The large intestine

After every useful, digestible ingredient other than water has been wrung out of your food, the rest — indigestible waste such as fiber — moves into the top of your large intestine, the area known as your colon. The colon’s primary job is to absorb water from this mixture and then to squeeze the remaining matter into the compact bundle known as feces.

Feces (whose brown color comes from leftover bile pigments) are made of indigestible material from food, plus cells that have sloughed off the intestinal lining and bacteria — quite a lot of bacteria. In fact, about 30 percent of the entire weight of the feces is bacteria. No, these bacteria aren’t a sign you’re sick. On the contrary, they prove that you’re healthy and well. These bacteria are good guys, microorganisms that live in permanent colonies in your colon, where they:

• Manufacture vitamin B12, which is absorbed through the colon wall
• Produce vitamin K, also absorbed through the colon wall
• Break down amino acids and produce nitrogen (which gives feces a characteristic odor)
• Feast on indigestible complex carbohydrates (fiber), excreting the gas that sometimes makes you physically uncomfortable — or a social pariah When the bacteria have finished, the feces — perhaps the small remains of yesterday’s copious feast — pass down through your rectum and out through your anus. But not necessarily right away: Digestion of any one meal may take longer than a day to complete.

After that, digestion’s done!