Researchers are nurturing a growing suspicion that body mass index, the height-weight calculation that distinguishes those with "normal healthy weight" from the overweight and obese, is not the whole picture when it comes to telling who is healthy and who is not. Two new studies drive that point home and underscore that BMI offers an incomplete picture of an individual's health.
Fitness matters, as does fatness. And the BMI is an imperfect measure of both.
In one study published Monday, researchers found that in a group of more than 1.5 million Swedish military recruits, men who had poor physical fitness at age 18 were three times more likely to develop Type 2
Muscle strength and, especially, aerobic capacity of males at age 18 were highly predictive of developing Type 2 diabetes in their 50s or 60s, the authors of the study found. Even men with BMIs that pegged them as "normal healthy weight" in their 50s or 60s were far more likely to develop diabetes if they had shown poor level aerobic conditioning and muscle strength at 18.
In a second study that tracked almost 55,000 Canadian adults, middle-aged and older people with low BMIs and those who carried a high percentage of body fat were most likely to die during a follow-up period of roughly seven years. All but 5,000 of the participants in the Canadian study were women.
The average woman with a BMI of 20 — the low end of "normal, healthy weight" — was even more likely to die of any cause in the follow-up period than was the average woman with a BMI of 40 — considered "morbidly obese." And women with just 20% body fat (as measured by a precise gauge of bone density) were just as likely to die of any cause during the follow-up period as were women with 45% body fat.
On average, women having a BMI between roughly 26 (overweight) and 35 (well into the obese category) and body-fat composition between 32% and 38% were least likely to die of any cause during the study.
Both studies were published Monday in the Annals of Internal Medicine.
Both articles are likely to help galvanize general-care physicians, who are expected to play a growing role in treating obesity, to recognize that in assessing a risk of Type 2 diabetes and heart disease, it's not enough to know a patient's BMI.
Patients who fall into the "normal healthy weight" category are far from healthy if they're flabby and out of shape, the studies suggest. And patients who are overweight or obese but who have a past and/or present record of physical fitness may be at less dire risk than has been warned.
The new studies suggest that these caveats about BMI are especially true for people as they age beyond their 50s and enter seniority, said Mayo Clinic cardiologist Francisco Lopez-Jimenez, who researches obesity's health effects. In the Canadian study and others that have raised what's called "the obesity paradox," Lopez-Jimenez said it's possible that older people who carry a few extra pounds are protected by having a reserve of excess weight they can afford to lose during an illness.
As the Swedish study suggested, when much of that extra weight is lean muscle mass and not fat, it's less likely to disrupt metabolism and increase inflammation — factors that over time can boost cardiovascular risk.
"At older ages I think we may really need to rethink what's a desirable weight," said Lopez-Jimenez. "If it turns out that BMI between 25 and 30 [the BMI range that defines overweight] might actually be normal and protective in people this age, then we should not be labeling these people unhealthy."
For physicians who counsel obese older adults, said Lopez-Jimenez, the new studies also suggest that a focus on fitness might be the best use of limited time and effort.
"It's much easier to make a person fit than to make a person lose 40 pounds," said Lopez-Jimenez.
Although obesity prevention and better nutrition remain societal imperatives, he said, physicians might do the most good by counseling obese patients to focus on improving their strength and aerobic fitness, whittling their waists to reduce cardiovascular risks and even measuring body composition so they can track changes in body composition that convert some fat to lean mass.
"Maybe talk about that factor more than fat and fat alone," said Lopez-Jimenez.
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