The causal relationship between diets and weight gain can also be tested by studying people with an external motivation to lose weight. Boxers and wrestlers who diet to qualify for their weight classes presumably have no particular genetic predisposition toward obesity. Yet a 2006 study found that elite athletes who competed for Finland in such weight-conscious sports were three times more likely to be obese by age 60 than their peers who competed in other sports.

To test this idea rigorously, researchers could randomly assign people to worry about their weight, but that is hard to do. One program took the opposite approach, though, helping teenage girls who were unhappy with their bodies to become less concerned about their weight. In a randomized trial, the eBody Project, an online program to fight eating disorders by reducing girls’ desire to be thin, led to less dieting and also prevented future weight gain. Girls who participated in the program saw their weight remain stable over the next two years, while their peers without the intervention gained a few pounds.

WHY would dieting lead to weight gain? First, dieting is stressful. Calorie restriction produces stress hormones, which act on fat cells to increase the amount of abdominal fat. Such fat is associated with medical problems like diabetes and heart disease, regardless of overall weight.

Second, weight anxiety and dieting predict later binge eating, as well as weight gain. Girls who labeled themselves as dieters in early adolescence were three times more likely to become overweight over the next four years. Another study found that adolescent girls who dieted frequently were 12 times more likely than non-dieters to binge two years later.

My repeated dieting eventually caught up with me, as this research would predict. When I was in graduate school and under a lot of stress, I started binge eating. I would finish a carton of ice cream or a box of saltines with butter, usually at 3 a.m. The urge to keep eating was intense, even after I had made myself sick. Fortunately, when the stress eased, I was able to stop. At the time, I felt terrible about being out of control, but now I know that binge eating is a common mammalian response to starvation.

Much of what we understand about weight regulation comes from studies of rodents, whose eating habits resemble ours. Mice and rats enjoy the same wide range of foods that we do. When tasty food is plentiful, individual rodents gain different amounts of weight, and the genes that influence weight in people have similar effects in mice. Under stress, rodents eat more sweet and fatty foods. Like us, both laboratory and wild rodents have become fatter over the past few decades.

In the laboratory, rodents learn to binge when deprivation alternates with tasty food — a situation familiar to many dieters. Rats develop binge eating after several weeks consisting of five days of food restriction followed by two days of free access to Oreos. Four days later, a brief stressor leads them to eat almost twice as many Oreos as animals that received the stressor but did not have their diets restricted. A small taste of Oreos can induce deprived animals to binge on regular chow, if nothing else is available. Repeated food deprivation changes dopamine and other neurotransmitters in the brain that govern how animals respond to rewards, which increases their motivation to seek out and eat food. This may explain why the animals binge, especially as these brain changes can last long after the diet is over.