Recently I joined a health club. The instructor calculated my "body mass index" and told me I was 10 percent overweight. It sounded very technical and I really did not understand what she was talking about. Can you explain?

ANSWER - Unfortunately, such terms are flying about fast and loose these days, often without being clearly defined. Body mass index (BMI) is a single figure describing the relationship between height and weight. It relies on two easily obtainable measurements and is regarded as a reasonable approximation of results that could be obtained with more accurate but technically more difficult and expensive methods.The calculation can be done on any pocket calculator. First, convert your weight in pounds to kilograms by dividing it by 2.2. Then convert your height in inches to meters, by multiplying it by .0254. Then square your height (multiply it by itself). Finally, divide your weight in kilograms by the square of your height in meters. There you have it: your BMI.

Here's an example: a woman 5-feet 5-inches tall and weighing 140 pounds would have a body mass index of 23.4. The question is, what does that mean? In general, a BMI of 24.8 is about 10 percent above desirable weight. A figure of 27.2 is 20 percent overweight, and is considered in the obese range.

There is an important exception, however. Extremely well exercised athletes may have excess weight in relation to height and a BMI suggesting that they are overweight. In fact, the excess is explained by muscles and not by extra fat.

QUESTION - Is a microwave oven more fuel efficient than cooking in a conventional oven?

ANSWER - "It depends," is the equivocal answer, according to a recent review of evidence by the Institute of Food Technologists Expert Panel on Food Safety and Nutrition. Factors affecting whether a microwave oven will consume more or less energy than a conventional gas or electric oven include the quality, type and shape of the food being heated. In general, however, when it comes to preparing a single item, microwave ovens consume as much or more energy than electric cooktop ranges, but less than conventional electric ovens. On the other hand, conventional ovens have the advantage of preparing several items together in a shorter period of time.

But these pieces of evidence do not constitute a clear and consistent set of guidelines for maximum energy efficiency. With the limited evidence available at present, it is often convenience and quality factors, rather than potential energy savings, that provide the motive for favoring one method over another.

QUESTION - Some people say that one benefit of exercise when trying to lose weight is that it increases resting metabolic rate. Is that true?

ANSWER - Current scientific evidence doesn't support that effect. Resting metabolic rate is the energy the body uses just to maintain itself. The earliest studies of that question, conducted more than 25 years ago, did find an increase in metabolic rate lasting as long as 48 hours after the exercise session, but there were serious flaws in the study design. The results of more recent studies have not been completely consistent, but in general the effect of exercise on raising metabolic rate has proven to be insignificant. For instance, in one investigation, the elevation in resting energy expenditure for 3 hours, following 30 minutes of bicycling at a moderately intense level, was just 14 calories, hardly an impressive amount.

However, there are certainly plenty of other reasons to exercise regularly, even if this is not among them.

QUESTION - Could you please tell me why it is that oatmeal breads really seem to have very little oats in them?

ANSWER - The problem is that oats contain no gluten. Gluten-rich flour, the type of wheat flour used in bread baking, is essential for the production of a dough that becomes strong and elastic as the yeast causes it to expand. In general, it takes about 2 1-2 cups of bread flour to support a half-cup of uncooked oatmeal in a loaf of homebaked oatmeal bread.