Which Matters More, Muscle or Fat?

Researchers at UCLA, America, recently decided to explore a stubborn riddle. Why is that some studies suggest that being a little bit heavier than “ideal” can translate into lower risk of death from heart disease and other factors, an observation often called the “obesity paradox”?

Their hypothesis was that the effect may be due to muscle, not fat. By some estimates, muscle is responsible for clearing about 85 per cent of the blood sugar from your circulation, so there’s reason to believe that having lots of healthy, active muscle could have far-reaching health benefits.

Previous studies have offered some support for this hypothesis, but the new study in the American Journal of Cardiology used a more accurate method for estimating body composition. They used data from 6,451 participants in the National Health and Nutrition Examination Survey who already had heart disease and who had undergone “dual x-ray absorptiometry” (DXA) scans to assess muscle and fat, and then tracked which ones died between 1999 and 2006.

Using the DXA data, they assessed fat around the trunk and muscle on the arms and legs. For each measure, they split the subjects into two groups based on whether they were above or below the median value for fat or muscle. That produced a total of four groups: low muscle/low fat, low muscle/high fat, high muscle/low fat, and high muscle/high fat.

The results showed that one group—the high muscle/low fat group—fared significantly better than the other three. Compared to the low muscle/low fat group, the high muscle/low fat group had a “hazard ratio” of 0.32 for heart disease-related deaths and 0.38 for death from any cause, which is roughly equivalent to saying that members of that group were only about a third as likely to die during the study period.

In other words, as Steve Martin’s grandmother might say, it’s better to have some muscle than to not.

That’s not a huge surprise, but it’s useful to confirm. And it also supports the idea that muscle mass could be what’s behind the obesity paradox, rather than fat itself having some mysterious protective properties. Two-thirds of people in the high muscle/low fat group would be classified as overweight (a body mass index, or BMI, of 25-30), and yet they had the best survival numbers. It’s okay to be heavy, in other words, if the extra weight is muscle.

In truth, I think you’d have trouble finding anyone who really argues the opposite—that a BMI of 26 in someone with lots of muscle and not much belly fat is a cause for alarm. As the authors note, the other big example of a factor that trumps BMI is cardiorespiratory fitness: If you’ve got a decent VO2max, that’s probably more important than whether your BMI is high or low.

The crucial point, of course, is that it’s not an either/or question. Ideally you’d like to have lots of muscle and not too much belly fat. Fortunately, you can choose—or at least aspire to—both.

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