requirement of energy of a person whose physical activity is rather low. We can accumulate "practically unlimited" stocks of energy in the form of fats. More precisely, there are certain limits, but they are far beyond the boundaries of common sense and requirements of survival. For instance, 9 to 10 kg of fat can provide enough energy for 40-45 days of life without food. But for life, we need many other things in addition to energy, therefore, living without food for more than 50 days is hardly possible. So, a stock of fats over 8 to 10 kg does not make sense. Sometimes this is formulated differently: a stock of fats over 12% of the body mass does not make sense. But people may have 30 or more kilograms of it, sometimes, much more. 

When stores of fat in the fat tissue are too large, the cells that store fat enlarge so much that they cannot support their own integrity and integrity of their own external membrane. Then they die, releasing the fat they have accumulated right into the organism. This causes chronical inflammation, which, in turn, stimulates further accumulation of fat.

Fat tissue can be subcutaneous (under the skin) and visceral (inside the body). The same fat tissue, only the places of its deposition are different. 

Subcutaneous fat is immediately visible, and this is the fat many people want to get rid of. But it does not make much of a health problem, even though its excess does not increase health. Besides, its excess looks unaesthetically. But there are completely healthy people with large stocks of subcutaneous fat (and almost no visceral one). A certain stock of subcutaneous fat provides energy safety and substantial independence from the time of the next meal. 

On the contrary, visceral fat is very undesirable. It accumulates in the fat tissue around internal organs and interferes with their normal operation. Its mere presence in large quantities indicates that something is wrong with the organism. 

The worst visceral fat is fat that is deposited right in the liver and other organs, not even around them. Over time, it leads to the fatty liver disease. This, in turn, may lead to the cirrhosis of the liver. Earlier this happened only to people who consumed too much alcohol for a very long time. But now, non-alcoholic fatty liver disease is observed even in children. Whether this disease alcoholic or non-alcoholic, is only a matter of its cause. But the outcomes are practically the same.  This kind of fat is called ectopic fat. It can be deposited not only in the liver, but also in other internal organs, and even in muscles. It is deposited not in the fat tissue, but right in the organs.

A limited amount of fats can be stored right in the muscle cells. This increases their ability to react to the necessity to elevate physical activity.  Obese people store more fats not only in the fat tissue, but also in the muscle cells. This does not mean that obese people are capable of higher physical activity than lean people. On the contrary, that excess fat storage in muscle cells poisons these cells, because muscle cells of obese people cannot use the fat accumulated in them as fuel effectively. Well-trained athletes can store about as much fats in muscle cells as obese people, but the cells of athletes can use these fats effectively, and these fats to not poison them.

We do not have stocks of proteins. All proteins are necessary for something. And they are constantly renewed, being made of amino acids we obtain from food. And those amino acids that are not used for making new proteins are not stored, they are converted into glucose. 

And at the same time, we have stocks of proteins. In the situations of acute shortage of proteins or food in general, our organism has to determine priorities. Some of the proteins are of lower priority, and they can be disassembled into spare parts (amino acids) to build more prioritized proteins out of them. In this respect, muscles can be considered as a stock of proteins. When there is nothing to eat for weeks, running, jumping and doing hard physical labor is usually out of the question. Therefore, large muscles in this situation become not particularly necessary, and proteins from them can be used to support the cardio-vascular system, nervous system and other prioritized systems. 

Besides, when there is a shortage of proteins, the organism is more thorough about recycling of proteins. Damaged or broken proteins are disassembled into amino acids, but are not converted into glucose. Instead, they are used for making new proteins to replace the broken ones. 

We do not have stocks of ketone bodies, and we almost do not get them with food. They are produced by our liver out of fatty acids and are used in all cells having mitochondria (and these are all cells, except for erythrocytes) as a fuel, when there is not enough glucose. When there is too much glucose, ketone bodies are not produced. More precisely, they are not produced when the level of insulin is too high, with some exceptions. But when there are ketone bodies in the blood, mitochondria can "burn" them for charging ATP even at a high insulin level.

What are those exceptions? They are so-called medium-chain triglycerides – fats with saturated fatty acids, the carbon chains of which contain 8 or 10 atoms of carbon (and also 6 atoms, but triglycerides with such short chains are rare in food). It is believed that, when such fats are consumed with food, the liver produces ketone bodies out of them even when the insulin level is elevated.

It is believed that ketone bodies are the best fuel for mitochondria. 

This control is done with the help of hormones. The key hormone in controlling the storage is insulin. It is produced by special cells in the pancreas (while other cells of the pancreas produce some other hormones) . The main function of insulin is to maintain concentration of glucose in blood within the normal limits. Both excess of glucose and its shortage can cause serious consequences, including coma and death.

When concentration of blood glucose increases, pancreas produces more insulin, and its concentration also increases. An elevated level of insulin is a command to cells of muscles and lever to take glucose from the blood and accumulate it as glycogen. Simultaneously, this is a command to the fat tissue cells not to release fats for its conversion into fuel for mitochondria, and if there is fat in blood, to accept it for storage. Because it is necessary to eliminate the excess of glucose, use of fats as the source of energy gets blocked. But when the "warehouses" of glycogen are already fully stuffed, and blood glucose is still high, high level of insulin is preserved, and this is a command to the liver to convert glucose into fats and a command to fat tissue to accumulate these fats (this is quite possible because there are practically no limits for storing fats). 

When concentration of glucose in blood decreases, after some time, insulin level also decreases. The baseline (normal) level of insulin is a command to the cells of muscles and liver to stop taking glucose from blood and to use the stocks of glycogen. Simultaneously, low level of insulin is a command to the fat tissue cells to release fats into blood and a command to the liver to produce fatty acids and ketone bodies out of these fats, so that other cells could use them as fuel for charging ATP instead of glucose. When the blood glucose level becomes even lower, the still lower level of insulin is a signal to the liver to release glucose from its stores of glycogen or to produce it out of fats or amino acids to maintain the normal level of blood glucose..

