Studies that bolster the obesity paradox have been faulted for not adequately controlling for the impact of smoking, and for misclassifying people who lost weight after their diagnosis.
Among smokers in the Harvard study, being too heavy increased the risk of death, although the pattern was not as pronounced as for nonsmokers.
The new research, published in last week's New England Journal of Medicine, won't settle the debate. Like previous analyses, it is based on observing a group of people, then deducing connections between certain characteristics and certain health outcomes.
The Harvard researchers mined two past studies - one that followed 8,900 nurses for 36 years and another that tracked 2,500 health professionals for 26 years.
"These are epidemiological studies," said endocrinologist Rexford Ahima, a diabetes researcher at the University of Pennsylvania. "They don't prove cause and effect. We are making inferences. My argument would be: Why don't we design experiments to study the direct effect of weight management in these groups? We could see how physically fit people are, how much muscle they have, not just how much they weigh. Those studies need to be done."
While obesity is clearly a modern public health crisis, one of the key measures of the crisis - the Body Mass Index - is a crude tool for assessing individual risks. It doesn't measure the proportion of fat to muscle, or whether fat is concentrated around the waist, a factor that raises the chance of heart disease.
In any case, over the last decade, studies of patients with heart failure, heart disease, end-stage renal disease, high blood pressure, and diabetes have found that normal weight, based on the BMI, correlates with shorter life.
One theory, Ahima said, is that chronically ill people tend to lose their appetite and become malnourished, "so if you have extra pounds, that's protective."
The Harvard researchers don't buy it.
"Given the relationship of overweight and obesity to . . . cardiovascular disease and cancer, the maintenance of a healthy body weight should remain the cornerstone of diabetes management," they said.