Not all of the components of the food we eat can be completely broken down and absorbed. Dietary fiber, for instance, passes through the stomach and the small intestine intact. What remains goes into the large intestine, or colon, where most of the fluid as well as some sodium, calcium and a tiny amount of zinc are absorbed. About two quarts of liquefied material reaches the colon each day. Most of it is absorbed.
Billions of bacteria play a critical role in the large intestine. The bacteria that reside here decompose carbohydrates (mostly fiber) that were not digested by the small intestine and synthesize vitamin K and certain B vitamins. The fermentation process by which the bacteria decompose the carbohydrates produces intestinal gas.
By the end of the passage through the colon — which measures about 5 feet — only a small amount of what you originally consumed remains, which constitutes the feces. The bulk of the feces — about 75 percent — is water and the remainder is solids made up of bacteria, undigested organic matter, and dead cells that are shed from the lining of the intestines. The feces travels to the very end of the colon, the rectum, where it accumulates until the urge comes, and then it is excreted.
It's a Gas
Scientists estimate that the average adult releases between 12 and 122 cubic inches of intestinal gas each day. Most of that gas is made up of hydrogen and methane produced by the bacteria as they ferment the fiber that was not digested in the stomach or small intestine.
Many fruits and vegetables, especially beans, contain these polysaccharides, which cannot be broken down by the enzymes in the stomach or small intestine. As the bacteria decompose these complex sugars, gas is released. Most of the gas is passed without notice.
Common wisdom tells us that beans give us a lot of gas — more than could pass without notice. Ever questing for eternal answers, scientists in one study measured the intestinal gas produced by people who ate a control diet and compared them to those who ate a meal that was half pork and beans. The folks in the control group released .9 cubic inches per hour of flatus (intestinal gas) while the pork and beans group released a whopping 10.7 cubic inches.
For more information about the digestive system, see the next page.