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:
The difference is barely significant
Overweight people were just 6 per cent 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.
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.
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 per cent higher likelihood of death). Focus on the big picture: avoiding morbid obesity.
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. 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.
Fat helps you survive some diseases
"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.
Fat protects you against injury
Many old people die from falls. Chubby people "have more padding to protect bones, should a patient take a tumble, reducing the risk of a life-endangering hip fracture," says the Los Angeles Times.
Overweight gets you more medical attention and intervention
Doctors' belief that fat signals a health risk makes them more likely to scrutinise heavier patients for disease symptoms or risk factors. Lots of evidence suggests doctors treat these patients more aggressively, thereby reducing mortality.
Medicine has made fat less harmful
"New pharmacological therapies and invasive treatments for existing disease may prolong survival," the JAMA editorial points out. These and other advances, particularly those that reduce cholesterol and blood pressure, "may account for the weakening of associations between obesity and mortality."
On one level, these explanations sound weak and weaselly. Dogmas, even in science, don't surrender easily to contrary evidence. Science, in its grudging way, does evolve.
The explanations offered today in defence of the fat-is-bad doctrine are actually modifications of it. (William Saletan/The Washington Post)