Despite never having been overweight, I went on my first diet at the age of 12. That is tragic enough in of itself, but it's even worse that it's not even uncommon. Many of my friends have struggled with body image and disordered eating, just as I have. We have been taught to base our self-worth on the size of our jeans, not our intelligence, creativity, or anything more worthwhile.
In fact, dieting appears to promote binge eating and future weight gain, in addition to eating disorders. I will write more about dieting and intuitive eating (i.e., the practice of not dieting) in subsequent posts.
There appear to be two additional salient factors in the development of obesity:
1) Genetic predisposition. Research has shown that obesity may be largely genetic, not necessarily a result of dietary and lifestyle patterns. Many people are able to maintain a normal weight despite a bad lifestyle. I am one of those genetically skinny people, and was underweight during my childhood despite eating junk food all the time. Other people I know eat much better than I did and are still overweight.
2) Modern lifestyles. Obesity was rare before the second half of the twentieth century. Our bodies are designed for leanness, and we have been thin for the vast majority of history. Before the twentieth century, obesity was rare. According to one study, the average BMI* of white men in America in the late nineteenth century was 22.8. (Today, the average white man has a BMI of 28.0.) The average Okinawan centenarian has a BMI between 18 and 22, and this is typical for other societies where people lead traditional lives. The Kitavans, a Polynesian people, are even thinner: the average BMI for 40-year-old men and women is 20 and 19, respectively. According to conventional BMI classification, the Kitavans are borderline underweight!
These examples reinforce the idea that modern lifestyles are a necessary factor for obesity to be developed.
In other words, genetic predisposition + lifestyle (+ dieting) -> obesity; both genetic predisposition and a modern lifestyle are necessary to cause obesity, and dieting may contribute to the development of obesity. Since we can't control our genetics, the only way to prevent or treat obesity is by lifestyle modification. Genetic factors are less obvious and less easily measured than lifestyle factors, which contributes to the unfortunate popular understanding that lifestyle factors directly cause obesity, and that obesity is best treated by dieting.
Some fat acceptance activists have questioned the need for lifestyle modification aimed at preventing or treating obesity, arguing that obesity is primarily an aesthetic problem. Unfortunately, the list of conditions that are related to excess weight is long and includes such serious ailments as cancer, coronary heart disease, and type 2 diabetes. The claims that overweight people have lower mortality rates than people of normal weight are fueled by an incomplete understanding of statistics.
Interestingly, the healthy weight range (BMI of 18.5-25) includes higher weights than were achieved by most people on ancestral diets. This may be because the upper range of "healthy" weights are incompatible with optimal health. This observational study of white men found that men with a BMI between 24 and 25 had a 50% higher chance of developing type 2 diabetes than men with a BMI lower than 24. (The same study also reported that severely obese men (BMI ≥ 35) had a rate of diabetes more than forty times higher than men with a BMI lower than 24. Type 2 diabetes is tightly correlated with excess weight; see here for a graphic representation.)
As for myself, I plan on controlling my weight by regular exercise and avoiding industrially processed "foods," which are closely correlated with obesity and disease. But I will not forget the role that my genetics, socioeconomic class, and other advantages have played in determining my weight.
*A measure of body weight, in kg/m2. Someone who is 5'8" and 150 lbs has a BMI of 22.8.