Proper acidity is critical to proper digestion and for absorption of zinc and ferrous iron. In addition, when the stomach's acid level is low, bacteria and viruses can colonize the GI tract. Many enzymes exist in the stomach and small intestine as inactive enzymes, or proenzymes, that lie dormant until needed. In the stomach, pepsinogen is activated by hydrochloric acid (HCL) secreted from parietal cells, and is converted to active pepsin for protein digestion. In the small intestine, the pancreatic pro-enzymes, trypsinogen, chymotrypsin and carboxypeptidase are converted to their active enzymes as well.
As we age, our ability to maintain stomach acidity decreases. In fact, after age sixty, over 50 percent of people suffer from low stomach acidity (hypochlorhydria),442 associated with a number of conditions such as asthma, celiac disease, chronic autoimmune disorders
Normally, the pancreas secretes about 2.5 liters of juices a day—a rather large volume. Meanwhile, the liver and gallbladder each secrete about 700 ml of bile a day. Pancreatic secretion is controlled by the hypothalamus via the vagus nerve and special locally secreted hormones such as secretin and choleocystokinin. These hormones are activated by the presence of fats, protein, and chyme in the duodenum. Several of these hormones may be deficient in the aged. This can lead to maldigestion.
Even small decreases in pancreatic enzyme activation can produce serious digestion problems, which can lead to abdominal bloating, upset stomach, cramping pains, and diarrhea. It also leads to malnutrition, since absorption of vital nutrients, including vitamins and minerals, is impaired.
Numerous glutamate receptors also exist in the pancreas, and insulin control is strongly regulated by glutamate in the blood. As yet, we do not know the function of these receptors on digestive enzyme function. If glutamate can affect their secretion as well, then diets high in MSG could cause significant digestive problems. Intestinal muscle also contains glutamate receptors which, when overstimulated, can result in increased spasm of the intestine. This would explain the diarrhea and cramping seen in many people following a meal high in MSG.
Once food enters the small intestine, it undergoes further churning and mixing with digestive enzymes and is mixed with bile produced in the liver. Bile helps emulsify the fats in your food so that they can be digested and absorbed properly. The pancreas secretes most of the digestive enzymes in the duodenum, including proteases to break down proteins, amylase for carbohydrate digestion and lipases to break down diglycerides and triglycerides into long-chained fatty acids. Fat-soluble vitamins, such as vitamins A, E, and D, are absorbed through dissolution in the emulsified fats.
Several enzymes inside the wall of the intestine further digest oligosaccharides and proteins. These special cells also control absorption of nutrients to prevent large molecules of undigested foods from entering the blood stream. If this occurs, food allergies may result. Drugs, food additives, and inflammatory bowel diseases all can damage these delicate cells. Frequent use of nonsteroidal anti-inflammatory medications, such as ibuprofen, is a common culprit when these cells are damaged.
Diseases such as sprue, Whipple's disease, Crohn's disease, Giardiasis, cryptosporidiosis, lactose intolerance, and eosinophilic gastroenteritis all impair proper digestion and absorption of nutrients. But it should be appreciated that nutrient absorption utilizes different pathways in the intestine, so that isolated nutritional deficiencies can occur. For example, one may see fat malabsorption alone, or impaired absorption of vitamin B12, iron, or zinc, while other nutrients are absorbed normally.
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