The perennial question: is obesity your fault? First there's the metabolic factor:
For example, the authors explain, when an individual reduces food intake and his or her body size diminishes, so does the amount of energy needed to maintain and move it. "Therefore, additional weight loss can only be achieved by a more severe diet or a more arduous physical activity routine," they write. "Most individuals do the opposite: After having achieved some weight loss, they resume their original diet and exercise habits. Consequently, weight gain recurs rapidly."
Then there's the wealth factor:
"[S]mall changes in lifestyle would have a minor effect on obesity prevention," they write. But the huge energy imbalance most Americans experience is "far beyond the ability of most individuals to address on a personal level." Instead, they say, changes in the food supply and social infrastructure and more stringent regulations of the food industry will be needed.
Katan elaborated in an e-mail: "Studies show that even the most motivated, thoughtful, strong-willed people have a hard time losing weight when huge portions of cheap, tasty, convenient food are available at every turn of the road, and when walking and other forms of exercise are superfluous or impossible."
Our bodies are designed to live in harsh conditions with scarce resources, which means they get fat when exposed to safe conditions with plentiful food. That is: a wealthy society is a fat society.
It's not that there isn't a certain level of willpower that will allow a person to lose weight, it's just that the willpower bar is so high that not many people can reach it despite the best intentions. When we fail to reach the bar, our bodies fall into natural states dictated by their metabolisms and environment.