Being overweight may not extend life after all

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Is it good to be a little bit overweight as you get older? That’s the message from a series of large, reputable studies (including one that made lots of news two years ago). The theory is that having a few extra pounds is good as you get old and frail, so that you don’t waste away to nothing if you get sick or break a hip. And the data seems to back up this theory.

But wait a sec. A new study in the Journal of the American Geriatrics Society from researchers at Loma Linda University in California (abstract here, press release here) shows the opposite:

[M]en over 75 with a body mass index (BMI) greater than 22.3 had a 3.7-year shorter life expectancy, and women over 75 with a BMI greater than 27.4 had a 2.1-year shorter life expectancy.

Why the contradiction? Actually, there are some very good reasons. The authors start by explaining some of the potential flaws with the earlier studies that seemed to show that being a bit chubby helped you live longer. The key problem is that the earlier studies measured weight just once, at the beginning of the study, and then waited to see how quickly the subjects died. But people who were skinny at the beginning of the study might have been skinny because they were already sick, possibly without having being diagnosed yet. If you’re skinny because you’re partway through a dramatic weight loss than ends with your death, that’s not quite the same as being thin with a long-term stable weight and healthy lifestyle.

The new Loma Linda study is able to address this problem, along with a few others. It relies on two large studies of Seventh-Day Adventists in California, one in 1960 and the second in 1976. Weight was measured on both occasions, and mortality was monitored until 1988. By looking only at the people whose weight stayed the same (within 5 kg) between 1960 and 1976, you get to study the effects of having extra weight, rather than the effects of gaining or losing it for whatever reason (both of which could have significant effects, but are separate questions).

Another quirk of the study is that Seventh-Day Adventists are forbidden to smoke. So to further eliminate confounders (e.g. people who were thin because they were puffing on cancer sticks), the analysis was able to completely exclude any past or present smokers, plus anyone with any history of coronary heart disease, stroke or cancer at baseline, and still have a total of 6,030 people aged 25-82. The point here is that we’re looking just at the effects of extra weight, stripped away of as many confounders as possible.

And sure enough, higher BMI corresponded to higher risk of death for 75- to 99-year-olds: above 22.3 was bad news for men, and above 27.4 was bad news for women. Obviously this study can’t tell us why there’s a difference between men and women, but the researchers suggest that women may benefit from having a bit more fat because that’s where estrogen is produced after menopause; too little estrogen may leave them susceptible to everything from hip fracture to insulin sensitivity problems. Here’s how the “hazards ratio” (basically your odds of dying relative to the healthiest group) looks as a function of BMI:

So what does this mean? Well, we’re not going to rewrite the medical textbooks based on a few Seventh-Day Adventists in California. It’s just one study, etc. etc. But given the methodological differences between this study and the previous ones, it certainly seems reasonable to wonder whether the apparent paradox of overweight people living longer is simply an artifact of the confounders this study managed to eliminate. I’m not saying you should panic if you’re overweight — I’m just saying you shouldn’t panic if you’re not.