Digestion is the process by which the food we eat is broken down into smaller substances that can be absorbed through our gastrointestinal tract. Proper digestion is extremely important for our health, and when digestion is impaired in any way, disease can develop.

The food we eat (besides vitamins and minerals) consists of three basic groups: carbohydrates, fats and proteins. Carbohydrates, fats and proteins need to be broken down into small enough chunks that can be absorbed by the body. Digestion is a combination of mechanical and chemical processes that allows food to be absorbed by our body.

Digestion of Carbohydrates

Human food contains only three sources of carbohydrates: sugar or sucrose, which is a disaccharide derived from cane, lactose which is a disaccharide in milk and complex carbohydrates (polysaccharides) which are found in most grains and potatoes. Cellulose is technically also a polysaccharide, but cannot be digested by the human GI tract. Digestion of carbohydrates starts in the mouth where food is mixed with saliva, which contains the enzyme alpha-amylase that breaks down carbohydrates into disaccharides such as maltose. The acid secretion of the stomach will inhibit further digestion of complex carbohydrates, so any polysaccharide that has not been digested at that time, will be digested when the pancreas secretes alpha-amylase enzyme. Maltose, lactose, sucrose and other small disaccharides are further broken down into monosaccharides by enzymes present in the intestinal lining. Monosaccharides are then quickly absorbed into the blood stream.

Digestion of fats

Fats are an important component of our diet since fats form the basis of many hormones and cell membrane building blocks. The most common fats in our diet are neutral fats (triglycerides), which are composed of glycerol linked to 3 fatty acid molecules. The amount of fat that is digested by the stomach is very low, and most of the fat is digested in the intestine. However, fat molecules form a so-called micelles (globular balls) in a water containing environment, so in order for digestive enzymes (which are water soluble) to gain access, fats need to be emulsified. Emulsification of fats occurs when the gall bladder secretes bile salts. Fats mixed with bile salts can be broken down by pancreatic lipase into fatty acids and mono-glycerides.

Digestion of proteins

Proteins in our diet are derived either from meat or vegetables. Proteins are long chains of amino acids held together by peptide bonds. Proteins are pre-digested in our stomach by gastric pepsin at a very acidic pH. When the pH is not low enough, protein digestion is impaired. The bulk of the protein digestion occurs in the small intestine, where proteolytic enzymes secreted by the pancreas further break down the proteins. The partially broken down proteins (peptones, polypeptides) are attacked by the pancreatic enzymes trypsin, chymotrypsin and carboxypolypeptidase and turned into (mostly) di-peptides. Di-peptides are turned into individual amino acids by enzymes present in the lining of the intestine. Amino acids are then absorbed into the blood.

As you can see, digestion is a complex process of many enzymatic reactions combined with the mechanics of chewing, stomach churning and movement of food particles through the intestinal tract. Many things can go wrong with digestion. Even when our food is completely digested, it still has to absorbed through the intestinal wall into the bloodstream.