So, you've just resolved to lose weight this year, when up pops a story that makes you think twice. Indeed, the headline in the New York Times -- "Study Suggests Lower Mortality Risk for People Deemed to be Overweight" -- might well lead you to open another bag of chips. But, while it literally represents the study's findings, published in the Jan 2 issue of the Journal of the American Medical Association (JAMA), it is a bit misleading, as it might be construed to mean that risk is higher for people who are "normal" weight, and lower for everyone else. But that's not what the study found.
The researchers did a meta-analysis of 100 studies of the risk of mortality associated with BMI, or body mass index, an indicator of how much fat a person is carrying independent of his or her height. A 'meta-analysis' pools results from many different studies, each of which may be too small in size to be definitive, in order to have larger samples and gain greater statistical power. In this case, the total included about 3 million people. Of course, one must assume that the different study samples are similar enough in their risk characteristics for this kind of pooling to be legitimate.
Be that as it may, the authors in this case found that subjects with intermediate BMI's, who are considered overweight but not grossly obese, with a BMI between 30 and 35, had lower risk of all-cause mortality than those with "normal" BMI of 25-30, or grades 2 and 3 obesity, BMI over 35. According to the JAMA press release about the study:
The researchers found that the summary HRs indicated a 6 percent lower risk of death for overweight; a 18 percent higher risk of death for obesity (all grades); a 5 percent lower risk of death for grade 1 obesity; and a 29 percent increased risk of death for grades 2 and 3 obesity.Why? Well, the researchers have numerous possible explanations, again as quoted in the press release:
“Possible explanations have included earlier presentation of heavier patients, greater likelihood of receiving optimal medical treatment, cardioprotective metabolic effects of increased body fat, and benefits of higher metabolic reserves.”That is, in part, BMI is likely to be a confounder, standing in for ('correlated with') other unmeasured variables that are the actual reasons that mortality is lower among overweight and moderately obese people, such as that they go to the doctor more and receive more treatment than thinner people.
And, the story in the NYT quotes physicians saying that it's not BMI that counts anyway, but other health indicators such as cholesterol levels, blood pressure or blood glucose levels. If those are normal, a person doesn't need to worry about losing weight for the sake of their health -- but, they are more likely to be elevated in people who are overweight. Illness can cause weight loss, too, so the population of people with "normal" BMI may well include some who are seriously ill. And, the location of the fat, whether in the belly or superficial, seems to be important too, and it varies from person to person.
The researchers do suggest that body fat might have protective benefits as well, and that may be so. But, the upshot of this study seems to us to be that the association of body weight with mortality risk is not straightforward. It's possible that in some people body fat is even protective, and more is better -- up to a point. And, well, if you know all sorts of other things, including whether the person already has symptoms or problems, then BMI becomes a useful predictive measure. In other words....despite expensive and extensive decades of research and countless news stories, and countless counselors, fad diets, exercise and reducing programs, and all the magazine and infomercial pressures on body weight, well, we're not so far from square one. Except, is it reasonable to say, for a lot of researchers who have made a lot of hay out of this for decades?
In the end, the decision as to whether or not you keep your resolution to lose weight should not be decided on the basis of this one study. We'll have more to say about this tomorrow.