BMI Blues

Q. According to the BMI chart, I’m officially overweight with a score of 27. But I’m only 200 pounds at 6 feet tall.  I run five times a week, do calisthenics three times a week and feel great. I would need to drop 15 more pounds just to be considered normal weight. Is there something off about this system?

A. Scientists need ways to measure what they study in order to make objective comparisons. The Body Mass Index  is a tool to estimate how fat a person is. A person with a BMI of 25 or above is considered overweight; a BMI of 30 and above is considered obese.

The average person tends to jump on a scale and use their weight as a gauge of their fatness. BMI tends to be more precise than just considering body weight alone because a person’s height makes a difference. Being 185 pounds, for example doesn’t say much unless you know more about the person’s shape. A person who is 5-foot-3 and 185 pounds, for example, is overweight and probably has health risks associated with their extra body fat. But a person who is 6-foot-3 and 185 may be underweight and therefore be less prone to obesity-related health problems. 

The best way to gauge a person’s level of fatness is to measure it directly, such as when using lab equipment such as underwater scales, dual-energy X-ray absorptiometry (DXA) and other methods. But these methods require pricey equipment and experienced lab technicians. Plus, some studies are so large, involving thousands of subjects, that it’s not practical to obtain this information.

So researchers who have studied obesity or the health risks associated with being fat often use the BMI as an indicator of excess body fat because it’s convenient and gives a better picture than just taking body weight alone.

Because the only data that is needed to determine a person’s BMI is their height and weight, and this can be measured directly—or even relayed verbally by a study participant in a telephone interview—it’s a measure that’s often used, especially in large-scale, epidemiological studies. Links can easily be made between the BMI and eating habits or health risks of, say, 50,000 people who are surveyed.

The BMI works well for research purposes, but doesn’t necessarily translate precisely to the individual. Unfortunately, it tends to convey that people that exercise regularly, for example, are overweight, when they are not actually overfat.  A fit person tends to have more muscle, so their body weight is a reflection of body fat as well as muscle and other lean tissue. 

Since the problem with being overfat is that health risks are increased, a BMI in the overweight range is probably not a negative indicator for a fit person. Regular exercise, low body fat and increased muscle mass are all factors that tend to outweigh any health risks suggested by a higher BMI.

In other words, if you know you are fit, not fat, you don’t need to worry about your BMI. But the Centers for Disease Control (website) points out that those with a BMI in the obese range (higher than 30) do tend to be overfat, even if they are fit. So, this may increase disease risk—although research does show that being fit and fat is still healthier than being thin and unfit.

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Martica is a Manhattan-based exercise physiologist and nutritionist and an award-winning fitness instructor. She has written for a variety of publications including Self , Health , Prevention , The New York Times and others. Martica is the author of seven books, including her latest, - Cross-Training for Dummies . (Read her full bio.)