WEDNESDAY, Oct. 26 (HealthDay News) -- Overweight people who diet and successfully shed pounds only to gain the weight back again within a year can blame their hunger hormones, new research suggests.

In a small study published in the Oct. 27 issue of the New England Journal of Medicine, Australian scientists found that after overweight and obese patients followed a low-calorie diet for 10 weeks, their appetite and hunger hormone levels changed. While some hormone levels increased and others went down compared to before they dieted, nearly all of the changes favored the body's efforts to regain the lost weight.

The scientists used blood tests to measure levels of nine different hormones at the start of the study, at week 10 when the diet period ended, and again a year later.

The hormone levels did not revert to baseline values within 12 months after the initial weight reduction, said study senior author Dr. Joseph Proietto, a University of Melbourne professor of medicine at Austin Health in Victoria.

For example, in follow-up blood tests, one hormone called ghrelin, an appetite stimulator produced by cells in the lining of the stomach, increased after weight loss and continued to do so throughout the study. On the other hand, levels of the hormone leptin, which suppresses appetite, went down.

"The implication of these findings is that subjects who have lost weight need to remain vigilant and understand that once they have lost weight, the battle is not over," said Proietto. The maintenance phase may be indefinite, he said.

The new study confirms previous findings, said endocrinologist Dr. David Heber, director of the Center for Human Nutrition at the University of California, Los Angeles. "There's been a lot of research to show that once people lose weight their natural hormones are pushing them to regain that weight, which is what this study shows. The reason is we're well adapted to starvation and poorly adapted to over-nutrition," Heber said.

While many studies have looked at hormonal levels short-term, this study's demonstration that changes in appetite hormones can persist up to a year is notable, said Dr. Gary Foster, director of the Center for Obesity Research and Education at Temple University, in Philadelphia. "It's interesting that hormone effects last such a long time," he said. But he notes that the study is very small and had no control group.

Proietto says some participants dropped out, and four others did not lose the 10 percent of body weight required for the follow-up phase of the study, so only 34 of the 50 people who started are included in the final results.

In addition to the having their hormone levels measured, the participants also rated their appetites. A "significant" increase was reported as time marched on, the authors said.

"It's difficult, though, to draw conclusions from the patients' subjective appetite ratings," Foster said. "Boy, what a loaded question. Hunger can mean different things to different people. When you say you're hungry, does it mean you're dizzy or just hungry, or did you just see a piece of carrot cake and now you want it so that makes you hungrier? Hunger isn't the driving force for most people who overeat," Foster said.

Dr. Kimberly Brownley, an assistant professor in the department of psychiatry at the University of North Carolina at Chapel Hill, said the study is interesting but only offers one perspective -- one set of data. "A limitation is that they didn't look at brain changes, too. The brain is always in control, in the driver's seat," she said.

The authors said the findings also imply that drugs to suppress hunger may be useful in the long term, but behavioral factors linked to overeating should also be tackled, Heber noted. "Food addiction needs to be addressed. We've found that 50 percent of overweight people have food-addiction issues," he said.

Overweight individuals shouldn't be discouraged by the new research suggesting hormones are a big player in weight regain, said Heber. "The power is in your hands to get to a healthy body weight by eating a healthy diet and exercising and receiving proper medical supervision. We have lots of evidence showing people have lost weight this way," he said.

"There will never be a magical pill that allows you to eat what you want and lose weight. The idea of this is really a disservice to society," he said.

More information

The Yale Rudd Center for Food Policy and Obesity provides information on food and addiction.