Willett—the Harvard nutrition professor who strongly opposes the notion of an obesity paradox—largely agrees with the new findings and the idea that birth period shapes obesity-related mortality risk. “This is likely to become even more important as the current cohort of adolescents and young adults gets older, as it is true that the adverse effects of overweight are cumulative over a lifetime,” he says. Willett thinks, however, that there are many other spurious causes for the obesity paradox, too, including the fact that as people get sicker, they often lose weight, a trend that associates slimness with poor health even though in these cases slimness hasn’t caused poor health. (Masters attempted to control for this so-called “reverse causation” by excluding people from his analysis who had a BMI under 18.5.) Other researchers try to account for this effect by excluding sick people from their analyses: In a study published online in August, for instance, Harvard nutrition researcher Chandra Jackson and her colleagues analyzed data from the general U.S. population and found that mortality risk appeared to decrease with increasing BMI among people with diagnosed diabetes; when they excluded those with diabetes, excess weight increased death risk. “It is imperative to address the methodological concerns that could lead to misleading conclusions,” says Jackson, who agrees that the obesity paradox is likely specious and that birth period could have important confounding effects, too.
Others are not convinced by the new findings. Flegal, the lead author of the CDC meta-analysis, says she does not think that her findings were biased by cohort effects, in part because many of the studies included in her analysis did not pool mortality estimates from wildly different age groups. She also says there’s little evidence that her findings were affected by selection bias. Neil Mehta, a professor of global health at Emory University who also studies the impact of obesity on mortality, says that the statistical method used by Masters and his colleagues “is questionable,” in part because it does not properly separate certain interrelated variables. The new findings ultimately inflate the risks of obesity in old age because older people, he says, suffer from many health problems and tend to die of a variety of causes. Yet Mehta agrees that “obesity duration matters a lot to mortality—the longer you are obese the higher the risk of dying,” a notion that his research supports, too.
Eric Reither, a sociologist at Utah State University and a co-author of the new studies with Masters, notes that their research is not necessarily at odds with Flegal’s work. “They also find that obesity contributes to premature mortality,” he says. “I would say that we build upon their research.” And although the new studies do little to settle the overall obesity paradox controversy, they do suggest that the impact of weight may be importantly dependent on age. “When people talk about the effect of [being] overweight or obesity on mortality risk, they often think it exists in a vacuum—that to be overweight at age 20 is to be overweight at age 60 is to be overweight at age 75,” Masters says. Instead of simply focusing on how age might influence the impact of obesity, he explains, scientists should consider that obesity might fundamentally shape the way people age—even if, right now, there’s no consensus on how it does so or even the nature of its influence.