Some people eat whatever catches there fancy and still remain as thin as ever, while some gain five pounds if they as much as look at that enticing chocolate cake.

A woman's waistline may have less to do with rigorous exercise and abstaining from sweets than it does with the genes of her parents, according to a recent study from Tel Aviv University.

Gregory Livshits and collaborators from King's College, London, have found a scientific link between the lean body mass of a woman and her genes.

Thinness, like our smile or eye colour, is an inheritable trait, he concludes.

"Many of our physical features, including our weight, are dependent on our genes. The good news is that women still have an opportunity to go against their genetic constitution and do something about it," said Livshits.

He conducted his study on more than 3,000 middle-aged women in Britain who were either identical or a fraternal twin pair.

He measured their "total lean mass", one of the three major components of body weight, and compared it to markers in their genes.

Those without the lean genes, however, will always find it harder to stay slim, predicted Livshits.