Most of us don’t think about our digestive tracts beyond what we put into it, although those who have a digestive disease or disorder often worry a bit more about what comes out. Compare this to the skin, where a healthy complexion is closely linked to our perception of beauty. The musculoskeletal and the cardiovascular systems are on our minds while we exercise. Our nervous system pays a great deal of attention to itself as we learn and think. At least half of the adult human population spends a lot of time thinking about the reproductive system. However, the complex organs that provide life’s raw materials usually escape notice unless they’re associated with pain or disease. So let’s take a few minutes to chew over the main aspects of digestion.

It’s difficult to grasp digestion without a basic understanding of food’s chemical components. Consider a meal of steak, mashed potatoes, roasted asparagus, and apple pie à la mode. The steak contains protein and fat. Mashed potatoes and asparagus are composed mostly of carbohydrates, with small amounts of protein; however, fat, such as butter, might have been added to the potatoes. The ice cream and pie contain carbohydrates, fat, and a little protein. Finally, each item on the menu is made mostly of water.

So what is a carbohydrate? It’s a term for all sugars, starches, and fibres. Fructose, glucose, lactose, and sucrose are simple sugars made up of one or two sugar molecules. Starch and fibre are complex carbohydrates made up of long chains of simple sugars. The primary function of carbohydrates is to provide energy to every cell within your body.

Proteins perform the chemical and mechanical tasks associated with life; they make muscles contract, hold tissues together, and are the molecular machinery of the body. Proteins are composed of long chains of 22 different simple molecules called amino acids.

Fats consist of long chains of carbon and hydrogen atoms, called fatty acids. These fatty acids are rich sources of energy and perform other functions in the body, such as helping brain function, cushioning internal organs, and aiding in the absorption of some vitamins.

Carbohydrates, proteins, fats, and water make up 99.9% of what we eat. To nourish our cells, our bodies need to break food down into its simplest components. Every bite needs to end up a simple sugar, amino acid, fatty acid, or water molecule. The first step is chewing, which divides food into manageable chunks. Saliva lubricates and eases the passage of the mouthful of food (now called a bolus) into the esophagus. Saliva also contains an enzyme, or protein that aids chemical reactions, called salivary amylase, which breaks up starch into shorter chains of simple sugars.

Now that we understand the main chemical components of the foods we eat, let’s go over how the digestive system extracts them from our food! The interior of the esophagus, stomach, and intestines are technically outside your body – it’s like a 10 metre long tunnel that communicates freely with your external environment. As well, this tunnel is lined with a type of epithelial cell that perform a similar barrier function to the outer cell layers of your skin.

The esophagus is more than just a hollow tube connecting the mouth to the rest of the digestive system, it has muscular walls that push the bolus into the stomach through a process called peristalsis. There are two layers of muscle along the entire digestive tract; a circular layer that squeezes and pushes the bolus of food forward, and a longitudinal layer that shortens the esophagus overlaying the bolus. Peristalsis mixes food and advances it through the entire digestive tract using coordinated waves of contraction, which travel along the circular and longitudinal muscle layers. When food reaches the end of the esophagus, a ring of muscle called the lower esophageal sphincter relaxes to allow the bolus to enter the stomach.

In the stomach, the bolus becomes a liquid soup-like mixture called chyme. The stomach’s secretions are about ten times as acidic as vinegar. This low pH destroys the complex three-dimensional shapes of most proteins in a process known as denaturation. A digestive enzyme called pepsin, which is found in the stomach, requires an acidic environment for activation and optimal function. Pepsin breaks down other proteins, in particular collagen, the main component of the tough web of proteins that hold animal tissues together. The stomach also contracts and relaxes in a peristalsis-like fashion to mix its contents thoroughly. Small amounts of chyme then slowly move through the stomach’s pyloric sphincter into the duodenum, the first part of the small intestine.

The remaining carbohydrates, proteins, and fats are chemically broken down in the duodenum and jejunum. Although chewing, saliva, and the stomach do play a role, the vast majority of digestion occurs following the stomach. The pancreas produces enzymes necessary to break complex carbohydrates down into simple sugars, proteins into individual amino acids, and fats into free fatty acids. It secretes these enzymes, dissolved in a basic fluid that neutralizes the acidity of the chyme, into the duodenum. Finally, proteins on the small intestinal cell surface convert simple sugars and paired amino acids into single sugars and amino acids respectively, and then transport them into the cells and bloodstream. Fatty acids need one more step before they can be absorbed. As we all know, oil and water don’t mix, and this is true of fat droplets and the rest of the water-based liquid chyme in the small intestine. This is where bile comes in. Produced in the liver and stored in the gall bladder, bile helps digest fats through emulsification. In this process, bile secreted into the duodenum combines with large drops of liquid fat to form tiny molecular-sized spheres. In these small spheres, or micelles, pancreatic enzymes can break down triglycerides (fat) into free fatty acids. Micelles are also small enough to be absorbed by intestinal cells. All absorbed nutrients enter the bloodstream via vessels in the walls of the intestine. The remaining unabsorbed materials now enter the colon, which receives 0.5-1.5 litres of fluid daily. The colon reduces daily stool volume to only 0.1 litre by absorbing excess water. As well, it hosts a huge population of bacteria that offer protection from other, more harmful microbiological bugs that we might unintentionally ingest. At this point, feces are formed from the remaining materials that cannot be absorbed and then passed from the body. This completes a basic look at digestion.

