DEAR DR. DONOHUE: You and others refer to BMI to determine a person’s obesity. I want to tell you how inaccurate it is. My daughter (22) and I (50) are both 5 feet 4 inches and weigh 150 pounds. She wears a size 8; I, a size 14. She sinks like a rock in water, and I float like a cork. She is all muscle. I am all lard. Our BMIs are identical. Surely there must be a better way of determining obesity. – T.N.

ANSWER: Body mass index – BMI – is a more accurate method for determining how much of the body is fat and how much is muscle and bone than is weight read from a scale. It’s not perfect, and there are better ways of arriving at body composition, but they are complicated and expensive.

Comparing body weight underwater with body weight on dry land, for instance, gives a good estimate of what percentage of body weight is fat. It is fat weight that leads to artery hardening, diabetes, heart attacks and strokes, not muscle weight.

To obtain body mass index, divide weight in kilograms by height in meters squared. For those not at ease with metric measurements, BMI is obtained by multiplying body weight in pounds by 703. The result obtained by doing that is then divided by height in inches. That result is divided once more by height in inches. A healthy BMI lies between 18.5 and 24.9. Numbers from 25 to 29.9 indicate overweight; 30 and above, obesity. You and your daughter have a BMI of 25.7.

You make a valid point. BMI is falsely high in people who are very muscular, like your daughter. Blacks and Polynesians often have BMIs that are higher than is their true percentage of body fat. The BMI doesn’t accurately reflect the body composition of very short people. All the same, it is a useful tool for most.

A waist measurement serves as a good marker of health and can substitute for BMI. A man’s waist should be less than 40 inches, and a woman’s, 35 inches. Abdominal fat is a marker for future heart and artery problems. The waist is measured at the level of the navel. See how your daughter comes out using this criterion.

DEAR DR. DONOHUE: My husband says aluminum and cast-iron skillets and cookware are dangerous to health. He will not eat anything cooked in or on them. We have been married only one year. My background is one where cast iron and aluminum were used. Until he sees it in writing, he won’t change. Can you solve this dispute for us? – S.G.

ANSWER: Acid foods, like tomatoes, draw iron from iron pots and pans. Even nonacid foods, when cooked in ironware for a long time, do the same. That’s not dangerous. It’s a way for many people to meet their daily iron requirement. The digestive tract has an inborn governor that limits the amount of iron absorbed so people don’t get an overdose. Only those who have a defective governor have to be careful about the amount of iron they eat. People with hemochromatosis, a genetic disorder, are examples.

Aluminum is a subject that generates a great deal of heated discussion. The brains of Alzheimer’s patients have more aluminum than do normal brains. Some people, therefore, jump to the conclusion that there must be a link between aluminum exposure and Alzheimer’s disease. However, no one has proven such a link. Neither the World Health Organization nor the Environmental Protection Agency has issued any warning about discarding aluminum cookware. Furthermore, exposure to aluminum from such sources is minute, and most authorities feel that fear of aluminum cookware is not warranted.

DEAR DR. DONOHUE: I am 80. When I was 8, a big, stray dog jumped on me and scratched my upper lip. I was set up for a series of rabies shots. The doctor told me that the serum came from the spine of a raving-mad jackrabbit. Have times changed? – J.F.

ANSWER: Louis Pasteur, in the 19th century, made the first rabies vaccine from the spinal cord and brain of a rabies-infected rabbit. It was a medical milestone. For many years after, rabies vaccines were produced from the nervous systems of animals. They still are in some countries, but not in the United States or Canada.

Our vaccines are produced on cells grown in the laboratory or in fertilized eggs. Times have changed.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475.