The Recommended Dietary Allowances (RDAs) were created in 1941 by the Food and Nutrition Board, a subsidiary of the National Research Council, which is part of the National Academy of Sciences in Washington, D.C.

RDAs originally were designed to make planning several days’ meals in advance easy for you. The D in RDA stands for dietary, not daily, because the RDAs are an average. You may get more of a nutrient one day and less the next, but the idea is to hit an average over several days.

For example, the current RDA for vitamin C is 75 mg for a woman and 90 mg for a man (age 18 and older). One 8-ounce glass of fresh orange juice has 120 mg vitamin C, so a woman can have an 8-ounce glass of orange juice on Monday and Tuesday, skip Wednesday, and still meet the RDA for the three days.

A man may have to toss in something else — maybe a stalk of broccoli — to be able to do the same thing. No big deal. The amounts recommended by the RDAs provide a margin of safety for healthy people, but they’re not therapeutic. In other words, RDA servings won’t cure a nutrient deficiency, but they can prevent one from occurring.

RDAs offer recommendations for protein and 18 essential vitamins and minerals, which include:

• Vitamin A
• Folate
• Vitamin D
• Vitamin B12
• Vitamin E
• Phosphorus
• Vitamin K
• Magnesium
• Vitamin C
• Iron
• Thiamin (vitamin B1)
• Zinc
• Riboflavin (vitamin B2)
• Copper
• Niacin
• Iodine
• Vitamin B6
• Selenium

The newest essential nutrient, choline, won its wings in 2002, but no RDAs have yet been established. Calcium also has an Adequate Intake (AI) rather than an RDA.

What nutrients are missing from the RDA list of essentials? Carbohydrates, fiber, fat, and alcohol. The reason is simple: If your diet provides enough protein, vitamins, and minerals, it’s almost certain to provide enough carbohydrates and probably more than enough fat.

Although no specific RDAs exist for carbohydrates and fat, guidelines definitely exist for them and for dietary fiber and alcohol. In 1980, the U.S. Public Health Service and the U.S. Department of Agriculture joined forces to produce the first edition of Dietary Guidelines for Americans.

This report has been modified many times. The latest set of recommendations, issued in the spring of 2005, sets parameters for what you can consider reasonable amounts of calories, carbohydrates, dietary fiber, fats, protein, and alcohol. According to these guidelines, as a general rule, you need to:

• Balance your calorie intake with energy output in the form of regular exercise.

• Eat enough carbohydrates (primarily the complex ones from fruits, vegetables, and whole grains) to account for 45 to 65 percent of your total daily calories. That’s 900 to 1,300 calories on a 2,000-calorie diet.

• Take in an appropriate amount of dietary fiber, currently described as 14 grams dietary fiber for every 1,000 calories.

• Get no more than 20 to 35 percent of your daily calories from dietary fat. Therefore, if your daily diet includes about 2,000 calories, only 400 to 700 calories should come from fat.

• If you choose to drink alcoholic beverages, do so in moderation, meaning one drink a day for a woman and two for a man.

Because different bodies require different amounts of nutrients, RDAs currently address as many as 22 specific categories of human beings: boys and girls, men and women, from infancy through middle age. The RDAs recently were expanded to include recommendations for groups of people ages 50 to 70 and 70 and older.

Eventually, recommendations will be made for people older than 85. These expanded groupings are a really good idea. In 1990, the U.S. Census counted 31.1 million Americans who are older than 65. By 2050, the U.S. Government expects more than 60 million to be alive and kickin’.

You wouldn’t want these baby boomers to miss their RDAs, now would you? But who you are affects the recommendations. If age is important, so is gender. For example, because women of childbearing age lose iron when they menstruate, their RDA for iron is higher than the RDA for men.

On the other hand, because men who are sexually active lose zinc through their ejaculations, the zinc RDA for men is higher than the zinc RDA for women. Finally, gender affects body composition, which influences RDAs.

Consider protein: The RDA for protein is set in terms of grams of protein per kilogram (2.2 pounds) of body weight. Because the average man weighs more than the average woman, his RDA for protein is higher than hers. The RDA for an adult male, age 19 or older, is 56 grams; for a woman, it’s 46 grams.