Vitamins are small organic molecules that act as coenzymes. Coenzymes activate enzymes and help them function. Because vitamins generally cannot be synthesized by the body, a diet should include the proper daily amounts of all vitamins. Like enzymes, coenzymes can be reused many times. Thus, only small quantities of vitamins are needed in the diet.
Vitamins dissolve in either water or fat. The fat-soluble vitamins include vitamins A, D, E, and K. The water-soluble vitamins are vitamin C and the group of B vitamins. Because the body cannot store water-soluble vitamins, it excretes surplus amounts in urine. Fat-soluble vitamins are absorbed and stored like fats. Unpleasant physical symptoms and even death can result from storing too much or having too little of a particular vitamin.
The only vitamin that the body can synthesize in large quantities is vitamin D. This synthesis involves sunlight converting cholesterol to vitamin D precursors in the skin. People who do not spend a lot of time in the sun can get their vitamin D from food.
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