11.10: Summary

Vitamin research started by looking for microbes, because the results of deficiencies look very much like infectious disease. Through various experiments, scientists discovered that certain elements in the diet prevented these “diseases.” But it was uncertain for some time just what role these vitamins played in the body.

The chemical reactions that occur in the body are collectively called metabolism. Metabolism includes the chemical reactions that release energy in the process of breaking down the energy-providing nutrients (carbohydrates, fat, protein) to carbon dioxide and water. Metabolism also includes the energy-requiring reactions in which larger molecules are made from smaller ones (e.g., proteins are made from amino acids).

Many complicated reactions are required for these processes, and B vitamins act as coenzymes in many of these reactions. This is why metabolism is impaired when any of these B vitamins is lacking in the diet.

Carbohydrate is made by plants, and is taken in by our cells as glucose when we eat it. The breakdown of glucose to provide energy is called glycolysis and produces pyruvate. Glycolysis includes many chemical reactions, each requiring its own enzymes and coenzymes. One of these steps requires a coenzyme that contains the B vitamin niacin.

The reactions of glycolysis take place in the cytoplasm, and don’t need oxygen—the process is thus called anaerobic metabolism. Glycolysis can produce energy quickly, but only for a limited time. When the need for lots of energy is prolonged (as in sustained, strenuous exercise), the pyruvate forms lactic acid.

The usual route for pyruvate is through the aerobic cycle, which takes place in the mitochondria. Pyruvate undergoes a series of chemical reactions (involving coenzymes that contain the B-vitamins thiamin, niacin, and riboflavin) to form acetic acid. Acetic acid attaches to Coenzyme A (which contains the B vitamin pantothenic acid) to form acetyl CoA.

Acetyl CoA combines with oxygen to produce carbon dioxide, water, and lots of ATP. In effect, the original process, in which plants gathered energy from the sun and through photosynthesis used carbon dioxide and water to build the molecules we call food, has been reversed, returning to carbon dioxide and water and releasing the stored energy.

The energy released from fat (i.e., fatty acids) comes exclusively from the breakdown of acetyl CoA. In other words, fat can’t release any of its energy without oxygen—fatty acids don’t take part in glycolysis.

Protein (i.e., amino acids) must have their amino groups removed before they can be broken down for energy. (The amino groups are removed as urea in urine.) This leaves a carbon skeleton, which enters the energy-producing reactions as pyruvate or acetyl CoA.

Athletes can be roughly divided into two groups: (1) endurance athletes, who rely mainly on fatty acids and aerobic metabolism for energy and (2) power athletes, who rely mainly on glucose and anaerobic metabolism (glycolysis). They tend to have a predominance of muscle cells best suited for their event: Fast-twitch cells depend mainly on glycolysis, and slow-twitch cells depend mainly on aerobic metabolism. Which cell type predominates is genetically determined.

When not needed for energy production, acetyl CoA and fatty acids are converted to triglycerides and stored as body fat. Since all the energy-providing nutrients go through acetyl CoA in their energy-releasing breakdown, excess calories—whether from carbohydrate, fat, or protein—can be made into body fat.

The body continually needs some glucose for energy. If the body doesn’t get enough carbohydrate, it makes glucose from amino acids. (The body can’t use fatty acids to make glucose.) During starvation, a relentless conversion of amino acids to glucose hastens the loss of protein from both muscle and vital organs. When carbohydrate is lacking (because of starvation or a low-carb diet), the body uses the acetyl CoA made from fatty acids to produce ketones for use as an energy alternative to glucose.