ATTENTION! This is NOT about too low or zero level of insulin. Zero or almost zero level of insulin is type one diabetes – a very dangerous autoimmune disease. When it occurs, special cells of the pancreas lose the ability to produce insulin. People with this disease need to inject insulin. Otherwise, they may quickly die. 

Normally, the level of insulin starts growing with a little delay after the increase of the level of glucose. But if we eat something sweet, the organism may start increasing insulin in advance, because something sweet usually means glucose (and, maybe, fructose), and this means that blood glucose will increase soon, so, the organism forestalls this. 

Soon, the level of insulin exceeds the threshold of spending fats. When insulin is above this threshold, fats cannot be spent. On the contrary, they must be stored. Also, glucose must be stored as glycogen or converted into fats by liver, and these fats must also be stored. 

When all food has been digested, and nutrients from it have been absorbed into the blood, the level of glucose gradually decreases, and, following it, the level of insulin decreases too. When the level of insulin becomes below the threshold of spending fats, the organism can spend glucose stored in glycogen and use fats for making fatty acids and ketones as fuel for mitochondria, until the next meal, if the control system works properly. 

We have considered how processes of accumulation and spending of energy are controlled by insulin. But the amount of food we it also needs to be regulated.

For this, there are additional control mechanisms in the organism. They evaluate the stocks of energy and intensity of its spending, and also periodicity of replenishment of stocks, and then send necessary control signals in the form of special hormones.

There is a hormone that signals that a certain time has already passed since the previous meal, and, perhaps, it is time to eat some more. It does not mean that we are running out of energy, and we need urgently to eat to prevent our death. It is just a reminder. The signal of this hormone is perceived as a light hunger or appetite, but it does not require an immediate action. Some people can easily ignore this signal or even not notice it, if they are occupied with something. Others feel almost panic and hurry to eat. But generally this is not a critical signal, and reaction to it can be trained.

There is a hormone signaling satiety, when a sufficient amount of energy has already been absorbed. This signal is "calculated" by the body considering the typical energy expenses, intervals and amounts of replenishment of stocks, possible emergencies that may either abruptly increase spending of energy or abruptly decrease its replenishment. That is, this signal is sent when the stocks of energy are replenish sufficiently, considering possible emergencies. As energy is spent, this signal weakens, and the feeling of satiety goes away. Then the signal that it would be nice to eat something is felt stronger.

And there is a signal that is sent when the level of glucose in blood drops quickly. Even when the glucose level is still high, but decreases quickly, this can be potentially dangerous, because who knows when this sharp decrease ends. Potential danger does not mean imminent danger, since glucose may drop sharply because insulin has increased sharply and "pushes" excess of that glucose into all cells that can accept it. In a normally working system, glucose will lower soon, and so will insulin, and further decrease of glucose will not be too sharp or will stop. But this signal of a potential danger is rather strong, and it is stronger than the signal of satiety. And then the desire to eat becomes almost unbearable. 

This is not all yet. There are also stresses that require sharp changes of physical activity (the "fight-or-flight" reaction to save life). In such cases, it is better to sacrifice something to save what is most important. This is also regulated by both the nervous system and hormones. And there are other situations requiring processes of acquisition, accumulation and expenditure of energy. For instance, before we wake up, the organism increases the level of blood glucose a little (and the level of insulin follows) to ensure availability of glucose in muscles by the moment of increase of physical activity after waking up. By the way, because of this effect, it is not reasonable to measure blood glucose immediately after waking up. It is better to wait for about an hour. 

Our stocks of energy in ATP are very small, less than for two minutes of life, and they need to be constantly replenished.

We store glucose in the form of glycogen, mostly in the liver and muscles. These stocks are enough for about one and a half to two days of life at a moderate physical activity. 

We store fats in the cells of the fat tissue, and these stocks can be very large, even much larger than it is necessary for any practical purposes. Out of fats, our liver can produce fatty acids and ketone bodies as fuels for mitochondria. Also, a part of fats (up to about 10%) can be converted into glucose. The fat tissue can be subcutaneous and visceral, and can also form in the liver. A large amount of visceral fat is bad. Fat tissue in the liver is very bad. Too much of subcutaneous fat may also be bad (though not as bad as visceral fat), and also it is unaesthetic. However, the requirements to its amount is very individual, and there are completely healthy people with large amounts of just subcutaneous fat. Also, this stock is necessary for the energy safety of the organism, however, keeping a stock exceeding 8 to 10 kg makes no practical sense. 

We do not have special stocks of proteins, however, in situations of prolonged shortage or absence of food, we can use proteins of our muscles as a source of amino acids for making the most prioritized proteins and as a source of energy. Also, we can use some "leftover" amino acids (those that have not been used to produce our proteins) for producing glucose. 

Insulin maintains the normal level of blood glucose. When blood glucose increases, so does insulin. A higher level of insulin "instructs" the cells of muscles and liver to take glucose out of blood and store it in glycogen. If muscles and the liver are fully stocked with glucose and cannot take more, the liver converts extra glucose from blood into fats. Also, an elevated level of insulin "prohibits" to spend fats and "instructs" to accumulate fats, because "it's not the time" for spending fats, because it is necessary to reduce the blood glucose to the normal level. 

Regulation of stocks also includes regulation of consumption of food. For this, there are signals of satiety and of hunger. Signals of hunger can be deceptive, when it is too far to the shortage of energy. And some of these signals can jam the signals of satiety, thus making us to make more than we need.