If you think weighing in at the "right" number means you're healthy, think again, warns a new study: if you carry fat around your middle, you may actually be at higher risk of mortality than your obese peers.
Researchers have known for 20 or so years that belly fat, or central obesity, is linked with adverse health outcomes (cardiovascular disease, type 2 diabetes and breast cancer, to name a few). But this is the first study to show that central obesity appears to be riskier than obesity.
"I was expecting it to be bad, but not worse," said lead author, Dr. Francisco Lopez-Jimenez.
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The researchers used data from 15,184 participants in the Third National Health and Nutrition Examination Survey to analyze the mortality risks for normal-weight adults with central obesity (defined by their waist-to-hip ratios) compared to risks for those who were obese according to their BMI.
And those normal weight people with larger-than-average bellies were at double the mortality risk as those who were measured as overweight or obese by BMI.
While the World Health Organization has recognized the central obesity problem , and some countries such as Canada routinely screen for it, it's largely been ignored in the United States.
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The attitude among many in the U.S., the researchers said, appears to be that obesity is a big enough problem, and there are "less and less people with normal weight anyway." Obesity experts have told Francisco, "If someone who has central obesity is obese anyway, why add another number to that?"
Major health-specific U.S. organizations, including the American Heart Association, the American College of Cardiology and The Obesity Society, do not recommend measuring waist-to-hip ratios because there's not enough data, Dr. Paul Poirier of Laval University in Canada, writes in an editorial accompanying the study published in the Annals of Internal Medicine.
Now that there is a U.S.-based study showing that risks may be greater for normal-weight people, some expect those attitudes to change. Here's how that would help patients, Poirier says:
"I will describe to you my brother," Poirier says. "He's 250 pounds and 6 feet 6 inches tall."
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"Then I say to you, he's a pro football player for the NFL," he says. "In your mind, he went from being an obese man to a football player in good shape. I just changed your perspective by giving you more information."
Just as doctors refine what types of cholesterol patients have to better assess their condition and treatment options, waist-to-hip ratios can help "tease out which patients are at risk," Poirier said.
And that percentage may be significant: the number of Americans with central obesity may be as high as 15 percent, and of those, up to a third may be at a normal weight, the study notes.
Researchers aren't sure exactly what causes central obesity, although some combination of genes and environment is a likely bet, Francisco says. An educated guess as to why the mix of central obesity and normal weight appears to come with a heightened risk is that subcutaneous fat has been linked to some protective effects.
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A person of normal weight with central obesity often has visceral or abdominal fat, skinny arms, legs and buttocks with very little muscle mass, Francisco says. That potentially protective fat is missing.
As for environment, certain segments of the population seem to be at higher risk of developing normal weight central obesity: smokers, for example, and people taking HIV medication.
Once a person is identified with central obesity, however, lifestyle changes can work well to reduce the weight-to-hip ratio. Visceral fat tends to respond well to exercise, which should include resistance training for legs, arms and core, Francisco says, and diet, which should be high in fruits and vegetables and low in sugar and refined carbohydrates.
"For me it's been so clear for so many years," Poirier said. "Now we've got the U.S.-based study with U.S. data from U.S.-based researchers. I do think it has the potential to change the practice."