Q: Is it true that soft-drink consumption is still rising in this country?

A: Unfortunately, it is. Figures from the U.S. Department of Agriculture indicate that per-capita soft-drink consumption rose 7 1/2 percent between 1983 and 1984. That brought it to just over 44 gallons per person per year (almost 16 ounces a day), of which nine gallons is artificially sweetened. Considering that some people consume none, there are many who are downing much more than that amount.

In 1970-74, soft-drink consumption was a not inconsiderable 25.5 gallons a year.

Clearly Americans are spending a lot of money for flavored water and, except for those who choose the artificially sweetened "diet" varieties, empty calories.

Q: In a column about genetic influences on weight, you mentioned that the measure of fatness was something called "the body mass index." Could you please explain this?

A: Body mass index (BMI) is a single figure describing the relationship between height and weight. It is often used in clinical research as an approximation of more accurate, but technically far more difficult, measures of body fatness.

The calculation is relatively simple: weight in kilograms divided by the square of height in meters. A BMI of 22.5 or slightly less is considered normal weight. (That figure corresponds to the midpoint of weight for a person of medium frame on the Metropolitan height and weight tables.) A BMI of 24.8 is 10 percent above desirable weight, and a figure of 27.2 is 20 percent overweight, and considered obese.

To work out your own BMI, you need to know 2 metric conversions: that .0254 meters equals 1 inch, that 1 kilogram equals 2.2 pounds. First, convert your weight to kilograms and your height to meters. Second, multiply the height figure by itself. Then divide the weight by that number.

For example, a woman 5 feet, 3 inches (63 inches) weighing 125 pounds would stand 1.6 meters tall and weigh 56.82 kg. To calculate the BMI, multiply 1.6 by itself (2.56) and divide the weight by that figure. This woman has a BMI of 22.2 and falls within the normal weight range.

One word of caution: Extremely well-exercised athletes may have excess weight in relation to height, and therefore a BMI that indicates they are overweight. But that weight is related to muscle, and they are not fat.

Q: Recently a friend who is a dietitian tried to convince me to write to my legislators encouraging them to support a nutrition monitoring act. Frankly, I was skeptical that this might be just another layer of government.

What do you think?

A: We wholeheartedly support this legislation. The nation has been waiting a long time for a coordinated nutrition monitoring program. It will ensure more timely, complete and accurate information on which to base health, nutrition and dietary policy, and help identify and target resources to meet existing needs. A coordinated effort will benefit all of us, not just special groups within our population who are at particular risk.

The bill is called the National Nutrition Monitoring and Related Research Act, or HR 2436 and S 1569. As for your concern about adding to the cost of government, this legislation is urgently needed to promote efficiency and reduce duplication of efforts at the federal level. That will save money in the long run.

The White House Conference on Food, Nutrition and Health in 1969, which was chaired by Dr. Mayer, recommended a national nutrition surveillance program. We hope that now, 17 years later, that recommendation will at last be realized.

We join your dietitian friend in encouraging you, and all other concerned citizens, to write to your legislators to support passage of the legislation.