Will Extra Pounds Give You Extra Years? Maybe

For years, health researchers and advocates have been debating whether it's possible to be both overweight and healthy. A new study comes down on the affirmative side, finding that slightly overweight adults may live longer than normal-weight peers whose weight fluctuates. The research, published in the American Journal of Epidemiology , tracked more than 10,000 middle-aged adults between 1992 and 2008. During this period, those who started overweight and stayed overweight were less likely to die than obese subjects or those whose body mass indexes went up and down.

Body mass index is a ratio derived from weight and height that's used to designate whether someone is normal weight, overweight, underweight or obese. BMIs above 18.5 and under 25 are considered "normal," while BMIs between 25 and 30 are considered overweight and BMIs above 30 are considered obese.

In the study, people who maintained a stable overweight BMI had lower death rates than those in several other weight categories. "The results were robust" after controlling for demographic and socioeconomic status, smoking, mobility limitations and a wide range of chronic illnesses, the researchers wrote.

Headlines have been proclaiming that "gaining weight helps you live longer" and you should "ditch the diet" because "a few extra pounds" can extend your life. But this misconstrues the study, which should come with a couple important caveats.

1. The research did not include normal-weight individuals who maintained a mostly-consistent weight over the 16-year period. It also did not include overweight or obese individuals who lost weight during this time. Rather, researchers compared people who maintained overweight status to a) people who maintained obese status, b) obese and morbidly obese adults who gained weight during the study and c) normal weight adults who gained or lost weight during the study. So the study actually tells us nothing about whether its better to be overweight than to be a normal weight and maintain it or to be overweight and lose weight.

2. The researchers controlled for "a wide range of chronic illnesses." This essentially means that they factored out the effects of extra weight on the development of chronic disease. Yes, it is totally possible to be overweight with a clean bill of health. But since countless studies have shown extra weight to be a risk factor for chronic conditions (from heart disease to cancer to arthritis), excluding these illnesses from conclusions about weight status and health seems to present, at best, an incomplete picture.

The real takeaway here seems to be that "BMI trajectories were more predictive of mortality risk than was static BMI status." In other words, weight gain and loss was more closely correlated to risk, no matter what weight you started out at, then simply starting out middle-age as normal or overweight. The researchers didn't speculate as to why, but there's a lot of evidence that yo-yo dieting can be bad for the body. Maintaining a stable weight (even if it's technically 'overweight') avoids putting your body through this stress, and perhaps reflects a more consistent pattern of healthy eating and behavior than frequent weight fluctuations.

And lead researcher Hui Zheng, an Ohio State University sociology professor, points out that the results definitely don't apply to younger people. “Our other research suggests that the negative effect of obesity on health is greater for young people than it is for older people, so young people especially shouldn’t think that being overweight is harmless,” he said.