Vitamins are compounds needed in relatively small amounts in the diet for optimal body health. Vitamins primarily work to help special proteins, known as enzymes, speed up chemical reactions in the body. If you do not consume enough vitamins in your diet, you may develop signs of a vitamin deficiency; conversely, some vitamins are toxic in high doses.
Guidelines
The recommended daily allowance, or RDA, of a vitamin is the amount that is sufficient to maintain the health of between 97 and 98 percent of all healthy individuals, the Institute of Medicine explains. In some cases, there is not enough scientific evidence to determine a recommended daily allowance, in which case another value, titled "Adequate Intake," or AI, is used. Some vitamins also have a tolerable upper limit or TUL, which is the maximum daily dose that can be taken without causing health problems.
Fat Soluble Vitamins
Some vitamins, including vitamins A, D, E and K, are fat-soluble vitamins because they dissolve more easily in a fat- or oil-based environment. For vitamin A, the RDA is 900 mcg for men and 700 mcg for women, and the tolerable upper limit is 3,000 mcg per day, the Merck Manual explains. The RDA for vitamin D is 200 International Units per day, except for adults over 51, who require 400 RDA per day. The tolerable upper limit for vitamin D is 2,000 International Units. The RDA and tolerable upper limit for vitamin E is 15 mg and 1,000 mg, respectively. The RDA for vitamin K is 120 mg for men and 90 mg for women and there is no tolerable upper limit.
Other Vitamins
The RDA for folate is 400 mcg with a TUL of 1,000 mcg. The RDA for niacin is 16 mg for men and 14 mg for women with a TUL of 35 mg. For men, the RDA of riboflavin and thiamine is 1.3 mg and 1.2 mg, respectively; for women, these values are both 1.1 mg. Men need 1.3 mg and 2.4 mg of vitamins B-6 and B-12, respectively, whereas women require 1.5 mg and 2.4 mg of these two vitamins. The RDA of vitamin C is 90 mg for men and 75 mg for women, with a TUL of 2,000 mg.
Considerations
The RDA for vitamins may vary from person to person. Larger people typically require higher doses of vitamins, for example. Pregnancy and lactation may also change your recommended daily intake of vitamins. People who smoke often need more vitamin C than non-smokers because tobacco smoke interferes with vitamin C absorption. In addition, the use of some drugs can interfere with vitamin absorption, so you should check with your doctor if you are concerned about your vitamin intake.
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