Carbohydrate Metabolism

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What happens to the carbohydrates that we eat? What our body does to carbohydrates is a series of mechanical and chemical breakdown known as carbohydrate metabolism.

Carbohydrate metabolism in the mouth

The metabolism of carbohydrates begins in our mouth. The first step is mastication -- chewing. When we chew, food is mechanically divided into smaller pieces and is mixed with an enzyme in our saliva (salivary amylase) that chemically breaks down carbohydrates into its simpler components.

A carbohydrate may be compared to a chain with many links. Each link is a monosaccharide (literally one carbohydrate or one sugar). A series of links forming a chain is a polysaccharide (several carbohydrate or sugar).

Chewing mechanically cuts the chains (polysaccharides) into more manageable chunks while salivary amylase cleaves one link from the divided chunks, releasing a monosaccharide.

Carbohydrate metabolism in the stomach

Once we swallow our food, it passes through our chest via the esophagus into the stomach. In the stomach, the mechanical breakdown of carbohydrates continues. It takes about four hours before a meal we eat completely passes out of the stomach. During that time, food is ground by repetitive and forceful contraction of the stomach and transformed into a semi-liquid substance called chyme.

Chyme, to continue our analogy, is made up of very short chunks of chain composed of one to five links each. The very short chunks facilitate the further chemical breakdown of carbohydrates in the small intestine.

Carbohydrate metabolism in the intestines

Chyme is slowly released from the stomach into the small intestines. Here the chemical breakdown of carbohydrates, which started in the mouth, is completed. Enzymes from the pancreas flow into the small intestine together with chyme. The enzyme break the very small chunks of carbohydrate into individual links. The long chain of carbohydrate which was mechanically broken down into smaller chunks in the mouth and stomach are now broken down into separate links (monosaccharides). Monosaccharides, of which glucose is well known, are taken up by the cells of the intestines and go into the bloodstream.

Carbohydrate metabolism in the blood

Once in the blood, glucose (and other monosaccharides) is used by the cells of the body as a source of energy. A molecule of glucose may be taken up by the cell, broken down further into its components and, by a series of chemical reactions, transformed into ATP (adenosine triphosphate), the primary energy currency of cells.

The formation of ATP marks the end of the catabolic metabolism (breakdown) of carbohydrates. Cells of the pancreas, liver, and fats use glucose in unique ways.

Carbohydrate metabolism in the pancreas

After a meal, the glucose level in the blood goes up. This high level of glucose causes the pancreas to release insulin, a hormone that affects the metabolism of glucose in the liver and fat tissues.

Carbohydrate metabolism in the liver

Insulin promotes the uptake of glucose by liver cells. In addition to using glucose as a source of energy, liver cells store the excess glucose as glycogen. Glycogen is converted back to glucose when the sugar level goes down between meals.

Carbohydrate metabolism in fat cells

Fat cells take up glucose from the blood and convert it to ATP. When there is an excess of ATP, as when caloric intake exceeds caloric requirement, fat cells convert ATP to fat. This is the reason why any excess in our caloric intake, be it protein, carbohydrates or fat, increases our body fat and, consequently, our body weight.

Our understanding of how the body uses carbohydrates highlights our need to eat a balanced diet to meet our daily caloric requirement.

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