Page 207 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN017G-116 August 2, 2001 18:14

Vitamins and Coenzymes 515

FIGURE 10 Coenzyme A and its constituent components, which include the vitamin pantothenic acid.

II. NUTRITIONAL RECOMMENDATIONS adequate amounts of all of the vitamins. There are excep-
tions. Vitamin B 12 is not made by plants and strict veg-
It is difficult to establish the amount of any vitamin that is etarians may become deficient if their diet does not con-
essential to a human being. Even for animals the amount tain yogurt or other products of fermentation. Thiamin
required for good health must exceed that needed for sur- is very labile, especially at high pH. Cooking at a pH
vival. The enormous individual variation among human above 8 quickly destroys this vitamin. Alcoholism is
beings ensures that any conclusion about the requirement another cause of thiamin deficiency, sometimes leading
for a nutrient will be incorrect for at least some individuals. to the characteristic Wernicke’s disease or Wernicke–
The need for a vitamin will be affected by the age of the in- Korsakoff syndrome, conditions with specific symptoms
dividual, by differences in the ability to take up the vitamin of encephalopathy. Because of the lability of thiamin,
from the digestive tract, and by the ability to convert it to the number of turnovers, i.e., the number of times that a
appropriate coenzyme forms. Also important is the ability thiamin diphosphate molecule can pass through its cat-
of numerous enzymes to hold the coenzyme correctly into alytic cycle, seems to be limited. For this reason, the
their active sites. With many thousands of possible sites for recommended daily allowance is increased by 0.5 mg for
mutation of the DNA encoding these proteins, there are each 1000 kcal (Cal) consumed beyond that for an aver-
many possible reasons why some individuals may need age person. Riboflavin is destroyed by light. Diets in many
larger amounts of a vitamin than does the average person. parts of the world are deficient in vitamin A and also in
Recommended dietary allowances (Table I) are based on the plant carotenes, which can be converted into vitamin
studies by panels of nutritional investigators. They vary A in the body. Folate deficiency may occur if there is in-
somewhat from one country to another and are revised adequate intake of fresh vegetables and fruit. Prolonged
periodically. To put the needs for vitamins in a better per- cooking can also destroy the vitamin.
spective, Table II lists in concise form the other known Specific dietary deficiencies sometimes affect large
human nutritional requirements. populations. In the past, beriberi was a widespread con-
Most of the B vitamins are synthesized by plants, sequence of consumption of polished rice without vita-
fungi, and bacteria. Meat and dairy products also contain min supplementation. During the early decades of the last
vitamins that have been obtained from these sources. As a century, pellagra was widespread in southern regions of
consequence, a well-balanced human diet usually supplies the United States because the diet was low in protein