The digestive system is the organ system responsible for processing the food we ingest. To most of us, it sounds like all that this system does is just take the nutrients in our food and then get rid of the waste. However, the digestive system is more than a mere food delivery driver. There is no better way to illustrate the genius behind this system than on the example of a Muslim observing Ramadan.

If you ever stopped to wonder what happens in your body as you fast. The simple answer is a lot. But if you are a science buff, like myself, look no further than this article.

Regulation of the metabolic rate, appetite & thirst

Metabolism is defined as all the chemical reactions (transformations of chemicals) taking place in the body to sustain its functions (e.g. make energy). The energy use of the body is defined by its total metabolic rate. The metabolic rate is mainly regulated by a region in the brain called the hypothalamus (region responsible for the body's “internal equilibrium”). 

Did you know that even relaxing our muscles requires energy?

This explains why after a while, the cadaver of a person becomes rigid (rigor mortis). Physicians (pathologists) use this, together with the estimated surrounding temperature, to determine the time of death.

*An enzyme (biocatalyst made of proteins) is like motivation for students; it (e.g. a score) makes them prepare for an exam. Students could potentially prepare for an exam without motivation but it would take forever. Just like scores, enzymes speed up chemical transformations that would otherwise not take place (they would need years to take place without enzymes).

Enzymes, like students, won’t do their job properly if they are not provided with the proper conditions. Each student likes a subject but won’t study other subjects. The pH and temperature of the body are like subjects. So each enzyme requires an optimum condition.

During the day, the empty stomach of a faster secretes ghrelin, a hormone (a hormone is a “chemical message” released into the bloodstream to communicate with a target tissue) made of a few amino acids (amino acids are the building blocks of proteins). Ghrelin stimulates the hypothalamus to release neuropeptide Y (a neurotransmitter [a signal secreted by nerve cells] made of tens of amino acids), thereby signalling hunger. Neuropeptide Y is also regulated by the activity of fat cells. These cells secrete adipokines (e.g. leptin) which are hormones that tell the hypothalamus “what is up with the fat''. When there is fat, more of these hormones are released, thereby slowing down the release of neuropeptide Y and stimulating the release of hormones, which decrease appetite (e.g. melanocortins, see later). Insulin (released by the pancreas) also stops the release of neuropeptide Y & thus decrease appetite. 

The blood pressure of the faster decreases and so starts to experience thirst. Low blood pressure means that blood perfusion in the kidneys decreases. As a result, the kidneys (juxtaglomerular apparatus) then secrete the enzyme renin*. This enzyme (renin) converts a protein in the blood plasma (called angiotensinogen) into another protein (angiotensin I). This other protein is then converted to another protein (angiotensin II) by an enzyme produced in the lungs (called ACE or angiotensin-converting enzyme). Angiotensin II increases blood pressure (by stimulating the release of the hormone aldosterone and making the lumen of blood vessels smaller; vasoconstriction), stimulates thirst and stimulates the release of antidiuretic hormone (responsible for making the Ramadan observer urinate less).  Antidiuretic hormone is made by the hypothalamus and secreted by the neurohypophysis/posterior pituitary, while aldosterone is secreted by the zona glomerulosa of the adrenal cortex. Doctors may control (e.g. using drugs that inhibit an enzyme in the pathway) this renin-angiotensin pathway to treat hypertension.

When the sun goes down and we break our fast, we start feeling satiated because the intestines release peptide YY that tells the hypothalamus to stop secreting neuropeptide Y, which stimulates appetite, & to release appetite-suppressants (melanocortins like melanocyte-stimulating hormone). Cholecystokinin, released by the initial horse-shoe shaped part of the intestine, the duodenum, in response to the presence of fats, signals satiety and suppresses appetite. Pancreatic polypeptide (PP) also exhibits a similar effect.

Breaking one’s fast also leads to a sense of pleasure (euphoria) by activating a reward circuit similar to the one activated in drug addicts; the dopamine reward circuit.

The adrenal gland is made up of the adrenal medulla and cortex. The adrenal medulla is, among other things, responsible for releasing adrenaline :)

Ingestion, digestion & elimination of a date

Muslims break their fast either with dates (or with water). It appeared to me as if explaining the process of digesting such a small fruit would be easy, but it turned out that finding the right words to convey my message with scientific accuracy was a mission impossible. Which is why I decided to leave out some steps (like absorption, see later) in order to simplify things for you, the reader.

Dates are rich in antioxidants (phytochemicals) that destroy oxidants, reactive molecules such as free radicals. Oxidants damage DNA, proteins, and unsaturated fatty acids by stealing their electrons. Oxidants are generated by radiation, alcohol, tobacco, etc.  Dates are also rich in fibre. Dietary fibre (roughage; indigestible carbohydrates like cellulose) decreases cholesterol concentration in the blood and decreases the risk for cardiovascular disease and diabetes. Fibre also stimulates the feeling of satiety and the peristaltic movement (peristalsis, by the muscle layers, are unidirectional waves of muscular contractions that push the food in the gastrointestinal tract) of the bowel (which helps avoid constipation). 

Ingestion includes masticating/chewing & swallowing the date. These two processes involve the mouth (buccal/oral cavity) & the teeth. 

• A quadrant of human dentition is made up of 2 incisors, 1 canine, 2 premolars & 3 molars. These teeth pierce, cut, tear, crush and grind the food into bolus. 

Digestion (breaking down bigger molecules in the food into smaller ones our cells can use) of the date starts in the mouth. Here, the starch (large molecule made up of 100s of glucose [simple carbohydrate] units) contents of the date are broken into malt sugar (a disaccharide made up of 2 glucose units) by salivary amylase. Saliva also moistens the food. 

