Q: I live in a retirement home. Recently our meal schedule was changed so that we have dinner at noon and a light supper at 5:30 p.m. We were told this was "better for us." I am 88, my digestion is just fine, and I much preferred the old schedule. Is there a real health reason why we are better off with a main meal at noon?

A: No, there is actually no health advantage to altering the meal structure.

There may be two reasons for the change. One is financial: Because of staffing patterns, it is sometimes easier to control the costs of meal preparation when the main repast is served at noon. It is also true that, while you do not, many older people prefer their main meal at midday and then a lighter supper, to have time to digest their food before bed. However, the hour at which your last meal is served is certainly well before most individuals go to sleep.

You might wish to explore whether the decision is unpopular with other residents. If so, maybe the management can be persuaded to develop a more flexible system. Then those who prefer could eat more lightly at noon and have more generous amounts of food available at the evening meal. In addition, you could request that the home provide evening snacks for those who would like them.

Q: I am 62 and recently retired. I now have time for leisurely shopping and have begun to read food labels more carefully. According to the nutrition label on instant nonfat dry-milk powder, it looks pretty good. But before I switch from pasteurized, homogenized vitamin-D milk, I wanted to ask how the two compare.

A: As long as you are buying nonfat milk fortified with both vitamins A and D, you are not compromising your nutrient intake one bit and will be saving money besides.

What is missing from both a cup of fresh and dry nonfat milk as compared with homogenized whole milk is about 8 grams of fat containing about 72 calories. As you probably know, energy requirements drop with advancing age, both because the basal metabolism rate declines and because older people tend to be less active.

One important step toward maintaining normal weight is to keep as active as possible. The other is to consume no more calories than you burn. The trick is to pare away excess calories that do not compromise the nutritional adequacy of the diet. Trimming away fat calories without giving up a single bite of food is an excellent example of how to do just that. Moreover, substantial evidence suggests that Americans should cut down on the amount of fat, especially saturated fat, that they consume. Milk fat is highly saturated.

Q: What is the difference between RDAs and USRDAs?

A: In short, it resembles a parent-offspring relationship. The RDAs (Recommended Dietary Allowance) were established by the Food and Nutrition Board of the National Research Council in 1943, and have since been revised periodically. The most recent edition, scheduled to appear several months ago, was delayed because of controversies that could not be resolved by the revising committee. The RDA sets allowances for 15 separate age categories as well as for pregnant and nursing women. Such detailed information has many important uses, but is far more than could reasonably fit on many packages.

The USRDAs (U.S. Recommended Daily Allowances) were developed for use in the nutrition-labeling program. They are condensed into four groups: two for infants and very young children, and one for pregnant and lactating women. The fourth, for adults and children four years and older, is the one used most often on food labels.

To create a set of standards to cover the needs of such a broad range of individuals, the highest RDA for nonpregnant and nonlactating women and adult women was chosen, with a few exceptions, as the USRDA for each nutrient. Thus, the 18 mg RDA for iron for women, rather than the 10 mg RDA for men, was used as the USRDA for that nutrient. However, the USRDA for calcium was set at 1,000 mg, the midpoint between the nonpregnant, nonlactating adult RDA of 800 mg and the teen-age RDA of 1,200 mg.

Interestingly, USRDAs were set for several nutrients for which RDAs had not yet been established, including the B vitamins, biotin and pantothenic acid, and copper. The committee responsible for the most recent edition that appeared in 1980, several years after the USRDA's were established, felt there was still insufficient information to set RDA for these nutrients. They were included in the edition in a special category of "safe and adequate daily dietary intakes."