Table of Contents
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 body functioning and good health. A well-balanced diet should provide most of the vitamins you need to stay healthy and prevent disease. Vitamin D is the only exception. Supplements can be helpful in certain situations, such as pregnancy and certain illnesses. Strict vegetarians may benefit from vitamin B12 supplements.
Unlike carbohydrates, fats, and proteins, vitamins are not sources of energy. Instead, vitamins are chemical partners for the enzymes involved in the body's metabolism, cell production, tissue repair, and other vital processes.
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 vitamin C and the B complex vitamins. The body uses these vitamins very quickly. Excess amounts are eliminated in urine.
Guidelines for Adequate Intake of Vitamins
The Recommended Dietary 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 to prevent deficiency diseases. The RDA refers to an estimate of the average requirements of dietary components such as calories, vitamins, minerals, and proteins that are required to prevent deficiency. Different values apply to different groups based on gender and age.
The RDA is gradually being replaced by a new standard called the Dietary Reference Intake (DRI). The DRI represents a shift in nutritional emphasis -- from preventing deficiencies to lowering risks of chronic diseases, such as cardiovascular disease. The DRI values comprise four categories:
- The recommended dietary allowance (RDA). This is the current rating on most vitamins.
- The estimated average requirement (EAR). This is the amount that will meet the nutritional requirements of 50% of the population.
- Adequate intake (AI). This is an amount that will be used if there is insufficient data to calculate the RDA.
- Tolerable upper intake level (UL). This is the maximum dose likely to be safe in 98% of the population.
Food and supplement labels now typically list the Daily Value (DV). This is the percentage of the amount of a nutrient that experts believe people need in their daily diet. On food labels it is usually based on one serving size for a person who takes in 2,000 calories a day.
Review Date: 10/08/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine,
Harvard Medical School; Physician, Massachusetts General Hospital.
Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M.,
Inc.
A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)
