Vitamins - Introduction

Highlights	Phytochemicals	Resources
Introduction	Healthy Foods	References
Carotenoids	Dietary Health Benefits

Introduction

Vitamins do not share a common chemistry, but they do share certain characteristics. They are all organic nutrients that are necessary in small amounts for normal metabolism and good health. Most vitamins must be provided by the diet or by supplements; only three vitamins (D, K, and the B vitamin biotin) can be manufactured in the body from nondietary sources. Vitamins are not sources of energy as are carbohydrates, fats, and proteins. Instead, vitamins serve as chemical partners for the enzymes involved in the body's metabolism, cell production, tissue repair, and other vital processes.

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Vitamins are either fat soluble or water soluble. The fat-soluble vitamins, which include A, D, E, and K, are absorbed by the body using processes that closely parallel the absorption of fat. They are stored in the liver and used up by the body very slowly. The water-soluble vitamins include C and the B complex vitamins. The body uses these vitamins very quickly; excess amounts are eliminated in urine.

Guidelines for Vitamin Adequate Intake

The Recommended Daily Allowance (RDA) for vitamins, set by the Food and Nutrition Board of the National Academy of Sciences-National Research Council, has been used for years as a guide for determining the amount of vitamins needed for a healthy diet. The RDA refers to an estimate of the average daily requirement. It is not completely adequate, however, for informing people about the amounts of vitamins they may need.

The RDA is gradually being enhanced using a new standard called the Dietary Reference Intake (DRI). The DRI is based on the following ratings, which will eventually appear on labels:

The recommended daily allowance (RDA). This is the current rating on most vitamins.
The estimated average requirement (EAR). This is the amount adequate for 50% of all people, which will be put on labels when it can be calculated.
Adequate intake (AI). This is an amount that will be used if there is insufficient data to calculate the EAR.
Tolerable upper intake level (UL). This is the maximum dose likely to be safe in nearly all individuals. It will be included on labels if this amount is known.

Food and supplement labels now typically list the Daily Value (DV). This is the percentage of the amount of a nutrient that experts believe a person needs in their daily diet. On food labels it is usually based on one serving size for a person who takes in 2000 calories a day.