New York: Doctors at Columbia University Medical Centre believe they have discovered why some dieters find it difficult to maintain weightloss and pile the pounds back on. A hormone called leptin, which regulates the metabolism may be responsible.

Leptin levels fall when people diet making it increasingly harder to burn off calories, said the findings published in the Journal of Clinical Investigation.

Dr Michael Rosenbaum, who led the research believes the findings could lead to a new diet drug containing leptin.

He said that leptin helps the body believe it is happy with a lower weight and help dieters keep off the pounds.

The research examined the reactions of 10 healthy men and women. As they lost weight their leptin levels fell. They were given leptin and maintained their weightloss.

In previous studies it has been found that leptin acts as an appetite suppressant. Levels also fall if we do not get enough sleep making us want to eat more.