(This is the sixth in a series of health articles that are designed to help you gain a deeper appreciation for God’s amazing handiwork of the human body and a better understanding of how it works and how it can be better maintained by simple methods. George McDaniel is my father-in-law, and has been a registered nurse for many years, which, along with much research, has taught him many useful health principles. I pray that you are being blessed by these articles. Editor)
Nutrition – 2
Last month we studied the importance of eating a nutritious diet. This is the first aspect of nutrition.
The second aspect of nutrition has to do with how well the body processes the food and delivers it to where it is needed and how efficiently it utilizes it and removes waste products.
The digestive system consists of the mouth; esophagus; stomach; small intestine (with three sections: duodenum, jejunum and ileum); large intestine (also called colon); and rectum. Two other major organs involved with digestion are the pancreas and liver, both of which open into the duodenum.
The digestion of food starts in the mouth where it needs to be well chewed. The teeth are very important to digestion and should be in good shape. Teeth that are in poor condition can interfere with this important first step. Haste and carelessness can also contribute to poorly chewed food. The food needs to be thoroughly mixed with saliva. Saliva contains a digestive enzyme, called ptyalin, which begins the digestion of starch into simple sugars. Starch is composed of glucose molecules which are joined together chemically into long chains. The saliva enzyme begins the process of breaking down the starch into smaller units so the body can absorb and utilize it. This is one reason why food should be well chewed and not washed down with water or some other drink. The saliva also lubricates the food so it will more easily pass through the esophagus into the stomach.
The stomach produces hydrochloric acid and the digestive enzyme pepsin. Pepsin acts on proteins, breaking them down into amino acids. Pepsin works better in acidic conditions. The stomach also secretes mucous which protects the stomach lining from being digested by its own secretions and which lubricates the food mixture. The contraction of muscles in the stomach wall thoroughly mixes the food particles with the stomach secretions.
Food normally stays in the stomach from two to five hours; however, a high fat meal stays longer in the stomach. The presence of a large amount of fat causes a signal to be sent to the duodenum to slow down the release of food from the stomach. Only protein digestion occurs in the stomach. Fats and carbohydrates are unchanged. The salivary enzyme that digests starch is inactivated by the stomach acid. Once digestion starts, no more food should be eaten until the stomach is empty. If a snack is eaten while partly-digested food is in the stomach, the partly-digested food will be held until the more- recently-eaten food is processed. Fermentation of the undigested carbohydrate can occur, which results in toxic substances being produced.
The food mixture that is ready to leave the stomach is a thick liquid called chyme. The chyme is released in small amounts at a time into the upper end of the small intestine, called the duodenum. Duodenum simply means twelve, as it is approximately twelve inches long. Here the chyme is acted upon by secretions from the pancreas and liver. The main contribution of the liver at this point is bile, which acts to emulsify fats, or break them down into small particles so they can be acted upon more thoroughly by fat-digesting enzymes. This is the reason why a high-fat diet is retained longer in the stomach, so that it can be mixed more completely with bile.
The pancreas produces three kinds of enzymes— trypsin, amylase and lipase. Trypsin completes the digestion of protein into amino acids; amylase completes the digestion of starches into simple sugars; lipase digests fats into fatty acids and glycerol. The pancreas actually produces many kinds of digestive enzymes with specialized functions, but they fall into three main categories: protease (trypsin), amylase and lipase. For example, there are protein-digesting enzymes that specialize in cleaving simple amino acids off from the ends of peptide chains and there are some that break down bonds on the interior of peptide chains.
When digestion is complete, starch has become simple sugars, protein has become amino acids and fat has become fatty acids, ready to be absorbed and used by the body. Vitamins and minerals don’t need digestion, but the process of digestion releases them from the food so they also can be taken up by the body.
The lining of the small intestine contains millions of tiny finger-like projections, called villi, which greatly increase the surface area and enable the body to absorb large amounts of digested nutrients. What is not absorbed by the small intestine passes on to the large intestine, or colon. Most of the water and minerals are absorbed here. The waste matter that remains passes into the rectum at the end of the colon and from there leaves the body.
The next article will deal with what happens to the nutrients after they are absorbed from the intestine.