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Scientists from the University of Melbourne reported that overweight or obese people who lost a significant amount of weight – at least 10 percent of their body weight – and kept the pounds off for one year still produced high levels of hunger-inducing hormones, giving them a biological urge to keep eating.

The scientists recruited 50 people for an intense 10-week, weight-loss program, USA Today reported. The participants consumed between 500 and 550 calories per day and lost an average of 30 pounds during the 10 weeks. Only 34 participants lost the required 10 percent of their body weight and were available for analysis one year later.

Although most of the people still weighed less than when the study began, they gained back about half of what they lost in the year after the program. When the scientists tested their blood for levels of hormones associated with appetite, such as leptin and ghrelin, they found the levels of those hormones changed in a way that made their appetites stronger than when the study began, the New York Times reports.

Dr. Lou Aronne, director of the Comprehensive Weight Control Program at New York Presbyterian Hospital, said obesity researchers have noted for many years that the body’s chemicals make obesity so difficult to treat.

“It’s not that they don’t’ want to maintain their weight loss,” Aronne said. “When people go off a diet and regain the weight, blaming them for doing that is the wrong response. This is a coordinated physiological system that is designed to push weight back up.”

Obesity researchers say these hormonal responses to weight loss are not surprising when viewed through the lens of human evolution. When humans’ early ancestors lost weight, it threatened their survival, so the body developed a hormonal response to keep that from happening.

Dr. Rudoph Leibel, an obesity researcher at Columbia University in New York City, told USA Today, “this is probably more or less a permanent response.”