Digestion is the process by which the food we eat is broken down into smaller substances that can be absorbed through our gastrointestinal tract, a process that is extremely important for our health. 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.
Basic Digestive Process
Carbohydrates, fats and proteins are all large molecules (polymers) made up of repeated subunits (monomers) that are chemically broken down by a process, which is called hydrolysis. In this process, specific digestive enzymes promote the breakdown of the chemical bonds between the monomer units by using water, like this (where “R” represents a particular monomer)
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 further inhibit breakdown 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. For that reason, heartburn medications may have a negative effect of the digestion of proteins.
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.
Disorders of the digestive tract may impair digestion or absorption of essential nutrients
As the pancreas is one of the most important organs for digestion, anything that impairs its function can have a major impact on a person’s health. The most common digestion failure occurs when the pancreas fails to secrete its enzymes into the small intestine. As a result, large portions of the food are only partially digested, resulting in fatty feces and symptoms of fatigue, headaches and an impaired immune system.
Inflammation of the pancreas can be either acute or chronic. The most common causes of pancreatitis are alcohol abuse or a blockage of the pancreatic duct by a gallstone. When a gallstone blocks the duct, pancreatic enzymes back up and can eventually become activated. Pancreatitis is extremely painful. When it is not treated quick enough, pancreatic enzymes can digest all or portions of the pancreas.
Conditions causing impaired absorption of nutrients
Crohn’s Disease, Celiac Disease and diverticulitis are intestinal disorders where the lining of the intestine is chronically inflamed. Chronic inflammation of the lining of the intestines can seriously impair the proper absorption of essential nutrients, which can lead to secondary health problems1. Symptoms of celiac disease can often be improved on a gluten-free diet2.
As we age, the body produces less digestive enzymes and stomach acid. Discover how a digestive enzymes supplement can help.
1. Erickson, R.H. and Kim, Y.S. (1990) Digestion and Absorption of Dietary Protein. An Rev Med 41, 133-139.
2. Shan, L. et al. (2002) Structural Basis for Gluten Intolerance in Celiac Sprue. Science 297, 2275-2279.
Disclaimer: The above information is for educational purposes only and is not intended to substitute medical advice or care. Always consult a health care professional about any health problems or illnesses that you may have.