WASHINGTON — Fat is bad for you, right? That's what doctors tell us. But a review of nearly 100 studies, published this week in the Journal of the American Medical Association, confirms previous indications that the story is more complex. Being overweight or even mildly obese, as measured by body mass index, doesn't make you more likely to die than a person of normal weight. It makes you slightly less likely to die.
How can this be? Is fat good for you?
That's the wrong conclusion, according to epidemiologists. They insist that, in general, excess weight is dangerous. But then they have to explain why the mortality-to-weight correlation runs the wrong way. The result is a messy, collective scramble for excuses and explanations that can make the new data fit the old ideas. Here's what they've come up with:
1. The difference is barely significant. In the JAMA analysis, overweight people were just 6 percent less likely to die than normal-weight people. "It's probably only statistically significant because of the large number" of people in the combined data set, says one skeptic. Maybe. But if the correlation had gone in the other direction — showing a marginally higher death rate among the overweight — you wouldn't hear scientists arguing that what's statistically significant isn't really significant.
2. Death risk is the wrong standard. So what if fat doesn't correlate with mortality? It still correlates with many diseases, which may ultimately affect mortality. Some studies covered by the JAMA review tracked people for up to 15 years. Others tracked them for as few as five years. Anyone who made it to that point counted as a survivor, regardless of diabetes, heart disease, or other conditions that may have contributed to death after the study ended.