Among people whose weight is considered "normal," only 1 in 4 people have unhealthy levels of cholesterol, blood pressure, blood glucose, and other risks for heart disease. While possible in anyone, these risk factors are correlated with older age, low levels of physical activity, and proportionately larger waist circumference.
Among those considered "overweight," the incidence of cardiovascular risk factors doubles to 1 in 2. Among those considered "obese," the risk factor increases even more dramatically to 7 in 10.
These are the results of a study hailed by the fat acceptance movement, which traditionally dismisses such research. The reason? Unlike similar papers, the study's results were worded the opposite way, emphasizing the number of "normal" people who are unhealthy, and the number of "overweight" and "obese" people who are healthy. But if the fat acceptance advocates who praise this study do the math, the numbers are clear: being "overweight" makes you twice as likely to be unhealthy, and being "obese" makes you almost three times as likely to be unhealthy. In fact, the study clearly shows that an "obese" person is very likely to have risk factors for heart disease.
Fat acceptance is a difficult subject to deal with, because it is truly one of those situations where both sides are right in some sense. The fat advocates are absolutely correct when they say that the often-used body mass index (BMI) is a rather ridiculous measure of whether or not one is "overweight" or "obese." BMI tells people virtually nothing about their actual physical health. People have different proportions of fat and muscle, which weigh different amounts, and so to use a simple formula to calculate any sort of consistent "weight class" is absurd, except in the extremes. Kate Harding's BMI Project makes this abundantly clear.
Fat advocates (and feminists more broadly) are also absolutely correct in their critique of body image as it is promoted in Western culture, and through mass media. As the ongoing Human Variation Project shows, average people almost never resemble the ideals thrust upon them through advertising and celebrity culture. The size and shape considered "normal" varies throughout time and across cultures, with very few universals. Fat advocates certainly take the right course in their suggestion that people love their bodies, regardless of any perceived flaws. Size, shape, and even health do not make one who one is, they are simply attributes that are as positive or negative as we make them. There are a wide-range of aspects outside one's volitional control (from heredity to opportunity) that influence one's size.
Finally, as we can see from the study cited above, fat advocates are also absolutely correct that it is possible to be classified as overweight and even obese without suffering from cardiovascular risk factors and other health problems.
But the most important thing the fat acceptance movement has correct is that one's size ought never be cause for discrimination or hatred. Fat people, regardless of why they are fat and regardless of whether or not they are healthy, are still people. Even if being fat were the direct result of people's choices and made them extremely ill, it would be no cause for discrimination or hatred. It would be (again, at worst) a medical condition like any other. That being fat is often not entirely in one's control, and it does not mean that one is automatically unhealthy, is even more reason not to engage in fat-shaming and discrimination.
On the other hand, the fat acceptance movement is like any other movement in that it resists holding its ideology up to scrutiny. From the valid premise that we should love the bodies we have combined with the accurate evidence that one can be "fat but fit," the fat acceptance movement has unfortunately built an uncritical belief that there is a vast diet-industry conspiracy controlling scientific research to promote an obesity epidemic. Some books written by people outside medicine, such as The Obesity Myth, go so far as to say there is little or no evidence that obesity is associated with health risks at all by cherry-picking a handful out of the tens of thousands of studies carried out on the topic, or, as exemplified in the beginning of this post, rephrasing others to accentuate the positive while ignoring the negative. The overwhelming tide of evidence points to obesity indeed being associated with many health problems.
The line that must be tread is clear enough: variation in size is a part of the human condition, and should be celebrated rather than suppressed. Nevertheless, all people should be aware of the factual information regarding weight and health, for better or worse, and should be empowered to make their own choices regarding diet, exercise, or medical treatment, choices that ought to be respected. Size is not a character flaw.