Anyone who has ever tried to lose weight and keep it off knows how hard it is. While a lot of people manage to drop some pounds, most end up in a frustrating cycle in which they eventually gain it all back again. Why is that? Well, some new research is providing some clues.

Joseph Proietto of the University of Melbourne in Australia and colleagues studied 50 overweight or obese volunteers who agreed to go on a 10-week diet and undergo a series of blood tests for more than a year afterwards to see what was happening with hormones that are involved in regulating appetite, eating and weight.

Although the subjects managed to lose weight, the researchers found that a year after trimming down, levels of hormones such as leptin, ghrelin, peptide YY, cholecystokinin, insulin and amylin had still not returned to normal. Leptin, for example, which reduces food intake and increases energy burning, dropped dramatically when people lose weight, the researchers found. At the same time, ghrelin, which increases hunger, rose significantly and stayed at the elevated levels.

The findings indicate that people don’t just fail to keep the pounds off because of a lack of willpower, leading them to fall back into old bad habits. Instead, the research indicates that the body is programmed to try to stay at the higher weight “set-point,” and people are fighting powerful biological signals that are hard to resist.

“These mechanisms would be advantageous for a lean person in an environment where food was scarce, but in an environment in which energy-dense food is abundant and physical activity is largely unnecessary, the high rate of relapse after weight loss is not surprising,” the researchers wrote in Thursday’s issue of The New England Journal of Medicine.

“Taken together, these findings indicate that in obese persons who have lost weight, multiple compensatory mechanisms encouraging weight gain, which persist for at least one year, must be overcome in order to maintain weight loss,” they wrote.