You must read and accept the disclaimer to use this site. Updated for monthly, 2011

Recommended Daily Allowances (RDAs) are established by the Federal Government based on the recommendation of the Food and Nutrition Board. RDAs establish the nutritional requirements for specific age and gender groups to help determine what the average healthy individual needs, especially to prevent getting a short term deficiency disease like scurvy or rickets. It is much more difficult to establish a consensus among experts about the optimal nutritional requirements for people - for some experts, the only way to determine the optimum would be to test various dosages and combinations of nutrients on thousands of people over a lifetime, while controlling exactly the diet and behavior of each person. One goal of the RDAs is to help the Department of Agriculture evaluate whether the U. S. food supply is adequate to prevent short term deficiency diseases, and whether to fortify foods with additional micronutrients. Acute nutrient deficiencies are rare in the United States, because our food supply is fortified (like breakfast cereals) to ensure that even those who only eat well-balanced meals occasionally will still obtain the basic nutrition they need.

The RDAs are set at levels that provide a substantial margin of safety, two standard deviations above the average needs. Evaluation of American diets indicates that although very few people consume 100% of the RDA for all nutrients, the vast majority meet at least their average needs (77% of the RDA) for all of the 14 nutrients for which data are available. - "Position of the American Dietetic Association: Enrichment and fortification of foods and dietary supplements"

Daily values are similar to the RDAs, but a single value is selected for everyone, so that it is easier for us to read the labels on packages. For vitamin K, the Daily Value is 80 mcg, but the RDA is different for each group, for instance:

Furthermore, RDAs and Daily Values are not intended for the design of supplements, but are specifically to help you determine what to get in food. Food contains a variety of important nutrients, including many micronutrients that are not listed in the Daily Values, simply because we have not yet identified all the important nutrients necessary in a healthy diet. You need to strive for a well-balanced diet, a supplement cannot replace these important nutrients.

However, Daily Values are not designed to provide long-term optimal health, and recent clinical trials indicate that some micronutrients cannot be obtained in sufficient quantities in the diet. In a recent article in the Journal of the American Medical Association Volume 277: 1398-99 (1997) it stated

"nutrient supplements may be important for health promotion and prevention of certain chronic diseases. This view goes against the prevailing dogma in nutritional science that a balanced diet is sufficient to achieve all nutritional objectives."

What should you take in a supplement? When you eat well, you obtain the RDAs in your diet. Taking a supplement with those same RDAs gives you another dose, so you end up with 200% of the RDAs. Is this good for your health? (Click here to see the evidence that multivitamins are likely to harm most people!) More is not always better, and for some micronutrients it is possible that getting 200% of the RDAs each day may do more harm than good. The vitamin and mineral pages at this site point out some of those for which there may be a hazard. On the other hand, the Federal Government regularly determines how much of each nutrient the typical person obtains in their diet, and what the range of intake is for safe, proven levels of nutrients. By calculating the difference betweeen what is obtained from the diet, and what the safe, proven level is shown to be in carefully controlled clinical studies, it is possible to determine what nutrients are safe and likely to be beneficial in supplement form. Such a formulation will contain only a limited number of vitamins and minerals, in age and gender-based dosages, because both diet and physiology change with age and gender. Thus, a good diet with a well-designed supplement is the best way to obtain optimal nutrition.