It's well known obese or overweight patients struggle to retain weight loss, but new research finds that a year after losing a significant amount of weight, hormones in people who went on the diet stimulated hunger.

As previous studies have shown, heavy people experience hormone changes immediately after losing weight which slows their metabolism and increases their appetite. But research published Thursday in the New England Journal of Medicine found that as much as one year after weight loss, those levels of appetite-regulating hormones didn't go back to baseline levels, meaning people were hungrier than when they began the diet.

"The high rate of relapse among obese people who have lost weight has a strong physiological basis and is not simply the result of the voluntary resumption of old habits," researchers led by Dr. Joseph Proietto of Heidelberg Repatriation Hospital in Australia said.

Researchers were motivated to find the reasons behind the high rate of weight regain after diet-induced weight loss.

They enrolled 50 overweight or obese patients without diabetes in a 10 week weight-loss program.

Researchers wanted to observe results in participants who at least had a weight loss of 10 percent their weight. Only 34 percent of the patients lost that amount and continued in the study enough time to be analyzed.

They examined levels of hormones in those participants before weight loss, at the end of the diet and 1 year after.

The average weight loss among participants during the 10-week program was 29 pounds. They consumed from 500 to 550 calories a day using a supplement called Optifast and vegetables during the first 8 weeks and during the last weeks they gradually added their common meals.

All participants were given a diet intended to keep their weight loss.

A year later researchers saw that levels of the hormones that mediate appetite changed in a way that increased the patients' appetites and indeed they were hungrier than when they began the diet.

Patients were gaining back an average of half what they had lost, despite maintaining a diet.

"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," researchers wrote.

"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," they added.

Therefore, researchers noted that treatments are needed to "counteract these compensatory mechanisms and reduce appetite."

The study was funded in part by the National Health and Medical Research Council.