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Sigmoidoscopy

Endoscope views rectum and lower colon

Colonoscopy

Endoscope views rectum and entire colon

Figure 17C

One diagnostic test for colon cancer is a double contrast barium enema. The barium highlights the lower digestive tract, revealing in this radiograph an obstruction caused by a tumor.

Life-span Changes

Changes to the digestive system that are associated with the passing years are slow and slight, so most people can enjoy eating a variety of foods as they grow older. Maintaining healthy teeth, of course, is vital to obtaining adequate nutrition. This requires frequent dental checkups, cleanings and plaque removal, plus care of the gums. Tooth loss due to periodontal disease becomes more likely after age thirty-five.

Despite regular dental care, some signs of aging may affect the teeth. The enamel often thins from years of brushing, teeth grinding, and eating acidic foods. Thinning enamel may make the teeth more sensitive to hot and cold foods. At the same time, the cementum may thicken. The dentin heals more slowly and enlarges as the pulp shrink. Loss of neurons in the pulp may make it more difficult to be aware of tooth decay. The gums recede, creating more pockets to harbor the bacteria whose activity contributes to periodontal disease. The teeth may are of two general types — looking for clues in feces, or exploring the large intestine with an endoscopic device. The key to saving more lives is to know whom to test. For example, the more invasive procedures are usually reserved for individuals with strong family histories of colorectal cancer. But an experiment that performed colonoscopy on 3,121 healthy volunteers without strong family histories of the illness revealed that one percent of them had advanced colorectal cancer! More than a third had some abnormal growths in their colons. Therefore, existing screening protocols may miss some early cases.

An alternative to performing invasive screening procedures on the general population is to develop a fecal test that is more accurate than the standard fecal occult blood test, which has a ten percent false positive rate. Researchers at the Mayo clinic have developed a more accurate fecal test that screens the DNA from cells in feces for mutations that are associated with colorectal cancer. (A strictly inherited form of colorectal cancer entails mutations present in the sperm or egg; more common cancers arise from so matic mutations, which originate in cells of the affected tissues.) The fecal gene test for colorectal cancer will not replace colonoscopy — still the most accurate test—but will help physicians select patients for it.

Colorectal cancer develops gradually. First, a cell lining the large intestine begins to divide more frequently than others, and the accumulating cells enter a precancerous state. Next, the growth forms a polyp, which is benign. Months or even years later, the polyp becomes cancerous. Still other genetic changes control the cancer's spread.

Treatment for colorectal cancer is to remove the affected tissue, with surgical construction of a new opening for feces to exit the body. The free end of the intestine is attached to an opening created through the skin of the abdomen, and a bag is attached to the opening to collect the fecal matter. This procedure is called a colostomy.

The future is bright for early detection and even prevention of colorectal cancer. Some people may be more likely than others to develop col-orectal cancer because they inherit a susceptibility gene. If a susceptibility test can be developed, individuals can lower their risk by eating high-fiber foods and exercising.

A new class of non-steroidal anti-inflammatory drugs (NSAIDS) called cox-2 inhibitors may help to prevent colorectal cancer. These drugs block the enzyme cyclooxyge-nase-2, which is necessary to convert a substance in cell membranes (arachidonic acid) into prostag-landins, which cause inflammation. The drugs were initially approved to treat arthritis without the gastrointestinal side effects that previous NSAIDS cause because they also block cox-1. However, researchers noted that people who take NSAIDS — such as an aspirin a day— have half the normal risk of developing intestinal polyps. A clinical trial on people with inherited colon cancer indicated that cox-2 inhibitors cause active cases to regress, and seem to prevent the disease altogether in individuals who have inherited the disease-causing mutation but do not yet have symptoms. Clinical trials are underway to determine whether these drugs can prevent, or at least delay onset, of other forms of colorectal cancer. ■

loosen as the bones of the jaw weaken. On a functional level, older people sometimes do not chew their food thoroughly, swallowing larger chunks of food that may present a choking hazard.

A common complaint of older individuals is "dry mouth," or xerostomia. This condition is not a normal part of aging—studies have shown that the oldest healthy people make just as much saliva as healthy younger people. Dry mouth is common, however, because it is a side effect of more than 400 medications, many of which are more likely to be taken by older persons. These include antidepressants, antihistamines, and drugs that treat cancer or hypertension. In addition, radiation and chemotherapy used to treat cancer can cause mouth sores and tooth decay. It is a good idea for cancer patients to coordinate dental visits with other aspects of their care.

Once past the mouth, food travels through a gastrointestinal tract that declines gradually in efficiency with age. A slowing of peristalsis may result in frequent heartburn as food backs up into the esophagus. The stomach lining thins with age, and secretion of hydrochloric acid, pepsin, and intrinsic factor decline. Exit of chyme from the stomach slows. Overall, these changes may affect the rate at which certain medications are absorbed.

Because the small intestine is the site of absorption of nutrients, it is here that noticeable signs of aging on digestion arise. Subtle shifts in the microbial species that inhabit the small intestine alter the rates of absorption of particular nutrients. With age, the small intestine becomes less efficient at absorbing vitamins A, D, and K and the mineral zinc. This raises the risk of deficiency symptoms—effects on skin and vision due to a lack of vitamin A; weakened bones from inadequate vitamin D; impaired blood clotting seen in vitamin K deficiency; and slowed healing, decreased immunity, and altered taste evidenced in zinc deficiency.

Many people who have inherited lactose intolerance begin to notice the telltale cramping after eating dairy foods in the middle years. They must be careful that by avoiding dairy products, they do not also lower their calcium intake. Less hydrochloric acid also adversely affects the absorption of calcium, as well as iron. Too little intrinsic factor may lead to vitamin B12 deficiency anemia.

The lining of the large intestine changes too, thinning and containing less smooth muscle and mucus. A dampening of the responsiveness of the smooth muscle to neural stimulation slows peristalsis, ultimately causing constipation. Compounding this common problem is a loss of elasticity in the walls of the rectum and declining strength and responsiveness of the internal and external sphincters.

The accessory organs to digestion age too, but not necessarily in ways that affect health. Both the pancreas and the liver are large organs with cells to spare, so a decline in their secretion abilities does not usually hamper digestion. Only 10% of the pancreas and 20% of the liver are required to digest foods. However, the liver may not be able to detoxify certain medications as quickly as it once did. The gallbladder becomes less sensitive to cholecystokinin, but in a classic feedback response, cells of the intestinal mucosa secrete more of it into the bloodstream, and the gallbladder continues to be able to contract. The bile ducts widen in some areas, but the end of the common bile duct narrows as it approaches the small intestine. As long as gallstones do not become entrapped in the ducts, the gallbladder generally functions well into the later years.

Describe the effects of aging on the teeth.

What conditions might be caused by the slowing of peristalsis in the digestive tract that occurs with aging?

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