The reaction of America’s leading “obesity” experts to the latest study on the issue demonstrates yet again that our current definition of the word “overweight” makes no sense. Walter Willett of the Harvard School of Public Health’s fumed that the new findings were “rubbish.” His colleague JoAnn Manson found the study, authored by Katherine Flegal and others, and published in the prestigious Journal of the American Medical Association, “very puzzling.” After all, for “overweight” people to be healthier than “healthy weight” people just doesn’t seem logically or linguistically possible.

How did we get into this mess? First, the relevant definitions: According to public health authorities in America and around the world, people are “overweight” if they have a body mass index between 25 and 30 (for a 5’4” woman this is between 145 and 173 pounds; a 5’10” man fits the category if he weighs between 174 and 208 pounds).

A decade ago, when I began to study the relationship between weight and health, I was struck by the almost total lack of medical justification for labeling people in this weight range “overweight.” Since then, the situation has become considerably more absurd. It’s possible to have reasonable disagreements about the extent to which “obesity” (defined as a BMI of 30 or higher) is an independent contributor to ill-health and mortality risk. After all, epidemiology is a crude science, and the correlations between ill health and body weight among very fat people are inevitably open to multiple interpretations.

For example, suppose one follows two groups, each made up of 5,000 people, for ten years. People in the first group are at their supposed “ideal weight” for their height, while people in the second group are “obese.” After a decade, twenty people in the first group and thirty in the second have died of heart disease. Statistically, this means the fat people had a 50% increased risk of dying from heart disease than the thin people did. (This is typical of the sorts of risk ratios associated with obesity, and an example of how a handful of extra deaths, in the context of a tiny baseline risk, makes for scary-sounding headlines about obesity “raising the risk of a fatal heart attack by 50%.”) Does that mean the ten extra heart disease deaths were caused by fatness? Far from it. Perhaps the fat people were, on average, poorer; more stressed; more prone to diet and therefore to weight cycle; more likely to use diet drugs, many of which have been linked to cardiovascular disease; more sedentary; more discriminated against by the health care system and by society in general; and so on. Long-term observational studies of this sort can never control for more than a few of these sort of confounding variables, making it difficult to determine the extent to which, if at all, a particular correlation between a risk factor and a health outcome is causal.

But all this involves a very different question from that at the center of the controversy over whether being “overweight” is unhealthy. Flegal’s study has provided yet another rigorous demonstration of the fact that, if anything, people in the “overweight” category have, on average, better overall health and lower mortality rates than people in the absurdly mislabeled “healthy weight” category.

Flegal and her colleagues found that, for a whole range of diseases, from Alzheimer’s and Parkinson’s to infectious illness and most of the major respiratory ailments, “overweight” people face a lower mortality risk than “healthy weight” persons. In addition, they found no difference between the two groups in mortality risk from heart disease or cancer (the nation’s two biggest killers, and ones that many people tend to associate with being overweight). Thus, the relative mortality risk, and by extension the overall health, of “overweight” Americans appears to be better than that of “healthy weight” people.

In the context of America’s war on fat, the fact that being called “overweight” makes no medical or scientific sense is hardly a trivial point. How do our anti-fat warriors deal with this inconvenient truth?

Three rationalizations are getting prominent play. First, obesity researchers point out that while being overweight doesn’t correlate with increased health risk, being obese does, and “overweight” people are closer to being “obese” than “healthy weight.” “You should not take heart in the idea that if you are only overweight you are OK…because people gain weight as they age in this country,” said Robert Kushner, a professor of medicine at Northwestern. The problem with this argument is that, statistically speaking, people who are even slightly “underweight” face greatly increased health risks. Consider two women of average height, who weigh 110 and 150 pounds, respectively. The former “ideal weight” woman is roughly ten pounds away from a lower weight level that correlates with a doubling of her mortality risk, while the latter “overweight” woman would have to gain more than 100 pounds to move into a similar risk category. And while people generally gain weight in middle age, they usually begin to lose weight once they’ve reached retirement age--and nearly 80% of all deaths take place among people 65 and older. (Indeed, high weight has almost no correlation--or even a negative correlation--with mortality risk among the elderly, while weight loss has a very strong positive correlation, even when one controls for weight loss caused by eventually fatal illnesses). Given these facts, it seems odd to focus on the possibility of an “overweight” woman gaining 100 pounds rather than a thin woman losing ten.

Second, researchers talk about “quality of life.” After all, life expectancy isn’t everything. As Manson says, “health extends far beyond mortality rates.” According to a New York Times story, Manson is concerned that excess weight makes it difficult for people to move around, and therefore impairs their quality of life. That’s part of “the big picture in terms of health outcomes,” Manson says. The notion that an average-height woman who weighs between 146 and 175 pounds is going to find it difficult to move about is as good an example as one could hope to find of what eating disorder experts call “anorexic ideation.” Here again, we see how an argument which may make sense when talking about extremely fat people is transferred onto people who are “fat” only in the sense that they don’t conform to a radical preference for extreme thinness--a preference which is one of the key explanations for why we’re saddled with a scientifically bogus definition of what constitutes a “healthy weight.”

Finally, as a senior government scientist told me last week, “There’s this new argument going around that says overweight people are living longer because they’re going to the doctor more often and are therefore getting better medical care.” The scientist emphasized that, in a culture where access to medical care is closely linked to socio-economic status, and in which socio-economic status is inversely related to increasing body mass, this argument is, to put it politely, highly implausible.

Still, when the entire public health establishment has put its stamp of approval on a definition, those who have staked their professional reputations on the accuracy of that definition aren’t going to be deterred by something like, well, evidence. Predictably, Willett, who has been perhaps the most prominent proponent of the idea that people ought to try to maintain very low weights, was outraged by the latest refutation of his theories: “It’s just ludicrous to say there is no increased risk of mortality from being overweight,” he told The Washington Post.

What’s actually ludicrous is that Occam’s razor has yet to be employed to explain the “very puzzling” result that, once again, “overweight” people have been found to enjoy better health than “healthy weight” people. The definition of “overweight” promulgated by Willett, Manson, and their colleagues makes no sense. Many “puzzling” results cease to puzzle when one stops abusing the English language.

Paul Campos is a professor of law at the University of Colorado-Boulder.

By Paul Campos