Also on the committee is the executive system, which deals with planning and decision-making. “This is the area of the brain that you tend to think of as your secret weapon for weight loss,” Aamodt said. “Your secret weapon for weight loss takes a lot of vacations.” Willpower is very taxing for people—studies show that any task that you do that requires discipline and self-control makes it harder for you to resist urges later. The executive system doesn’t function as well when people are lonely, or stressed. “And guess what? It’s impaired when you’re hungry,” Aamodt said. “The basic answer to why people have so much trouble with dieting is they’re trying to use a system that tires easily to fight against brain systems that are always working, never take a day off.”

Also at the Aspen Ideas Festival, Kevin Hall, a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases (and an author on the Biggest Loser study) described another problem: When people go on a diet and begin to lose weight, their appetite increases substantially. And so people end up eating more calories without realizing it. “If you ask people if they’re doing anything different, they’ll say no,” Hall said. They feel like they’re exerting the same amount of effort toward their diet, because their appetite is so high that they’re still hungry even if they eat a little more. And it continues—“there’s an exponential decay of diet adherence,” Hall said.

Meanwhile, metabolism slows down, and the weight comes back, with some dieters eventually weighing more than they did before the diet. This happens over the long-term, months or years later, which means that in the short-term diets still seem appealing. “When I say diets don’t work, I mean they don’t work five years later, Aamodt said. “Two months later, they work great.”

Diets can permanently mess up not just your weight, but the way you eat. One study that followed thousands of girls and boys for two years found that dieters were more likely to binge-eat. (Female frequent dieters were 12 times as likely to binge eat, male frequent dieters were 7 times as likely to do so.)

“That’s probably just a biological response to repeated starvation,” Aamodt said.

None of this is encouraging. What are people supposed to do, just stop trying to lose weight? Well, yeah, maybe. Unless you want to diet for the rest of your life (while probably still feeling hungry a lot of the time). That’s what people do, who manage to successfully keep weight off. “They commit to counting calories forever, they exercise every day,” Aamodt said. “A large number of them are fitness professionals. That’s the level of commitment that it requires.”

But you don’t have to lose weight to improve your health and lower your risk of dying. One study of people of a range of BMIs, from normal to overweight to obese, found that the more healthy habits they adopted, the less likely they were to die during the course of the study. Those habits were regular exercise, not smoking, moderate drinking, and eating at least five fruits and vegetables every day. The higher someone’s BMI, the greater the benefit they saw from the habits, and by the time all four were adopted, there was almost no difference in the mortality risk for all BMIs.