Is Fat Good?
Epidemiologists struggle to explain a study that challenges a core belief: Fat will kill you.
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.
3. Overweight is too close to obese. We used to think that being overweight gave you a higher risk of death. Then we collected data that suggested overweight might be OK but obesity was still deadly. Now we have data that suggest even mild obesity may be OK. To account for this, weight-control advocates are adding another link to their long-standing sermon: If you’re overweight, you’re on the road not just to mild obesity but eventually to morbid obesity, which the JAMA analysis validates as a huge mortality risk (with a 29 percent higher likelihood of death). So don’t gloat that you’re a measly 6 percent healthier than thinner people. Focus on the big picture: avoiding morbid obesity.
4. The dangers of being underweight hide the dangers of being overweight. A JAMA editorial notes that people in the thinner half of the “normal” BMI range have a higher mortality rate than those in the plumper half. This thinner subset inflates the normal-weight group’s mortality rate, which makes the mortality rate among overweight and obese people look good by comparison. The solution is to shift the whole scale to the right, so that these thin people are recognized as underweight, while people presently labeled overweight are redefined as normal weight. Once we’ve completed this reallocation, it will be clear once again that overweight people—now defined to include many whose BMI would previously have put them in the obese category—are at higher risk of death than the newly reclassified normal-weight people are. The value of weight control will be reaffirmed, but the thresholds will have changed.
5. Some kinds of fat are worse than others. At the moment, scientists seem to agree that while belly fat is bad for you, butt and thigh fat might be safe or even beneficial. So instead of focusing on BMI, we should measure your waist-to-hip ratio, body-fat percentage, blood pressure, blood lipids, glucose, and cardio-respiratory fitness.
6. Fat helps you survive some diseases. The JAMA editorial points out that among people suffering “a wasting disease, heart disease, diabetes, renal dialysis, or older age,” higher BMI correlates with a lower mortality rate. “Even in the absence of chronic disease, small excess amounts of adipose tissue may provide needed energy reserves during acute catabolic illnesses.” That doesn’t mean fat makes you healthy. It means that once you’re unhealthy, fat might keep you alive, at least for a while.
7. Fat protects you against injury. Many old people die from falls. Chubbier people “have more padding to protect the bones should a patient take a tumble, lowering the risk of a life-endangering hip fracture,” notes the Los Angeles Times.
Will Saletan covers science, technology, and politics for Slate and says a lot of things that get him in trouble.