After taking a bite of a sandwich or chewing on some pizza, you probably do not give that food another thought. But after you swallow, the body kicks into digestion and metabolism of the lipids, proteins, and carbohydrates in the food. Each of these nutrients has a unique metabolic pathway for full breakdown of the food in order for it to be used throughout the body.
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"Lipid" is another name for fat. When fats are eaten, multiple organs in the body have to get involved to break it down. The pancreas creates lipase, the enzyme that can break through the fat bonds once it enters the small intestines. The liver makes bile, which is stored in the gall bladder until it's also released into the small intestines. The bile binds with the broken up fat pieces so they can be absorbed into the blood stream for use by cells.
Protein's chemical breakdown begins in the stomach. Here, hydrochloric acid and an enzyme called pepsin starts to break apart protein. Then, in the small intestines, four different enzymes that are created by the pancreas break the protein pieces all the way down to individual amino acids to be absorbed. Most of the protein has made it through to the bloodstream by the time it makes it half way down the small intestines. The amino acids and nitrogen are carried into the blood to be delivered to all cells.
Carbohydrates start their breakdown as soon as they enter the mouth, where the enzyme salivary amylase, which is part of saliva, starts to work. It continues this break down in the stomach and is completed in the small intestines by an enzyme call pancreatic amylase. Ultimately, sugars and starches get smaller and smaller until they are individual monosaccharides that can then be absorbed in the small intestines.
Some carbohydrates cannot be broken down all the way in the human body. These carbs, called fiber, travel down into the large intestines, where they are passed out with other waste materials that are not absorbed.
Similar But Different
Carbohydrates, protein and fat follow some similar processes for digestion. They all begin the breakdown process in the mouth, being broken up with chewing and are broken up even smaller with the churning of the stomach. The majority of these nutrients are absorbed into the blood stream in the small intestines. They also all rely on enzymes to break them down to small enough pieces in order to be used by the body for ultimate metabolism, where their energy and other components are used by individual cells.