Most people don’t think much about the food they eat once it’s swallowed, except perhaps to regret the occasional binge. The process of digestion is actually quite fascinating - your body is basically a factory that uses food as a raw material to produce energy and cellular building materials. Understanding the process gives you an insight into why certain foods may not agree with you, and it helps you make better food choices. Like any machine, your body only functions at optimum levels if it’s well-maintained.
Digestion actually begins as soon as the food hits your tongue. Chewing breaks the food into smaller pieces to allow digestive enzymes to work more efficiently on a larger surface area. Chewing also triggers the release of saliva, which contains an enzyme called amylase. Amylase begins breaking carbohydrates down into glucose, a basic sugar.
You can taste amylase at work - chew a bite of white bread for a minute or two, and it will begin to taste sweet as the starch becomes sugar.
After you swallow, the food travels down through the esophagus to your stomach, which serves as a sort of processing tank. Because you eat faster than your stomach empties, the stomach must be able to store food while it processes. The stretchy walls can accommodate quite a feast, but the more you put it, the longer it takes to move on.
The stomach mixes up the chewed food, swallowed saliva and its own enzymes to make a sort of slurry. The chemicals in the slurry get to work breaking the food pieces down further to give the later digestive enzymes even more surface area to work on. One of those enzymes begins breaking protein down into its individual amino acid components.
The interior wall of the stomach has a special thick mucous coating that protects it from the highly corrosive digestive juices. Some people may not produce enough of this mucus, and suffer from "sour stomach" or other similar complaints when the digestive juices irritate the stomach lining.
Eventually, your stomach empties the slurry of food and acid into the small intestine. Your pancreas sends over an enzyme called lipase, which begins breaking down fat. The diet pill Orlistat works by disabling this enzyme and forcing the fat droplets to pass through your system undigested.
Together with lipase, enzymes from the lining of the small intestine work to break the food down chemically into individual nutrients. Your gall bladder sends over some bile to help break down fat even more - fats are very large molecules, and take several steps to break down. This process takes a relatively long time, and fats are generally the last to leave the stomach.
Once the food in your small intestine has been broken down sufficiently, it gets absorbed into the bloodstream. Carbohydrates leave the stomach soonest and are broken down into glucose, which absorbs through the intestinal lining and is distributed throughout the body. Some of it is used to provide instant energy (which is why you feel better after you eat), and the rest is stored as glycogen in the liver and muscles. Glycogen is an energy reserve that gets called upon when glucose runs low.
Protein gets broken down into amino acids, which are small enough to be absorbed through the intestinal lining. They enter the bloodstream and are carried throughout the body to assist in cell repair. Ironically, these amino acids that your body worked so hard to wring out of giant protein molecules only remain individual amino acids until they reach their destination site - they are then reassembled into whatever large protein molecule is needed for the job.
Fats undergo a similar fate - they are broken down into fatty acids and cholesterol within the small intestine, and are then absorbed through the lining. They are then reassembled and taken to the nearby lymphatics, which sends certain fats to the blood vessels in the chest and the rest to storage sites throughout the body - most people have no trouble at all locating their bodies’ favorite fat storage site. Fat is also used as a source of energy, especially when the glycogen wells run dry.
Your body also absorbs vitamins from the food you eat in the same way. Once they absorb through your intestinal wall, they are transported through the bloodstream to wherever they are needed. Any water-soluble vitamins that aren’t needed get urinated out, but your body can store fat-soluble vitamins like A, D, E and K for a rainy day.
Your bowel movements are made up of whatever you ate that your body couldn’t absorb. Your body cannot absorb fiber, because the molecule is simply too large to break down in time with the available enzymes. Vegetable fiber is particularly difficult, which is why many people notice undigested vegetable matter in their stool. Certain fats may also make an appearance, even if you’re not taking Orlistat - a meal with a very high fat content may simply overload your system with more fat than your digestive enzymes can handle. Some fat will be absorbed, but the rest will appear in your stool.
This also explains why laxatives don’t work as a weight loss aid. Laxatives work in your large intestine, but once the food gets there, your body has already absorbed the calories.