(Health.com) -- Losing weight is hard, but keeping the pounds off can be even harder.
By some estimates, as many as 80% of overweight people who manage to slim down noticeably after a diet gain some or all of the weight back within one year.
A shortage of willpower may not be the only reason for this rebound weight gain. According to a new study in the New England Journal of Medicine, hunger-related hormones disrupted by dieting and weight loss can remain at altered levels for at least a year, fueling a heartier-than-normal appetite and thwarting the best intentions of dieters.
"Maintaining weight loss may be more difficult than losing weight," says lead researcher Joseph Proietto, Ph.D., a professor of medicine at the University of Melbourne's Heidelberg Repatriation Hospital, in Victoria, Australia. "This may be due to biological changes rather than [a] voluntary return to old habits."
Scientists have known for years that hormones found in the gut, pancreas, and fatty tissue strongly influence body weight and processes such as hunger and calorie burning. And the reverse is also true: A drop in body fat percentage, for instance, causes a decrease in the levels of certain hormones (such as leptin, which signals to your brain when you're full) and an increase in others (such as ghrelin, which stimulates hunger).
What wasn't so well known, until now, was whether these changes in hormone levels persist after an individual loses weight. To find out, Proietto and his colleagues put 50 overweight or obese men and women on a very low-calorie diet for 10 weeks, then tracked their hormone levels for one year.
The average weight loss during the initial diet period was about 30 pounds, which for most of the participants represented at least 10% of their starting body weight. (Seven people who did not meet this target were dismissed from the study.) Blood tests showed that average levels of several hormones (including leptin, ghrelin, and insulin) had changed as a result of the weight loss. As expected, the participants also reported being hungrier -- both before and after breakfast -- than they had been at the study's start.
At the 10-week mark the participants were allowed to resume a normal diet, but they continued to receive periodic advice from a dietitian and were also encouraged to get 30 minutes of exercise most days of the week. One year later, they'd regained about 12 pounds, on average, and follow-up tests showed that their hormone levels had only partially stabilized. Their hunger levels remained elevated as well.
The results aren't surprising, says Charles Burant, M.D., the director of the University of Michigan Nutrition Obesity Research Center, in Ann Arbor, who was not involved in the research. In fact, he says, the hormone changes seen in the study are a well-known evolutionary survival tactic.
"Multiple mechanisms have been developed, over eons of evolution, to get you to regain weight once you lose it...to tell your brain you're hungry and to ensure that you don't stop eating," he says. "If you don't have those drives, you wouldn't be alive."
But now that we live in a world where calories are so easily consumed and physical exercise -- the best way to burn off those calories -- is largely unnecessary for day-to-day survival, these biological drives are backfiring and contributing to obesity, Burant says.
That's not to say that weight regain is inevitable, or that these drives can't be overcome through willpower. Although the hormone changes noted in the study are very real physical effects, Proietto says, personality and psychological factors may play a role in an individual's ability to manage chronic hunger.
"This may explain why some people maintain weight loss for longer than others," he says. "Maintenance of weight loss requires continued vigilance and conscious effort to resist hunger."
Promising research is being done to discover ways to restore hormone levels in people who lose weight, Burant says. Preliminary studies from Columbia University, for example, have found that when dieters are injected with replacement leptin hormones, it's easier for them to maintain or continue weight loss.
"When diabetics don't have enough insulin in their bodies, we give them back insulin in order to maintain their blood glucose," Burant says. Researchers should be finding a way to do the same for people who have lost weight, he adds, "whether it's with a drug, a dietary supplement, or certain nutrients—something that will stimulate the release of these hormones."
Proietto agrees that finding an appetite suppressant of this sort is the next logical step in hormone and obesity research. Until then, he says, weight-loss surgery is a possible option for some severely obese people who have not been able to keep weight off by other methods.
Copyright Health Magazine 2011