The combination of the acidic environment in the stomach and the action of gastric enzymes convert the bolus into chyme, a liquefied mass that is squirted from the stomach into the small intestine once a follow-up meal assists in pushing it through. Carbohydrates tend to leave the stomach rapidly and enter the small intestine; proteins leave the stomach less rapidly; and hydrogenated fats linger there the longest. This leads to the theory that carbs should be eaten in twice the quantity of proteins and you should eat half that much fat.
The small intestine is the principal site of digestion and absorption. There, enzymes and secretions from the pancreas, liver, gallbladder, and the small intestine itself combine to break down the chyme to pull nutrients out so that they can be absorbed. The pancreas is a veritable enzyme factory, supplying enzymes to digest proteins, fats, and carbohydrates when necessary almost limitlessly. However, intestinal cells also supply some enzymes. The liver produces bile which is required for the emulsification of fat, and the gallbladder stores the bile until it is needed. Sometimes this process is backed up by eating hydrogenated oils like those in popular protein powders. This not only constipated but doesn't allow proper fats to process. The absorption of nutrients that are taken in from the small intestine are absorbed by tiny projections called villi, which are torn down by caffeine, alcohol, ephedra and artificial sweeteners. The nutrients are supposed to pass through the intestinal membranes into the circulatory (blood) system, which transports them to body tissues. This requires another meal so the chyme gets pushed that far down, which is why it's better to eat several meals a day. Nutrients are then absorbed once again by the cells, where they are used for growth, repair, healing and the release or storage of energy. The overall process is called metabolism. It is highly complex and has little to do with exercise or fat burners.
If you want the metabolism to speed, you need to eat right, speed up digestion, and keep the process going again and again. Undigested chyme will otherwise proceed from the small intestine into the large intestine (colon), where instead of it becoming concentrated in preparation for excretion, it dehydrates and clogs your drain. Bacteria builds up to cause fermentation, which facilitates nasty gas, and absorption of nutrients is minimal there. They needed to be absorbed earlier in the process. If the food isn't recognized to begin with, like most meal replacements powders aren't real foods, then the problems could occur as early as the stuff hits the belly. Belching, tummy aches, head aches....
The key points to remember about digestion are:
Foods must be broken down from and into their most natural components before they can be absorbed. The body really does care whether nutrients it absorbs through the digestive tract come from "natural" or synthetic food sources. The body's reaction to absorbed nutrients depends on their chemical structure not the source from which they were obtained. So-called "diet foods" do not contain nutrients that are found in a balanced diet of natural foods.
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