According to the United States government, nearly 7 out of 10 American adults weigh too much. (In 2010, the Centers for Disease Control and Prevention categorized 74 percent of men and 65 percent of women as either overweight or obese.)
But a new meta-analysis of the relationship between weight and mortality risk, involving nearly three million subjects from more than a dozen countries, illustrates just how exaggerated and unscientific that claim is.
The meta-analysis, published this week in The Journal of the American Medical Association, reviewed data from nearly a hundred large epidemiological studies to determine the correlation between body mass and mortality risk. The results ought to stun anyone who assumes the definition of “normal” or “healthy” weight used by our public health authorities is actually supported by the medical literature.
The study, by Katherine M. Flegal and her associates at the C.D.C. and the National Institutes of Health, found that all adults categorized as overweight and most of those categorized as obese have a lower mortality risk than so-called normal-weight individuals. If the government were to redefine normal weight as one that doesn’t increase the risk of death, then about 130 million of the 165 million American adults currently categorized as overweight and obese would be re-categorized as normal weight instead.
To put some flesh on these statistical bones, the study found a 6 percent decrease in mortality risk among people classified as overweight and a 5 percent decrease in people classified as Grade 1 obese, the lowest level (most of the obese fall in this category). This means that average-height women — 5 feet 4 inches — who weigh between 108 and 145 pounds have a higher mortality risk than average-height women who weigh between 146 and 203 pounds. For average-height men — 5 feet 10 inches — those who weigh between 129 and 174 pounds have a higher mortality risk than those who weigh between 175 and 243 pounds.
Now, if we were to employ the logic of our public health authorities, who treat any correlation between weight and increased mortality risk as a good reason to encourage people to try to modify their weight, we ought to be telling the 75 million American adults currently occupying the government’s “healthy weight” category to put on some pounds, so they can move into the lower risk, higher-weight categories.
In reality, of course, it would be nonsensical to tell so-called normal-weight people to try to become heavier to lower their mortality risk. Such advice would ignore the fact that tiny variations in relative risk in observational studies provide no scientific basis for concluding either that those variations are causally related to the variable in question or that this risk would change if the variable were altered.
This is because observational studies merely record statistical correlations: we don’t know to what extent, if any, the slight decrease in mortality risk observed among people defined as overweight or moderately obese is caused by higher weight or by other factors. Similarly, we don’t know whether the small increase in mortality risk observed among very obese people is caused by their weight or by any number of other factors, including lower socioeconomic status, dieting and the weight cycling that accompanies it, social discrimination and stigma, or stress.
In other words, there is no reason to believe that the trivial variations in mortality risk observed across an enormous weight range actually have anything to do with weight or that intentional weight gain or loss would affect that risk in a predictable way.
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