Last Updated: February 10, 2019
All of our energy and nutrition come by way of the gastrointestinal (GI) tract. It is an amazing machine well designed for digesting and absorbing nutrients and for shielding the rest of the body from microorganisms, viruses and other foreign matter. The GI tract spans from the mouth to the anus, with each separate organ having a role in digestion, absorption or excretion.
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In front of you is a turkey and cheese sandwich with lettuce, onion, red bell pepper, mustard and mayonnaise on multi-grain bread. If you really want that sandwich, you may notice that you’re salivating. The hungrier you are and the more you crave that sandwich, the more you will salivate. That’s because digestion actually begins in the brain. This is the cephalic phase of digestion, which is stimulated by the parasympathetic nervous system. When you see and smell food and even start thinking about eating, the brain readies the digestive tract for nourishment. This anticipation stimulates the secretion of saliva in the mouth and gastric juices in the stomach.
Once you take a bite of your sandwich, you’ve initiated mechanical digestion. This chewing reduces the size of the food particles and mixes them with saliva for easier swallowing. Before your food passes from the mouth and down your esophagus, salivary amylase, an enzyme in saliva, begins to digest the starch in your bread. That is the start of chemical digestion.
The mass of chewed sandwich is called a bolus. You voluntarily swallow it, but the rest of the digestion-absorption process is involuntary. The passage of the bolus through the esophagus to the stomach occurs by peristalsis, a series of wave-like muscle contractions.
As the bolus approaches the stomach, a ring-like muscle called the lower esophageal sphincter relaxes, allowing the chewed food to enter. Peristalsis mixes the bolus with gastric secretions containing hydrochloric acid (HCl), hormones and digestive enzymes to produce a thick slurry called chyme. Hydrochloric acid is so strong that it could dissolve metal. It doesn’t eat away at the stomach mucosa (lining) because you have cells that also secrete mucus to protect your stomach wall. The acid kills much of the microorganisms that piggyback on your food, and in this way the acid has prevented you from frequently becoming sick. To aid digestion, HCl denatures or unfolds proteins, making them more available to attack by digestive enzymes.
The digestive enzyme pepsin begins to breakdown the protein in your sandwich (largely the meat and cheese with smaller amounts in the bread and vegetables). Gastric lipase begins to digest the fats present in your cheese and mayonnaise, but does so only minimally. Digestion of the starch in your bread does not occur in the stomach because the salivary amylase that began chemical digestion in your mouth became inactive in the presence of stomach acid.
Bit by bit, the pyloric sphincter that separates the stomach from the small intestine allows the chyme to drop into the small intestine. Here is where most of the magic occurs. Though the stomach often gets the credit, the small intestine is the primary site for digestion, and its remarkable and expansive absorptive area allow us to harness the nutrients in our foods.
Because chyme was mixed with HCl in the stomach, it has a very low pH. The mucosa of the small intestine does not have as much protective mucus as the stomach, but it does have something else to shield it from the acid - pancreatic juices. The presence of chyme in the upper portion of the small intestine triggers the pancreas to secrete bicarbonate to neutralize the acid, lipase to digest fats, amylase to digest starches, and proteases to digest proteins. The cells of the small intestine secrete additional enzymes to complete digestion.
The gallbladder gets into the act here as well. When fat is present in the chyme (as it is from the cheese and mayonnaise in our sandwich), the gallbladder contracts and secretes bile into the small intestine. Bile acts like a detergent and emulsifies the fat, breaking it into small globules, aiding fat absorption.
You need a very large surface area to absorb all of the nutrients your body requires. If the lining of your small intestine were smooth like the inside of a rubber tube, you would not be able to absorb adequate nutrition. Instead the walls of the intestinal lumen are wrinkled and folded, which increase its surface area threefold. But this is still much too little. Therefore, the folds are covered with tiny fingerlike projections called villi, which, in turn, are covered, by microscopic projections called microvilli. This combination of folds and projections increases the absorptive area of the small intestine 600-fold to the size of a tennis court! And that allows for the absorption of far more nourishment than most of us would want to eat.
Once starches and sugars have been digested into monosaccharides - glucose, galactose or fructose - they are ready for absorption. They pass through the lining of the small intestine, into the bloodstream via the portal vein and to the liver. Proteins are broken down primarily to single amino acids. They follow a similar path to the liver, as do the monosaccharides. Because of their lack of water-solubility, the majority of the fats take a much different path. Most of the dietary fat comes in the form of triglycerides and is digested into two free fatty acids and monoglycerides. While within the intestinal cells, however, these components are resynthesized into triglycerides. They do not go through the portal vein. Instead, they are packaged with cholesterol and coated with proteins to form particles called chylomicrons. The lymphatic vessels transport the chylomicrons to the junction of the lymphatic and circulatory systems where they enter the bloodstream. Only a small portion of fats is absorbed into the portal vein. These are the smaller fatty acids that are more water-soluble than the long-chain fatty acids.
Peristalsis pushes food waste, fiber (from your bread and vegetables) and any foreign materials through the small and large intestines. More water and salts are absorbed from the large intestine. As the contents move downward, the feces - consisting of water, fiber, intestinal secretions and dead bacteria - are formed and eventually pushed through the anus. Though many people worry they are constipated if they do not have daily bowel movements, the National Institute of Diabetes and Digestive and Kidney Diseases has the following definition for constipation: having a bowel movement fewer than three times per week. Don’t worry; it’s also normal to have two or three bowel movements each day.1
You have about three pounds and hundreds of species of hard-working bacteria in your gut. Commonly referred to as intestinal microflora, bacteria ferment or digest some of the food waste that escaped enzymatic digestion. Those bacteria are thriving on the fiber from your bread and vegetables. One of the byproducts of this fermentation is gas.
Though no one likes it, having gas is perfectly normal. In fact, the average person produces one to four pints of gas daily.2 Other byproducts are vitamins K, B12, thiamin and riboflavin which the body absorbs and uses, and short chain fatty acids which provide fuel to the cells of the colon.
The proper functioning of the GI tract is even more intricate than described here with multiple mechanisms for communication and control. Both the endocrine and nervous systems are critical players. In fact, there are more than 80 hormones involved in regulating the GI tract. Additionally, the enteric, peripheral and central nervous systems largely regulate the contractions and secretions of the gastrointestinal tract.
You can see just how complex the GI tract is. Any problems with anatomy, inflammation, nerve disorders and more can hinder the breakdown and absorption of nutrients. Getting proper nutrition to the cells of your body is more than merely making wise food choices.
Continue your exploration of nutrition: Understanding Calories
NIDDK. Constipation. http://digestive.niddk.nih.gov/ddiseases/pubs/constipation/ ↩
NIDDK. Gas in the Digestive Tract. http://digestive.niddk.nih.gov/ddiseases/pubs/gas/ ↩