This has implications for treatment, said Drexel psychotherapist Michael Lowe, senior author of the study.
Therapists should "review [the patient's] weight history from before and during the disorder," he said. "They can argue that these drastic behaviors really aren't working" and may even be contributing to weight gain.
If a patient understands that Catch-22, she "may open up to considering healthier ways of controlling her weight," Lowe said.
Bulimia was formally recognized as an eating disorder in 1979. Since then, studies have found that women who develop it usually start out moderately overweight, then slim down by crash dieting until they are borderline underweight. (One study found a third of bulimics had a history of anorexia, the self-starvation disorder.)
"After six months to two years . . . their bodies can't take it anymore," Lowe said. "It leads to bingeing."
This pattern is not just a matter of overindulgence. Animal studies, as well as a study of conscientious objectors on hunger strikes, have shown that reintroducing food after starvation triggers wildly out-of-control eating.
"That creates physical discomfort, so self-induced vomiting is both psychologically and physically a relief," Lowe said, explaining how a normally disgusting behavior can become comforting. "Bulimics get stuck in a bind."
Depending how long they're stuck, they may damage their teeth, throat, and digestive tract, and induce life-threatening fluid and nutrient imbalances.
Therapists have long known that the binge-purge cycle does not prevent weight gain. Laxatives and vomiting reduce but don't stop food absorption, so consuming thousands of calories at a sitting eventually adds pounds.
Still, therapists have focused more on the harmful behaviors than the resulting weight gain because patients usually remain far from obese.
"I've been in this field for 27 years," said Susan Ice, medical director of the Renfrew Center of Philadelphia, which treats eating disorders. "Yet I had the impression that bulimics were usually near normal weight and weight was not the issue. They generally looked fine compared to anorexic patients, maybe even a little bit overweight. So the finding that they wound up rebounding and exceeding their highest weight was a surprise."
Lowe's team, which included Drexel doctoral student Jena Shaw and experts from Massachusetts General Hospital, analyzed data from two previous studies of bulimia patients, some of whom were in remission from the disorder. One study enrolled 78 women treated at Renfrew for two years. The other involved 110 women treated in Boston and interviewed at six-month intervals for eight years.
About 60 percent of the Renfrew group and 72 percent of the Boston group reached their highest weights while struggling with bulimia. Their gains left the women only moderately overweight, with Body Mass Indexes of about 26 to 27 (10 to 15 pounds over ideal weights). However, these women had been diagnosed at earlier ages and engaged in harmful behaviors longer than women whose highest weights were reached pre-diagnosis.
The researchers offer several possible explanations. Women who develop bulimia at younger ages may be particularly prone to weight gain. Continual bingeing and purging also appears to slow down the body's calorie-burning ability, perhaps as a self-preservation tactic.
In any case, the findings could help to improve treatment, which has traditionally assumed that bulimics, like anorexics, suffer from poor body image and an extreme fear of weight gain.
Bulimics' fear "may in part be a product of their actual past experience with rapid weight gain," the researchers wrote in their paper, published online last month in the International Journal of Eating Disorders.
Renfrew's Ice said the study also suggests that "the sooner they can let go of their eating pathology, the more their body will take care of itself."
"The message for patients is clear: Nip this thing in the bud," she said.
Contact Marie McCullough at 215-854-2720 or firstname.lastname@example.org.