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Why You Should Ignore Body Mass Index

At one time, a low body mass index, was the mark of a healthy person. But nowadays we know there's more to our bodies than what we read on a chart.

A few years ago, Katherine Flegal—a researcher on the epidemiology of obesity—and her colleagues, conducted a scientific review of past studies on body mass index that correlated it with the risk of dying of various diseases. Their “meta-analysis” was published in the January 2013 edition of JAMA (the Journal of the American Medical Association). The reason it made news was that Flegal’s study reported that men and women who were classified as overweight (but not obese) by official BMI standards tended to live longer than people who were classified as normal weight.

What’s Wrong With This Picture?

Many strong opinions about this surprising finding were expressed. A number of Flegal’s fellow obesity experts dismissed it as an erroneous result of faulty methodology. So-called fat advocates hailed it as final proof of what they had known all along. As for me, when I read about the paper I merely felt glad that I had stopped giving credence to the BMI scale long before.

BMI, in case you don’t know, is a number that represents the relationship between a person’s height and weight. Since taller people tend to weigh more, BMI was created as a tool that people could use to determine whether they were too heavy for their height. The problem with BMI is that while it effectively neutralizes the influence of height on body weight, Body mass index makes no distinction between body fat and lean body mass. For example, a lean football player with a BMI of 25.5 and a couch potato with a huge beer belly and a BMI of 25.5 are both classified as overweight by the BMI scale, which defies common sense.

BMI vs. Body Composition

By ignoring body composition, BMI sacrifices a lot of predictive power in relation to health outcomes. In recent years, medical researchers have performed a number of studies comparing the effect of BMI versus that of body composition (or body fat percentage) on the risk for various diseases. The conclusion is always the same. While the effect of higher BMI on the risk for lifestyle diseases such as heart disease than normal-weight men and women is muddled, the connection between body fat percentage and disease risk is much stronger. In fact, heavier individuals with a low body fat percentage tend to be healthier and to live longer than skinnier individuals with a higher body fat percentage. In other words, body composition is a far better predictor of health and longevity than BMI. Doctors recently coined the term “normal weight obese” to categorize men and women who fall within the normal body weight range but have more than 30 percent body fat. Studies have found that normal weight obese individuals have the same levels of circulating inflammation markers—a major heart disease risk factor—as those who are technically obese.

Muscle vs. Fat

A second reason why lean men and women are healthier than skinny people with more fat hidden inside them has to do with muscle. Recent medical research has shown that muscle mass is as beneficial to health as excess body fat is damaging to health. Having a little extra muscle has been shown to increase metabolism, reduce insulin resistance and diabetes risk, increase bone density and lower the risk of osteoporosis, and more. Having a little extra muscle even increases longevity. A number of studies have found that, among elderly populations, those with the most muscle strength live the longest.

In addition to telling members of the general population more about their health than BMI does, body fat percentage tells endurance athletes more about their fitness than BMI does. Research has demonstrated that endurance athletes perform best at a body fat percentage that is close to the minimum they can attain through focused training and healthy eating. Regularly monitoring body fat percentage makes a lot of sense for endurance athletes. A 2 percent drop in body fat could easily correlate to a big leap forward in performance, even if your body weight—hence BMI—stays the same.

So if you aren’t yet tracking your body fat percentage, start. And if you are paying attention to headlines about BMI, stop!

Racing Weight is a proven weight-management program designed specifically for endurance athletes. Revealing the latest research and drawing from the best practices of elite athletes, coach and nutritionist Matt Fitzgerald lays out six easy steps to help cyclists, triathletes, and runners lose weight without harming their training.