I'll preface this post by noting that one should always pay attention to the incentives operating behind any particular piece of news as it makes its way through the cultural landscape. In this case it is news about weight and mortality risk, and the incentive that always seems close to mind here is that, on balance, overweight people like to be told that they are not risking their health and longevity by being overweight. No one enjoys bad news, and bad news about general health always has that implied nagging edge of personal criticism. Good news is so much more welcome.
Thus there has been more press attention than usual given to the latest big metastudy on weight and mortality risk, as it claims that being slightly overweight is better than being of normal weight when it comes to mortality rates. Being obese still shows up as a bad lifestyle choice. The paper is open access and very readable, but here's the press release:
The researchers found [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. The authors note that the finding that grade 1 obesity was not associated with higher mortality suggests that that the excess mortality in obesity may predominantly be due to elevated mortality at higher BMI levels.
A summary of some of the analysis that goes on beyond presentation of the data can be found in a recent Slate article:
How can this be? Is fat good for you? That's the wrong conclusion, according to epidemiologists. They insist that, in general, excess weight is dangerous. But then they have to explain why the mortality-to-weight correlation runs the wrong way. The result is a messy, collective scramble for excuses and explanations that can make the new data fit the old ideas.
I can roll out some of the obvious theorizing myself without trying too hard: average weight is increasing and thus more of the people of normal weight in fact have something wrong with them that causes weight loss and higher mortality; wealth effects and corresponding access to medical services have some relation to weight and are thus distorting the picture; overweight people have higher medical expenditures and are getting value for that expense; and so forth. Many versions of this sort of pseudo-explanation are doing the rounds, including the standard objections to working with BMI as a measure in the first place, as it isn't necessarily a great proxy for visceral fat mass.
Beyond the fact that people want to hear that they are just fine the way they are, the issue here is a collision of large masses of data. On the one side there is a huge, mountainous pile of evidence to show that excess visceral fat is very bad for you. Even a modest amount above a normal weight raises the risk of later age-related disease and shortens your life expectancy. Calorie restriction in humans, which necessitates leanness, is shown to be very, very positive for all measures of health. It goes on.
On the other hand we have this meta-study and other similar items from past years to claim that irrespective of all this data showing that fat is bad for health, people who are slightly overweight still live longer. There are no doubt some researchers out there who see this as an opportunity: how to reconcile these two collections of research. If there is a consensus among scientists with an interest in the field, I'd have to say it looks like one that leans towards the evidence that fat is bad, and therefore something has yet to be explained in the structure or underlying threads of causation in these mortality rate studies.