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Your B.M.I. doesn't tell the whole story, experts say

NYT: Calculated by dividing one’s weight in kilograms by the square of one’s height in meters, body mass index, or B.M.I., doesn’t differentiate between fatty and lean tissue.
/ Source: The New York Times

A frequent question among people of a certain age, including yours truly, is “Why, when I weigh the same as or less than I did when I was younger, does my waist keep getting bigger?” Phrased another way, the question could be “Why, when my body mass index has not changed, am I fatter than I used to be?”

The simple answer is that the index, usually called B.M.I. for short, is a crude measure of fatness in individuals. Calculated by dividing one’s weight in kilograms by the square of one’s height in meters, it doesn’t differentiate between fatty and lean tissue.

“The B.M.I. tables are excellent for identifying obesity and body fat in large populations, but they are far less reliable for determining fatness in individuals,” explained Dr. Carl Lavie, a cardiologist at the Ochsner Heart and Vascular Institute in New Orleans.

Fat takes up about four times the space of muscle tissue, for example, so it is quite possible to look and feel fatter even if your height and weight remain the same. This is particularly common among women past 50 and men past 60, and the results are likely to show around the middle.

For children and the elderly, body mass values can be especially misleading because the relationship of lean body mass to height changes as they get older.

B.M.I. charts pop up all over the place, in popular publications, exercise facilities and doctors’ offices. The charts are widely used by doctors to determine if their patients are underweight, normal weight, overweight or obese. Thus, a body mass of less than 18.5 is considered underweight; 18.5 to 24.9 is considered a healthy weight; 25 to 29.9 is overweight; 30 to 39.9 is obese; and 40 or more is morbidly obese.

If you fall into the “healthy weight” or “underweight” range, you can easily be lulled into a false sense of security. But thinness is not necessarily healthy — recall the 97-pound weakling from the Charles Atlas ads of yore. A low B.M.I. could be indicative of malnutrition, anorexia, cancer or a wasting disease. On the other hand, if you are an athlete or body builder, your B.M.I. could mistakenly put you in the range for overweight or obese.

Degree of body fatness is a better way than body mass to classify individuals. Both the World Health Organization and the National Institutes of Health define obesity as more than 25 percent body fat in men and more than 35 percent body fat in women. So “a woman who is 5 feet 5 inches tall and weighs 120 to 125 pounds could be quite fat,” Dr. Lavie told me, “even though her weight and B.M.I. seem O.K.”

Among Americans in general, he said, “a six-foot, 250-pound man will be obese, but if he were an N.F.L. lineman of 6-foot-3 weighing 280 pounds, he might be solid muscle with only 2 percent body fat.”

Of course, most Americans with a body mass index in the overweight or obese range carry around too much fat in relation to muscle. And a study that followed 527,265 American men and women ages 50 to 71 in 1995-96 found that those rated overweight based on a body mass reading of 25 to 29.9 were 20 to 40 percent more likely to die within 10 years, and those rated obese, at 30 or higher, were two to three times as likely to die within a decade as those who had a lower reading in midlife.

In an editorial in the July issue of Mayo Clinic Proceedings, Dr. Lavie and colleagues discussed what has been termed the “obesity paradox” among patients with heart failure. The paradox refers to the repeated finding that while overweight people are more prone to heart failure, patients with heart failure have lower mortality rates if they are obese. The reason for this paradox is far from clear, though Dr. Lavie suggested that one explanation could be that once people become ill, having more bodily “reserve” could be to their advantage.

The editorial was prepared in response to a report in the same journal by Antigone Oreopoulos of the University of Alberta and her colleagues. The authors compared the B.M.I. of 140 heart failure patients with a more accurate, though more involved, measure of fat and lean body mass using a DEXA scan (DEXA stands for dual energy X-ray absorptiometry). They found that B.M.I. value alone misclassified the degree of body fatness in 41 percent of the patients.

Their conclusion: Having more lean tissue and less fat may more accurately predict a patient’s survival chances. Thus, among patients with heart failure, these authors stated, “body mass index may not be a good indicator of adiposity.”

Dr. Lavie and coauthors wrote, “Although B.M.I. is the most common method to define overweightness and obesity in both epidemiological studies and major clinical trials, clearly this method does not necessarily reflect true body fatness, and B.M.I./body fatness may differ considerably among people of different age, race and sex.”

A more reliable, but still relatively simple, assessment of fatness would rely on a skin-fold score based on measurements taken with a caliper at several areas (in men, the thigh, midchest and abdomen, and in women, the thigh, triceps and area above the hip bone) that reflects the amount of fat under the skin.

Or, since abdominal fat is more hazardous, simply take a tape measure around the widest part of the abdomen and another at the hips and calculate the waist-to-hip ratio. For men it should be no higher than 0.90, and for women no higher than 0.83.

An oversize abdomen is symptomatic of too much metabolically active visceral fat, which increases the risk of heart attack and premature death. If just waist measurements are used, Dr. Lavie said, a man’s waist should be less than 40 inches and a woman’s less than 35.

Exercise is the best way to minimize an age-related rise in body fat, the doctor said. Aerobic exercise, though important at all ages, is not enough. You must also do weight training to build and maintain muscle. And since the body’s production of testosterone, the hormone that favors muscle-building, diminishes with age in both men and women, you may have to increase the amount of strengthening exercises as you get older just to stay in place.

This article, "Weight Index Doesn't Tell the Whole Truth," first appeared in The New York Times.