6.7: Summary

The digestive tract includes the mouth, esophagus, stomach, small intestine, colon, and rectum. In digestion, food is broken down into a form that can be absorbed from the intestine into the body. Starch and double sugars must be broken down (digested) to single sugars; proteins to amino acids; and triglycerides must have at least two of their fatty acids removed. Digestive enzymes do this. Some substances (e.g., alcohol and cholesterol) don’t have to be digested; they’re absorbed “as is.”

Tooth decay is caused by mouth bacteria that colonize in soft plaques that cling to teeth. They break down sugars in our food and make lactic acid that dissolves minerals in the enamel, starting the process of decay. Saliva helps control tooth decay; it has antibacterial agents, and neutralizes some of the acid.

Saliva also has the calcium and phosphorus used to remineralize the demineralized areas of enamel. Fluoride reduces decay by making teeth more resistant to acid. Drinking water that has 0.7 ppm fluoride is recommended for maximum protection.

The microbe H. pylori is thought to play a role in stomach cancer. It’s a major cause of duodenal ulcers, and can be eradicated with specific medication and antibiotics. Among ethnic groups in the U.S., Korean Americans have the highest rate of stomach cancer.

Pancreas and liver play crucial roles in digestion. The pancreas provides digestive enzymes and sodium bicarbonate, which is alkaline and neutralizes the acid coming from the stomach. Liver provides bile, which emulsifies fat, making it more digestible. Bile is stored and concentrated in the gallbladder.

These pancreatic and liver secretions enter at the top of the small intestine through a tube called the bile duct. Digestion and absorption of food occurs mostly in the small intestine, which has an intricate lining for this purpose.

Lactase, an enzyme found in the lining of the small intestine, normally falls after early childhood, in which case lactose (the double sugar found in milk) can’t be digested as fast. When a person consumes more lactose than the enzyme can handle, some of the lactose doesn’t get digested and proceeds to the colon where bacteria break it down and make gas as a byproduct. This can cause discomfort—lactose intolerance. Those with low lactase needn’t necessarily avoid milk; they may just need to ingest it more slowly with other foods or switch to lactose-free milk.

The colon is rich in bacteria and is where residue from food and digestive secretions is stored until it’s excreted through the rectum. The colon extracts water from the residue, so if the residue goes through the colon too fast, it’s watery (diarrhea). If it stays in the colon a long time, it can become quite dry. This can be constipating and can contribute to the development of outpouchings of the colon wall (diverticulosis).

Insoluble dietary fiber (fibers that don’t dissolve in water) can help prevent constipation and diverticulosis by holding water, thus providing bulk and softness to the stool. Fiber might also help lower the risk of colon cancer, but this has not been proven. Because the mixture of substances that pass through the colon is so complex, it is extremely hard to identify and sort out the factors that contribute to or help prevent colon cancer. A healthy diet is also a colon-healthy diet—a diet rich in vegetables, fruit, and whole grains.

Screening for colon cancer by visually examining the lining of the colon (colonoscopy) and removing polyps is a primary means of prevention. Most polyps do not result in cancer, but colon cancer often begins as a polyp.