Swallowing (deglutition) is a complex neuromuscular reflex involving the soft palate, pharynx & larynx. The tongue propels the date bolus pushing the soft palate to the back, thus closing the path to the nose. The pharynx is also connected to both the oesophagus & the larynx, which is connected to the airways and consequently the lungs. As the bolus is propelled, the laryngeal muscles push the larynx upwards. As a result, a cap-like structure called the epiglottis closes the glottis (part of the larynx).

Now, we can voluntarily push the date down the oesophagus until it reaches the part we can’t control. The oesophagus is a muscular tube that moves the bolus into the stomach even when standing upside down. So you won’t have to worry about your date not reaching the stomach while praying Maghrib. Since the stomach was empty the whole day it was folded into a hot dog. As the date bolus enters, the stomach unravels. As food distends your stomach gastrin is released. Gastrin stimulates churning & the glands lining the walls of the stomach. These glands (made of so called parietal and chief cells) release stomach acid (HCl), a substance needed for vitamin B12 absorption called the intrinsic factor & pepsin (the  enzyme that will break down the protein content of the date into the smaller molecules called peptides).

There are several ways that protect the stomach from its acid. 

1. The stomach secretes a layer of alkaline mucus to neutralise the acidity of the acid along its lining.

2. Moreover, its cells fit tightly together, like a jigsaw puzzle to stop the acid from leaking between them and into the underlying tissue. 

3. If some of the cells become damaged, they are quickly replaced. So each minute about a half million of these cells are exfoliated and replaced.

Now, the dates exit the stomach in the form of the fluid-like chyme. Here [in the small intestine], the chyme will face chemical transformations like it has never seen before, as this is where most of digestion takes place. Bile, produced by the liver (the largest secreting organ; largest gland) & concentrated by the gallbladder, emulsifies the fat globules into droplets in the same way a detergent washes oil from dishes. Emulsification greatly increases the surface area of fat exposed to lipase (a fat digesting enzyme released by the small intestine and pancreas). Bile also activates the enzyme lipase. The products of fat & oil digestion include larger and smaller building blocks - they can even be absorbed without digestion since the conditions in the intestine aren’t exactly perfect (see *).

• In addition to mechanical digestion, the liver is responsible for controlling the amount of nutrients in the blood & storing them. Most importantly the liver detoxifies alcohol and other drugs (which are not allowed in Islam).

• Liver cells repackage cholesterol and fats. These fats are bound to proteins and are transported as high-density lipoproteins, or HDLs (“good” cholesterol), but most are transported by low-density lipoproteins, or LDLs (“bad” cholesterol). LDLs deliver cholesterol to the cells. LDL enters the cell, and its cholesterol and other components are used. When cholesterol levels are high, HDLs collect the excess cholesterol and transport it to the liver. HDLs decrease risk for cardiovascular disease. Cholesterol is a component of animal cell membranes, but when excess cholesterol in blood forms plaques on artery walls, thereby increasing the risk of cardiovascular disease. Omega-3 (found in dates) decreases LDL levels, decreasing the risk for heart disease.

The pancreas is a gland that secretes digestive enzymes, which break sugars, proteins & fats, and hormones, which regulate the level of glucose in the blood. The pancreatic ducts secrete an alkaline solution to provide the optimal environment for the enzymes in the small intestine. Pancreatic enzymes include trypsin, carboxypeptidase and chymotrypsin, which digest larger peptides to shorter peptides; pancreatic lipase, which degrades fats; pancreatic amylase, which breaks down complex carbohydrates, except cellulose, to carbohydrates made of 2 units; and nucleases, which split the nucleic acids (e.g. DNA) into their building blocks called nucleotides. Trypsin, secreted in an inactive form by the pancreas, is activated by an enzyme called enterokinase. The trypsin then activates chymotrypsin and carboxypeptidase and more trypsin. Dipeptidases, released by the duodenum, then split the small peptides into amino acids. The protein content of the date has been digested. The duodenum also releases enzymes that break carbohydrates (disaccharides made of two carbohydrate units) into a single carbohydrate unit. Now, the duodenum secretes a hormone called glucose-dependent insulinotropic peptide in response to glucose. This hormone stimulates the release of insulin. Insulin acts like a spoon for cells. It allows the cells to take in glucose from the blood. It is the only hormone that does this.

The duodenum also secretes other hormones. For instance, cholecystokinin is released in response to the protein & fat contents of the date (now, chyme). Cholecystokinin, in addition to signalling satiety, stimulates the release of digestive juices in the pancreas and gallbladder. There are other enzymes that can stop the stomach from emptying its contents (e.g. secretin). 

The final step would be absorption. Folds in the wall of the intestine (villi, and microvilli) increase the surface area of the small intestine by about 600 times. If we could unfold and spread out the lining of the small intestine, its surface would approximate the size of a football stadium. The digested molecules can now be absorbed by the (lymph & blood) vessels lining the intestine to be delivered to the liver. Due to the lack of space this article will not discuss the very interesting process of absorption. But if you are dying to know about it, check out this link: bit.ly/3DHtWcV. Most of the water you drink to hydrate your dry throat after 14 hours of fasting is absorbed in the small intestine. The concentration of water is higher in chyme than it is in cells lining the intestine. Meaning, water moves down its concentration gradient from the chyme into cells. The remaining water is then absorbed in the large intestine (it is large because of its diameter). 

The large intestine, which consists of the cecum, colon, rectum, and anus, is responsible for eliminating/egesting (defecation) undigested wastes. As chyme passes, it assumes the consistency of faeces (its brownish colour is due to bile). Bacteria in the large intestine feed on the undigested materials of the chyme. In exchange, they produce large amounts of vitamin K and certain B vitamins for us to use.