The epidemiology of overweight and obesity: public health crisis or moral panic?

Abstract

National and international health organizations have focused increasingly on a perceived obesity epidemic said to pose drastic threats to public health. Indeed, some medical experts have gone so far as to predict that growing body mass will halt and perhaps even reverse the millennia-long trend of rising human life expectancy.1 In response to such concerns public health agencies across the world have sprung into action, searching for policies or incentives to mitigate the alleged ‘disease’ of obesity. Yet even as the volume of alarm grows louder, a growing number of researchers, drawn from a broad array of academic disciplines, are calling these claims into question. The authors of this article come from this latter group. In our view the available scientific data neither support alarmist claims about obesity nor justify diverting scarce resources away from far more pressing public health issues. This article evaluates four central claims made by those who are calling for intensifying the war on fat: that obesity is an epidemic; that overweight and obesity are major contributors to mortality; that higher than average adiposity is pathological and a primary direct cause of disease; and that significant long-term weight loss is both medically beneficial and a practical goal. Given the limited scientific evidence for any of these claims, we suggest that the current rhetoric about an obesity-driven health crisis is being driven more by cultural and political factors than by any threat increasing body weight may pose to public health. Claim #1: 'Almost all countries (high-income and low-income alike) are experiencing an obesity epidemic … WHO, 2003 (p. 61).2 The claim that we are seeing an ‘epidemic’ of overweight and obesity implies an exponential pattern of growth typical of epidemics. The available data do not support this claim. Instead, what we have seen, in the US, is a relatively modest rightward skewing of average weight on the distribution curve, with people of lower weights gaining little or no weight, and the majority of people weighing ∼3–5 kg more than they did a generation ago.3 The average American's weight gain can be explained by 10 extra calories a day, or the equivalent of a Big Mac once every 2 months. Exercise equivalents would be a few minutes of walking every day. This is hardly the orgy of fast food binging and inactivity widely thought to be to blame for the supposed fat explosion. While there has been significant weight gain among the heaviest individuals4 the vast majority of people in the ‘overweight’ and ‘obese’ categories are now at weight levels that are only slightly higher than those they or their predecessors were maintaining a generation ago. In other words we are seeing subtle shifts, rather than an alarming epidemic. Biologist Jeffery Friedman offers this analogy: ‘Imagine that the average IQ was 100 and that five percent of the population had an IQ of 140 and were considered to be geniuses. Now let's say that education improves and the average IQ increases to 107 and 10% of the population has an IQ of >140. You could present the data in two ways. You could say that average IQ is up seven points or you could say that because of improved education the number of geniuses has doubled. The whole obesity debate is equivalent to drawing conclusions about national education programmes by saying that the number of geniuses has doubled.’ In the US, to take a much-cited example, the so-called ‘obesity epidemic’ is almost wholly a product of tens of millions of people with BMIs formerly in the 23–25 range gaining a modest amount of weight and thus now being classified as ‘overweight’, and, similarly, tens of millions of people with BMIs formerly in the high 20s now having BMIs just >30. This movement of population cohorts from just below to just above the formal definitions of overweight and obesity is what public health officials are referring to when they point out that rates of obesity have exploded over the course of the last generation. (Furthermore, there is some evidence that adult and childhood BMI may have ceased to increase, as shown by comparison of NHANES data from 1999 to 2000 and from 2001 to 2002).5 In any case the real question is whether these developments represent some sort of genuine health crisis. This is true only if crossing the threshold of BMI 25 or 30 is analogous to contracting a life-threatening disease. But this analogy holds only to the extent that overweight and obesity actually cause increased mortality. Claim #2: ‘Mortality rates increase with increasing degrees of overweight, as measured by BMI.’—WHO, 2003 (p. 61)2 This claim, central to arguments that higher than average body mass amount to a major public health problem, is at best weakly supported by the epidemiological literature. Except at true statistical extremes, high body mass is a very weak predictor of mortality, and may even be protective in older populations. In particular, the claim that ‘o