Then why are there these warnings about the dangers of excess weight? Last year researchers from the Centers for Disease Control and Prevention reported that people who are obese at age 45 are less likely to survive to age 65 than people at a healthy weight. A Harvard study, also published last year, showed that the risk of heart disease was more than twice as high among people with a body mass index over 30. Shouldn’t those findings alarm us?
A Studies like these find associations, not cause and effect. Unfortunately, there’s been a widespread rush to conclude that being fat causes illness. Neither of the studies you mentioned took physical activity into account, for example, yet it’s much more likely that being sedentary, not being fat, is the real problem. When researchers have looked at physical activity, the association between health and BMI gets much smaller. A 2008 study, done by Harvard researchers, looked into the effects of physical activity and body mass index on coronary heart disease and found that overweight women who walked more than four hours a week had a significantly lower risk of heart disease than those who didn’t exercise. This says to me that the most important risk factor isn’t being fat; it’s being sedentary.

What about type 2 diabetes? Its incidence is rising, and experts link it to excess weight.
A It is true that type 2 diabetes is more common in people who are heavier. However, researchers have been too quick to assume that being fat is the primary cause of insulin resistance, which in turn leads to type 2 diabetes. We know that insulin resistance causes people to gain weight. The bigger part of the story may be that weight gain is a consequence of insulin resistance rather than the other way around. Insulin resistance is related to many factors, such as genetics, what people eat and how much exercise they get. We’d be much more effective at addressing diabetes if we paid more attention to behavioral factors than to body size. There are millions of people who are defined as obese but are very healthy. One important study, an analysis of more than 5,000 participants in a national government sample, showed that 35 percent of obese women don’t have the constellation of health problems typically linked to higher weight. Specifically, they showed no more than one sign of these issues: high blood pressure, high triglycerides, low HDL [“good”] cholesterol, elevated glucose levels, insulin resistance and systemic inflammation. And there are some health concerns, such as osteoporosis, that are much less common in people categorized as overweight or obese.

Are you suggesting that being overweight or obese may actually be an advantage?
A There’s something researchers call the “obesity paradox”—that people who are heavier tend to survive longer than thinner people with the same disease. In one early study, researchers looked at patients with heart failure, assuming that obese people would fare worse than thin people. The opposite turned out to be true. Obese patients who suffered heart failure survived longer than nonobese people with the same history. If you look at people diagnosed with cardiovascular disease in general, those who are categorized as obese have greater longevity than people who are thin. The same holds true for chronic kidney disease, hypertension and even type 2 diabetes. Why isn’t entirely clear. But the findings call into question the widely held belief that being overweight or obese is always dangerous and suggest that it may be protective in some ways.