If Obesity Is a Disease, Why Are So Many Obese People Healthy?

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The decision of the American Medical Association (AMA) to classify obesity as a disease is great news for the pharmaceutical industry, as it is likely to increase pressure on the Food and Drug Administration to approve more weight-loss drugs and increase the odds that insurance companies will reimburse their cost. But it is deeply misleading.

Treating obesity as a disease implies that moving into the category of obesity, which for adults means moving from a body-mass index (BMI) of 29 to a BMI of 30, is equivalent to contracting a disease. But that is simply not the case.

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Yes, there are certain health risks associated with having an elevated BMI, such as type 2 diabetes and heart disease. More broadly, a higher BMI is associated with a greater risk of cardiometabolic abnormalities, as measured by blood pressure, triglycerides, cholesterol, glucose, insulin resistance and inflammation. Nonetheless, almost one quarter of “normal weight” people also have metabolic abnormalities, and more than half of “overweight” and almost one third of “obese” people have normal profiles, according to a 2008 study. That’s 16 million normal-weight Americans who have metabolic abnormalities and 20 million obese (or 56 million overweight and obese) Americans who have no such abnormalities.

One explanation for this discrepancy is that physical fitness and/or nutrition — rather than weight per se — may be what really matters. Several studies have shown that physically fit “obese” individuals have lower incidence of heart disease and mortality from all causes than do sedentary people of “normal” weight. A recent clinical trial published in the New England Journal of Medicine showed that adopting a Mediterranean diet reduced cardiovascular risk independent of weight loss.

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Some assume that the problem lies with BMI as a measure, which does not distinguish between fat, muscle and bone. While BMI is indeed a flawed measure, it is not clear that there are better ones. A 2009 study, using the National Health and Nutrition Examination Survey, estimated excess deaths for people of standard BMI levels as well as for those with comparable levels of percentage body fat, waist circumference, hip and arm circumferences, waist-to-hip ratio, the sum of four skinfold thicknesses and waist-to-stature ratio. They found no systematic differences between BMI and other variables. In other words, it is not just that BMI is a poor measure of obesity but that obesity is a poor predictor of health.

Some hope that designating obesity as a disease will remove the stigma associated with it, and obese people will no longer be blamed for their condition. Yet already it is being called the “fork to mouth” disease, and the disease categorization may reinforce blame by raising the stakes. If obesity is a disease, parents of fat children may not merely be silently judged as bad parents but also accused of neglect and child endangerment.

If the AMA’s goal is to address the serious diseases of type 2 diabetes and heart disease, it would be more productive and accurate for the association to urge doctors to focus on cardiometabolic risk, recognizing that there are both metabolically healthy and metabolically unhealthy individuals in all categories of weight. Rather than promote weight loss per se, doctors should instead encourage their patients of all sizes to incorporate physical activity and a balanced diet into their lives.