Biochemistry, Vol. 3 - A. Lehninger 1985

Selected Aspects of Human Biochemistry
Human Nutrition
Deficiency of certain vitamins can be life-threatening

Let us now examine The Role of Vitamins in Human Nutrition. In Chapter 10, we already discussed The Structure of VITAMINS AND THEIR coenzyme Functions. In this section, we will focus on human dietary requirements for vitamins and the health consequences of their deficiency.

Today we can state with confidence that all vitamins essential for the normal nutrition of humans and rats have been identified. Depending on the severity of health impairment caused by their deficiency, Vitamins can be divided into two classes (Table 26-12). Deficiencies of thiamine, niacin, riboflavin, folic acid, and ascorbic acid have a substantial impact on human health and represent a pressing issue in many countries. Marginal deficiencies of these vitamins are encountered relatively frequently even in highly developed nations. As for pantothenic acid, pyridoxine, biotin, vitamin B₁₂, and vitamins A, D, E, and K, deficiencies of these vitamins are extremely rare among residents of developed countries.

Class="center">Table 26-12. Vitamins Required by Humans

Individual requirements for specific vitamins can vary significantly depending on dietary patterns, gastrointestinal microflora activity, and genetic factors. For instance, the requirement for nicotinamide strongly depends on dietary protein composition and, specifically, on the presence of Tryptophan, which can be converted into nicotinamide in the body, whereas the requirement for pyridoxine increases with higher dietary protein content. At the same time, vitamins such as biotin, pantothenic acid, and vitamin B₁₂ are synthesized by intestinal Bacteria in quantities sufficient to meet normal human requirements. A deficiency of these vitamins may occur only under highly unusual dietary regimens.

Most Water-Soluble Vitamins must be supplied regularly in the diet because they are relatively quickly eliminated from the body or degraded during routine enzymatic reactions. When excessive amounts of water-soluble vitamins are consumed in the form of multivitamin supplements, the excess beyond daily requirements is simply excreted, since most water-soluble vitamins cannot be stored in the body. At the same time, excessive intake of Fat-soluble vitamins (A and D) can be toxic.