The New York Times reports a new study in JAMA on the mortality risk associated with different BMI classes. The study found that obesity and underweight classes faced a higher mortality risk, but that overweight people were just as well off as normal weight.
In our analysis, we did not find overweight (BMI 25 to <30) to be associated with increased mortality in any of the 3 surveys. Our results are similar to those of a previous analysis of NHANES I and II data that found little effect of overweight on life expectancy. Our finding is consistent with other results reported in the literature, although methodologic differences often preclude exact comparisons. In many studies, a plot of the relative risk of mortality against BMI follows a U-shaped curve, with the minimum mortality close to a BMI of 25; mortality increases both as BMI increases above 25 and as BMI decreases below 25, which may explain why risks in the overweight category are not much different from those in the normal weight category (Flegal et al. 2005:1866).
The authors speculate that there may have been a recent reduction in mortality associated with obesity and overweight because of increasingly successful treatment of chronic high blood pressure and high cholesterol. Likewise people with extra weight may have an advantage in maintaining bone density and muscle strength into old age compared to normal or underweight people.
Flegal KM, Graubard BI, Williamson DF, Gail MH. 2005. Excess deaths associated with underweight, overweight, and obesity. JAMA 293:1861-1867. JAMA Online