What do you mean by absorption of water and nutrients?

Absorption is the movement of nutrients across the inner digestive tract surface and eventually into the bloodstream. It occurs mostly in the small intestine; about 8.5 litres of fluid enter it per day, and only 0.5-1.5 litres exit. To accomplish this, the inner surface of the small intestine has many folds, increasing the surface area and allowing for greater intestinal wall absorptiveness. If it were just a smooth, hollow tube, only about half a square metre of intestinal surface, or an area of about 5.3 square feet, would come into contact with nutrients. With folding, the surface area is about 200 m2, or an area the size of a tennis court. There are three levels of irregularity of the inner intestinal wall that increase surface area: the plicae circulares are folds of tissue that can be seen by the naked eye; the villi are microscopic finger-like structures composed of many cells; and the microvilli are even smaller projections found on the surface of the small intestinal cell itself (see diagram below). Peristalsis mixes chyme to increase exposure to this magnified surface area.


Is anything absorbed in the stomach?

Alcohol and aspirin are among the few things absorbed in your stomach, which is why you can feel tipsy after a few sips of wine and why headache relief comes quickly after popping some aspirin. Water, as well as some electrolytes, can also be absorbed in the stomach. However, the majority of nutrients are absorbed in the small intestine, with a few vitamins and minerals absorbed in the large intestine.


Why don’t the stomach, pancreas, and intestines digest themselves?

Approximately 100 million nerve cells, dozens of hormones, and countless complex strategies work together to tightly regulate every function in the digestive tract, from enzyme secretion to peristalsis. Take the example of trypsin, which breaks down protein in the small intestine. It is stored in an inactive form so it doesn’t digest the pancreas and is activated by other enzymes once it reaches the intestine. It doesn’t digest the intestinal wall, though, partly because of a protective layer of mucus. Trypsin produces short amino acid chains through its enzymatic activity, and these short chains cause the release of a hormone that reduces pancreatic enzyme (and therefore trypsin) secretion. Finally, trypsin breaks other molecules of trypsin apart, which ensures that it doesn’t sit in the small intestine for extended periods of time. Protective strategies exist in the stomach as well; a thick layer of mucus coats the stomach wall to prevent the highly concentrated hydrochloric acid and protein-digesting enzymes from destroying the stomach lining.


Why do certain foods feel more filling than others do, even if I eat the same amounts of each?

It’s true that the quantity of food you consume does not necessarily determine how long you will feel full following a meal. The degree of fullness you feel after eating a particular food depends on three key factors, water content, fibre content, and the ratios of fat, protein, and carbohydrate. Foods that contain a lot of water, such as fruits and vegetables, will provide you with a feeling of fullness soon after you eat them but this will quickly diminish as the water is absorbed. High fibre foods, such as whole grains and lentils, take more time to digest than low fibre foods, giving you a more lasting feeling of fullness. The body digests each food component at a different rate, with carbohydrates digested faster than proteins and fats. Therefore, foods with a high fat content, such as a burger, slow down the release of chyme from the stomach. With chyme remaining in your stomach for a longer duration, your feeling of fullness will persist long after you’ve finished eating. (We’re not recommending burgers just so that you can feel full longer.)


Do you have to be sitting upright or standing in order for your food to digest?

The process of digestion does not rely on gravity, rather on the peristalsis; the muscles of the digestive tract are strong enough that you could eat upside down, or even in no-gravity space! However, lying down, or hanging upside down while digesting will likely cause acid-reflux in those individuals who have GERD.


Why do some foods affect the absorption of medications?

The explanation for this depends on the specific medication and food in question. One example is grapefruit or its juice, which interferes with the enzymes responsible for breaking down various drugs in our digestive system. Without the enzymes, the drugs cannot be properly digested and absorbed, and therefore are not able to perform their function in the body. It is important to always read the labels on your prescriptions for any food-drug interactions or ask your pharmacist.


Are some foods easier to digest then others?

Certain foods are easier to digest, depending on your disease or disorder. Some people suffering from irritable bowel syndrome might find lower carbohydrate diets helpful in alleviating their symptoms as well as cooking their fruits and vegetables instead of eating them raw. Physicians recommend their patients with ulcer disease avoid certain foods such as chocolate, tomato products, and alcohol while the ulcers are healing. Keeping a food diary can be helpful in determining which foods pass through easily and which foods worsen your discomfort.


Is there anything I can do to speed up my digestion?

Speeding up digestion is useful if you have less frequent bowel movements, as in the case of constipation. Studies show that exercise increases the rate of digestion in the body. Caffeine can also be a stimulant for digestion, although health professionals warn that a high daily intake of caffeine may lead to other negative health effects.

Andrew Vargo, MD
First published in the Inside Tract® newsletter issue 174 – 2010
Andrew is a freelance medical writer who generously provides his time and skills to the Inside Tract®