In this Chapter:
21.1. Water-soluble vitamins
21.2. Fat-soluble vitamins
Vitamins participate in metabolism by functioning as coenzymes or their components.
Most vitamins are not synthesized in the human body. Usually diet serves as their source. The only two vitamins that are produced in body tissues are vitamin D3 (in skin exposed to ultraviolet rays) and nicotinamide (from tryptophan). A number of vitamins (vitamin K and others) are produced by microorganisms in the large intestine. Under certain conditions varying severity of vitamin deficiency (hypovitaminosis, avitaminosis) may develop. The most common cause of the deficiency of vitamins is their low content in the diet. Besides, some pathological processes in the gastrointestinal tract may lead to failure of vitamin absorption. In some cases hypovitaminosis can develop as the result of an increased metabolic need for vitamins (for example, pregnancy, thyrotoxicosis, fever).
It is possible to compensate for vitamin deficiency by prescribing a diet with an adequate content of vegetables, fruit and products of animal origin. This is, undoubtedly, the most convenient and easy way to eliminate hypovitaminosis. However, in this case it is hard to determine the vitamin dose. Besides, the use of food vitamins is ineffective if vitamin absorption is impaired.
Vitamins produced by the pharmaceutical industry play an important role in hypoand avitaminosis therapy. They are convenient in many respects. First of all, their production does not depend on the season. It is possible to dose vitamin preparations exactly. If oral administration does not result in the necessary effect, the vitamin preparations may also be used parenterally. However, the possibility of hypervitaminosis - poisoning with vitamin preparations (especially with fat-soluble ones) - should be considered.