Most mammals cannot drink milk after infancy, including humans. Since most humans stop producing milk-digesting enzymes in their bodies by 5 years old, undigested milk sugar ferments in the colon and causes digestive upset. According to a 2009 article in "USA Today," less than 40 percent of the world's population can digest milk after childhood. Descendants from Northern Europe and Eastern Africa have the highest probability of producing enzymes for the digestion of milk into adulthood.
Enzymes
Many different digestive enzymes help you break down foods you eat into usable nutritional molecules that your body can absorb. Various cells in your body produce different enzymes, which catalyze numerous biochemical reactions during the digestion process. Digestive enzyme secretion begins in your mouth with saliva and continues with secretion from your stomach and small intestine. Also, your pancreas produces seven types of digestive enzymes.
Components of Milk
Milk contains water, carbohydrates, fat, protein, vitamins and minerals. Lactose is the carbohydrate sugar in milk that has to be broken by the enzyme lactase to become digestible. Milk has about 65 percent saturated fats, 29 percent monounsaturated fats and 6 percent polyunsaturated. With protein at 3.3 percent, milk contains all of the essential amino acids your body needs. All of the proteins, fats and carbohydrates in your milk need assistance from digestive enzymes to be nutritionally absorbed into your body.
Stomach Enzymes
Chief cells in the lining of your stomach secrete a digestive enzyme precursor called pepsinogen. The pepsinogen molecule is cleaved by a reaction with your stomach acid into the digestive enzyme pepsin. Pepsin breaks down the large proteins in milk, such as casein, into smaller proteins called polypeptides, which progress further into the digestive process for complete digestion later in the small intestine.
Pancreatic Enzymes
Your pancreas has numerous cells that secrete a combination of digestive enzymes into your small intestine. It secretes amylase, lipase, trypsin, chymotrypsin, elastase, carboxypeptidase and nucleases. Lipase digests milk fats with the help of bile from the gallbladder and nuclease digests consumed RNA and DNA. Amylase digests starches, which are not present in milk. The other four enzymes continue digestion of milk proteins.
Intestinal Enzymes
The final digestion phase of milk occurs in your small intestine. The small intestine has many microscopic finger-like projections on the surface called villi, which greatly increase the surface area of the small intestine. Cells on the surface of your villi secrete aminopeptidases and disaccharidases as digestive enzymes. Aminopeptidases, maltase, sucrase and lactase, convert milk proteins into to single amino acids, while disaccharidases break down the sugars.
Lactase Enzyme
Milk has about 4.9 percent of its total carbohydrate in the form of lactose, according to Cornell University, although the amount varies with the variety of milk. Since most adults do not secrete the lactase enzyme into their small intestine, undigested lactose moves into the colon where it causes gas, cramping and diarrhea. Dietary supplements with lactase enzyme may be beneficial, but have not been evaluated by the FDA. Raw milk does not offer you any advantage over pasteurized milk for lactose intolerance.
References
- USA Today; Sixty Percent of Adults Can't Digest Milk; Elizabeth Weise; September 2009
- Scientific American; African Adaptation to Digesting Milk is Strongest Signal of Selection Ever; Mikhil Swaminathan; December 2006
- Biology Pages; The Human Gastrointestinal Tract; J. Kimball; July 2011
- Cornell University; Nutritional Components of Milk; February 2007
- Colorado State University; Pepsinogens and Pepsins; R. Bowen; November 1996
- Clermont College; Effects of Antacids on Pepsin; J. Stein Carter; November 2004
