Dear Dr. Gott: I am a lady in her 90th year and hope you can answer a pertinent question. Until my 70s, I was a respectable five feet, six inches. Now I am one-and-half inches shorter. When checking height charts for healthy weight, which height do I use?

At 150 pounds, I am below the BMI of 25 if the higher height is used. I am just above the 25 mark if the lower height is used. I know that life is not fair, but should I be considered overweight at 90 when I was OK at 70?

Reply: At 90, you needn't be worried about your weight or Body Mass Index. Your weight and BMI are essentially normal and should not be a concern to either you or your physician.

As we age, most of us shrink and gain some weight. This is usually normal. I would be concerned only if your loss of height were coupled with back pain or increased fractures (signs of osteoporosis) or if your weight changed drastically. (Weight loss could signify cancer or malnutrition; gain may mean oedema or heart failure.)

BMI is calculated from a person's height and weight. It is considered a reliable indicator of body fatness yet does not measure it directly.

BMI is one of several tools used to identify possible weight problems for adults.

It cannot be used as a diagnostic tool, however. Well-muscled and highly trained athletes, for example, may have a high BMI but low body fat. This is because the test does not distinguish between fat and muscle, only weight-to-height ratios.

There are four categories: underweight (BMI under 18.5), normal (18.5 to 24.9), overweight (25.0 to 29.9) and obese (30.0 and up). Someone who is 5ft 9ins can have a weight range of 125 pounds to 168 pounds with BMIs ranging from 18.5 to 24.9. With respect to amount of body fat, it varies by age and gender.

Men and women with the same BMI often have different amounts of body fat because women tend to have more fat than do men.

This is also true for older people, who generally have increased amounts of fat over that of younger people.

It is important to remember that BMI must be used in conjunction with other tools, such as waist circumference. Other risk factors must come into play, such as smoking, inactivity, hypertension or high cholesterol.

Many people over the age of 80 can safely ignore most of the "rules" of good health. Mildly elevated cholesterol levels and blood pressures are common and generally inconsequential.

In those younger, however, preventive medicine is always the best medicine. Take care of your health while you're young so you can enjoy it when you're old.

I am sending you a copy of my Health Report "A Strategy for Losing Weight." Other readers who would like a copy should send a self-addressed, stamped number 10 envelope and $2 to Newsletter, PO Box 167, Wickliffe, OH 44092.

Doctor Gott is a retired physician and the author of the book "Dr. Gott's No Flour, No Sugar Diet," available at most chain and independent bookstores, and the recently published "Dr. Gott's No Flour, No Sugar Cookbook." Write directly to Dr. Gott c/o United Media, 200 Madison Ave., 4th fl., New York, NY 10016.