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Vitamins are a group of 14 organic essential nutrients that are required in very small amounts in the diet. The exact chemical structures of the first vitamins to be discovered were unknown, and they were simply identified by letters of the alphabet. Vitamin B turned out to be composed of eight substances now known as the vitamin B complex. Plants and bacteria have the enzymes necessary for vitamin synthesis, and it is by eating either plants or meat from animals that have eaten plants that we get our vitamins.

The vitamins as a class have no particular chemical structure in common, but they can be divided into the water-soluble vitamins and the fat-soluble vitamins. The water-soluble vitamins form portions of coenzymes such as NAD+, FAD, and coenzyme A. The fat-soluble vitamins (A, D, E, and K) in general do not function as coenzymes. For example, vitamin A (retinol) is used to form the light-sensitive pigment in the eye, and lack of this vitamin leads to night blindness. The specific functions of each of the fat-soluble vitamins will be described in later chapters.

The catabolism of vitamins does not provide chemical energy, although some of them participate as coen-zymes in chemical reactions that release energy from other molecules. Increasing the amount of vitamins in the diet beyond a certain minimum does not necessarily increase the activity of those enzymes for which the vitamin functions as a coenzyme. Only very small quantities of coenzymes participate in the chemical reactions that require them and increasing the concentration above this level does not increase the reaction rate.

The fate of large quantities of ingested vitamins varies depending upon whether the vitamin is water-soluble or fat-soluble. As the amount of water-soluble vitamins in the diet is increased, so is the amount excreted in the urine; thus the accumulation of these vitamins in the body is limited. On the other hand, fat-soluble vitamins can accumulate in the body because they are poorly excreted by the kidneys and because they dissolve in the fat stores in adipose tissue. The intake of very large quantities of fat-soluble vitamins can produce toxic effects.

A great deal of research is presently being done concerning the health consequences of taking large amounts of different vitamins, amounts much larger than one would ever normally ingest in food. Many claims have been made for the beneficial effects of this practice—the use of vitamins as drugs—but most of these claims remain unsubstantiated. On the other hand, it is now clear that ingesting large amounts of certain vitamins does indeed have proven health-promoting effects; most notably, the ingestion of large amounts of vitamin E (400 International Units per day) is protective against both heart disease and multiple forms of cancer, the most likely explanation of these effects being that vitamin E is an antioxidant and thus scavenges toxic free radicals. (See also the section on aging in Chapter 7.)

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Protein Activity and Cellular Metabolism CHAPTER FOUR

Protein Activity and Cellular Metabolism CHAPTER FOUR


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