Learning Center > Digestion & Nutrition

Digestion, Absorption & Elimination

Digestion is the process by which our bodies break down the food and liquid into small molecules that our blood can absorb. Absorption is the basis of nutrition: it is how our bodies pull into the bloodstream the required nutrients from the food and liquid we digest in order to provide energy and life to all of our cells. Elimination is the process whereby our bodies excrete that which is not absorbed or not useful; for example undigested food particles, toxins, and dead cells. Our bodies have built in elimination systems that encompass a range of activities from bowel movements and urination to sweating and breathing. In other words, our bodies turn food and drink into absorbable nutrients via our digestive system. When you eat an apple, your body is performing many different tasks in order to turn that apple into substances that it can use, and then efficiently eliminate what is not useable. It is essential to our health to keep our digestive and elimination systems functioning properly. Anything preventing proper digestion, absorption and elimination directly affects our bodies' ability to nourish our cells; when any part of this system is not performing, the system cannot absorb all the nutrients from the food being ingested and our bodies suffer.

The Digestive System - How it all comes together

The digestive system is composed of the organs of the alimentary canal (also called the digestive tract), and the pancreas, liver, and gallbladder. In addition, the nervous system and circulatory system play a major role in assisting the digestive process. The alimentary canal is the long hollow tube (about 30 feet long) that starts with your mouth and ends with your anus. This tube includes the mouth, the esophagus, the stomach, the small intestine (20 feet long, three main parts: duodenum, jejunum and ileum), the large intestine (5 feet long, six main parts: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum), and the anus. Inside this tube is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. For hours after eating, these organs work together to push food through the body while secreting juices that break the food into tiny nutrients that can be absorbed. Protein gets broken down into amino acids, starches into simple sugars, and fats into fatty acids and glycerol. Water is absorbed into the bloodstream to hydrate the body.

How food is turned into nutrients - from start to finish

Before food even enters the mouth the body secretes saliva (enzymes) to help moisten food and start the digestion of the starch in food into smaller molecules.

Food is chewed up and pushed down the esophagus in a small lump called a bolus. This lump of food is pushed down into the stomach, where three things happen: the lumps of food are stored (up to a half gallon can be stored), then mechanically blended with stomach acids and an enzyme that digests protein (pepsin), and then emptied as a liquid solution in spurts into the small intestine.

Things really get interesting as food is emptied from the stomach to the small intestine, where the majority of digestion happens. The pancreas secretes a variety of enzymes in a solution known as pancreatic juice. This helps digest proteins, fats and carbohydrates, and helps to neutralize the acids used to digest foods in the stomach.

The liver and gall bladder secret a substance called bile which helps the body break apart and absorb fat. Bile is composed of bile salts (produced by the liver's breakdown of cholesterol), cholesterol, phospholipids, bilirubin (the yellow by-product of dead red blood cells, which bacteria in the intestines modify, thereby causing the brown color of feces), protein and miscellaneous components.

The bile is stored between meals in the gallbladder. When food enters the small intestine, bile is squeezed out of the gallbladder into the bile ducts and then into the small intestine. Bile dissolves fat the same way detergents dissolve grease from a frying pan. After the fat is dissolved, it can be digested. Bile and pancreatic juices break down the food into tiny particles that can be absorbed across the wall of the small intestines into the blood stream.

The wall of the small intestines has a huge surface area (around 1,600 square feet), due to presence of villi, which are fingerlike projections in the lining of the small intestine (think of egg carton foam mattress pads, and their expanded surface area.)

At the bases of the villi are openings of intestinal glands, which secrete a watery intestinal juice containing digestive enzymes. On average, about 2 quarts of intestinal juice are secreted into the small intestine each day. As with the lining of the stomach, a coating of mucus helps protect the lining of the small intestine.

The digestive enzymes wear away the lining, which is replaced about every two days. These dead cells of the lining are pushed down the digestive tract to be eliminated with other fecal matter.

Each villi contains a lymph vessel surrounded by a network of capillaries. The nutrients absorbed into the lymph vessel pass into the lymphatic system. The particles that are absorbed into the capillaries empty into the portal vein and travel directly to the liver, where the liver filters the blood of toxins and sends the filtered, nutrient-rich blood to the heart for circulation around the body.

Simple sugars and amino acids pass through the capillaries to enter the bloodstream. Fatty acids and glycerol pass through to the lymphatic system.

It is important to note that the small intestine, contains an extensive immune system. The immune system protects the intestine from disease-causing viruses, bacteria, yeasts, fungi and parasites. Beneficial bacteria provide a defence against harmful organisms by taking up residence where harmful bacteria would otherwise settle in. Additional beneficial bacteria ("intestinal flora") live in the large intestine (we'll cover this later.)

Unabsorbed and undigested food, along with water, digestive juices and dead cells continue their journey through the small intestine where some of the substances are reabsorbed and recycled (like bile salts).

Whatever is not absorbed in the small intestine gets pushed through to the large intestine.

By the time food gets to the large intestine, digestion is largely completed. The primary goals now are the re-uptake of water, sodium and minerals; the absorption of Vitamin K and B created as by-products of bacteria that live in the large intestine; decomposition of undigested food (aided by bacteria); and the compaction and elimination of waste material.

The large intestine hosts at least 500 different types of bacteria. In fact, about 3-4 pounds of bacteria live in the large intestine. These bacteria produce methane (CH4), hydrogen sulfide (H2S), and other gases as they ferment their food.

Occasionally, some of this gas is released as flatus. As these bacteria digest/ferment left-over food, they secrete beneficial chemicals such as vitamin K, biotin (a B vitamin), and some amino acids, and are our main source of some of these nutrients. Beneficial, or symbiotic "friendly" bacteria have several roles in our bodies: to protect us from bad bacteria; to assist with decomposition and breakdown of undigested food; to synthesize vitamins B & K; to maintain proper pH; and assist with immune system functions. Ideally, about 85% of the bacteria in your digestive tract are beneficial, and have established large, healthy populations that keep the growth of bad bacterial in check.

However, higher amounts of bad bacteria are usually present due to a variety of reasons including poor/acidic diet, chlorinated water, and antibiotics. Over-colonization of disease causing micro-organisms like E. coli, Staphylococcus aureus, salmonella and Candida can harm the body by producing toxins and even carcinogens.

Higher levels of bad bacteria also account for the production of gases inside our intestine, which causes digestive discomfort, foul flatulence and may be implicated in some diseases. In the large intestine (a.k.a., the colon) water is absorbed back into the body via the walls of the large intestine.

Approximately 1.6 quarts of water per day flows through the large intestine, 85% of which comes from digestive secretions. 95% of this water is reabsorbed into the intestinal walls. But if the water is not absorbed, diarrhea can result.

The remains - rotting food, bacteria, dead epithelial cells from the walls of the intestines, dead red blood cells filtered by the liver and released in bile, and other toxins that were filtered by the liver and secreted with bile during digestion - proceed to the rectum, where they are stored until they are excreted as feces.

The evacuation of feces should occur at least daily, ideally two or three times daily shortly after each meal. Constipation is defined as having less than three bowel movements per week. Bowel movements should be comfortable and easy. If you are forcing them, or if you are not having a bowel movement for 1-2 days at a time (or for some people even longer!) then your digestive system is not working efficiently and you are likely over-exposed to rotting putrefying food in your digestive tract.