The growing interest in diet and health has made the Recommended Dietary Allowances, or RDAs, a part of the common vocabulary of nutrition. With the 10th edition of these guidelines due out soon, it's a good time to look back at their origins and examine their uses and limitations.

As Dr. A.E. Harper of the University of Wisconsin pointed out in the American Journal of Clinical Nutrition, the first "official" standard probably came in 1835, when Great Britain passed the Merchant Seaman's Act. It required that lime juice be carried aboard ship to prevent scurvy. Not long after, the British Privy Council, concerned about the health of workers during an economic depression, commissioned Dr. Edward Smith to set dietary standards. Smith based his recommendations of approximately 3,000 calories and 80 grams of protein on his own studies of respiration.

During the mid- to late-19th century, Germany, the United States and Britain each made other recommendations. All reflected about the same level of caloric intake, although protein standards were set as high as 125 grams a day. (By comparison, today's figures are 2,700 calories and 56 grams of protein a day for the typical adult male, who is less active than his counterpart of an earlier era.)

At the turn of the century, several minerals had been recognized as essential for good health. But it was clear that there were other important factors, namely, vitamins. Dietary guidelines began to emphasize the need for fruits and vegetables and the importance of milk for babies and young children. By the 1930s, many vitamins had been identified and their roles were delineated. Knowledge of minerals also was expanding. Scientists became aware of nutritional needs during different periods of life.

A committee of the League of Nations initiated a set of guidelines taking all these factors into consideration. For the first time, recommendations were formed for the sole purpose of preserving the health of the general population, not just the work force or the military.

But it took World War II and our government's desire for a national-defense guide for food supplies to launch the RDAs. These guidelines were developed under the auspices of the National Academy of Sciences. Since then, the scientific basis for the RDA for each nutrient has been reviewed (and, if necessary, revised) about every five years.

To date, RDAs have been set for 10 vitamins (A, D, E, C, B-6, B-12, thiamin, riboflavin, niacin and folacin), six minerals (calcium, phosphorus, magnesium, iron, zinc and iodine) and protein. The recommendations are divided by age (10 categories) and sex, with additional guidelines for pregnant and nursing women.

For some nutrients whose importance is certain, there is still insufficient information to establish definitive recommendations. In the current edition, ranges of "safe and adequate dietary intakes" were set for such nutrients, divided only by age. The 12 in this category include vitamin K, biotin, panothenic acid, copper, manganese, fluoride, chromium, selenium, molybdenum, sodium, potassium and chloride.

The RDAs are widely employed by health professionals and the lay public to judge nutritional status. But it also is important to know the correct way to use them.

Rather than reflecting the dietary needs of any one person, the RDAs represent the "average daily amount of a nutrient that population groups should consume." Wide variation in needs among individuals is assumed. The RDA for each nutrient is designed to meet or exceed the needs of most healthy people. The suggested intakes are not intended to be duplicated on a daily basis, but to be consumed over a period of time.

These allowances should not be confused with minimum daily requirements that your body needs to maintain normal functioning. They are generally based on such minimum needs and then increased to take into account individual variation and differences in food intake. How this "margin of safety" is set depends on the nutrient. In the case of vitamin D, which is toxic at levels not far above the requirements, the margin is set barely higher than what we need. For obvious reasons, the RDA for calories has no extra allowance.

The RDAs are designed to be met by diet alone. The notable exception is iron for women in their reproductive years. Many have trouble consuming 18 milligrams a day in their ordinary diet, so a need for an iron supplement is common.

The new edition of these guidelines will certainly show some changes, but none that cannot be fulfilled by a varied diet rich in fresh and lightly processed foods.