People who are obese have an increased risk of many diseases, including type 2 diabetes and heart disease and the evidence is that as many as 75% to 95% of previously obese individuals regain their lost weight - obesity is now promising to be a major health problem in most developed countries.

The researchers from Columbia University Medical Center in New York are suggesting that the hormone leptin that helps the brain resist food temptations may be the culprit.

According to lead researcher Michael Rosenbaum, following weight loss the body's metabolism becomes more efficient, so the body needs fewer calories, but the brain becomes more vulnerable to tempting food.

Rosenbaum says areas of the brain involved in regulating when a person eats, appear to become less active, making them more responsive to food and less in control.

Previous research revealed that when people lose weight, leptin levels fall as the body tries to protect its energy stores and it was the impact of this loss of leptin on the brains of people who had lost weight which was investigated by Rosenbaum - and whether replacing the hormone might help them keep off the weight.

Leptin it seems is a natural appetite suppressant secreted by fat cells in the body and the researchers suggest that weight loss creates the perfect circumstances for regaining weight and restoring leptin to pre-diet levels may resolve the problem.

For the study an imaging technique known as functional magnetic resonance imaging (fMRI) which shows activity in the brain, was used with six obese patients.

The fMRI scans were done before and after they were placed on a hospital-supervised diet that reduced their body weight by 10 percent.

The fMRI's revealed when the participants were shown pictures of food and non-food items, after weight loss, areas in the brain responsible for regulating food intake were less active, whereas areas in the brain responsible for emotion were more active.

When the researchers restored leptin to the levels before the dieting, these changes were largely reversed; they say similar results have been observed in people with a rare genetic condition in which their bodies do not make leptin.

Rosenbaum believes leptin could be a useful in developing treatments to help individuals maintain their weight loss.

The research is published in the Journal of Clinical Investigation.