Recent surveys shows that more and more food shoppers are reading - and using - nutritional labeling. This fact is also reflected in the many questions readers send us. On a nutritional label, the information on a number of nutrients is expressed in percentages of the USRDA - five important letters that spell good nutrition.

USRDA stands for "U.S. Recommended Daily Allowances." The USRDA's were developed by the Food and Drug Administration specifically for use in nutritional labeling, which was initiated in 1972. Now they are the official labeling standard of daily nutrient allowances.

Before nutritional labeling took effect, the FDA (with the help of nutritionists and some food manufacturers and supermarket chains) tested several ways to indicate nutritional values. The majority of consumers tested agreed that listing the nutrients and the percentage of daily requirements of those nutrients in the food was the most useful and informative process.

So the FDA turned to the Recommended Dietary Allowances (RDA) tables of the Food and Nutrition Board of the National Academy of Sciences, which have been the authoritative standard of dietary adequacy in the United States since they were first published in 1943. Scientists who serve on the Food and Nutrition Board and its committees constantly review the latest evidence on nutritional needs and regularly revise the recommended allowances. The eighth and latest edition was published in 1974, and the ninth is due in 1978.

Since the RDA's are broken down into too many categories to be of practical use on a nutritional label, the FDA simplified the categories and came up with the USRDA. For instance, instead of separate recommendations for males and females of a varying height and weight, plus 15 separate age groups and listings for pregnant and lactating women, the USRDA's have only four categories: infants, children up to 4 years old, pregnant and lactating women, and everyone else (4 years through old age). To be certain that almost everyone's needs are amply covered by teh USRDA, the FDA uses the highest recommended allowance in each category for almost every nutrient.

The USRDA should not be used as a rigid standard for everyone. For instance, a child 7 to 10 years old needs only 55 per cent of the USRDA for protein, and even an adult man needs only 90 per cent of it, according to the standards.

Also, neither the USRDA's nor the RDA's should be used as the only criterion for a complete, well-balanced diet. The USRDA covers 20 of the 40-odd essential nutrients.

The eight nutrients that are required to appear on a nutritional label were chosen because they are "indicator" nutrients. These are vitamins A and C, thiamin, riboflavin, niacin, three B vitamins, calcium, iron and protein. If your diet is varied, composed mainly of fresh and lightly processed foods, and is adequate in these eight nutrients, chances are good that you will be getting adequate amounts of all the other 20 unlisted nutrients as well.

How should you use USRDA's and nutritional labeling? For one thing, by checking the content of fat, carbohydrate, protein and calories per serving and the percentage of USRDA of various essential nutrients, you can compare the nutritional value of similar foods against both their caloric content and their price. Also, a consistent reading of nutritional labels will help you to learn the general sources of protein, fat, carbohydrate, vitamins and minerals, and will help you choose other unlabeled foods - such as fresh fruits and vegetables - with a better idea of their nutritional value.

Finally, keeping general track of the indicator nutrients in foods, both packaged and fresh, will give you the means to judge whether you and your family are getting sufficient amounts of all the essential listed and unlisted nutrients.