Are you mystified why your sporty friend eats whatever she wants and still fits into a skimpy dress and you, on the other hand, never seem to be the right shape to wear anything fashionable?
Blame your genes.
According to an Anglo Israeli study, a woman's muscle mass may have less to do with exercise and abstaining from sweets than it does with the DNA of her parents.
In the study, which focused on brawn, sinew and shape more than obesity, Prof Gregory Livshits from Tel Aviv University and Prof Tim Spector from King's College in London have found a link between the muscle mass of a woman and her genes.
"We have known that obesity is heritable - but this study shows the importance of genes determining how much muscle each of us has - which determines body shape and also athletic abilities," says Prof Spector.
"Finding the genes responsible will have major impacts on sports as well a s explaining why many people will never obtain the perfect figure." adds Prof Spector who led the research team.
In the study more than 3,000 middle-aged British women who belonged to either an identical or fraternal twin pair were examined. The team from the St Thomas' Twin Unit measured the volunteers' "total lean mass," one of the three major components of total body weight - along with bone - and compared it to markers in their genes.
Until now, scientists were not sure to what extent environmental influences and genetics played a role in muscle mass. When controlling for age, and fat, they found that genes account for over half of the differences in womens' body sizes.
Those without the lean genes, however, will always find it harder to stay slim, predicts Prof Livshits. "The bad news is that 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."
The study is published in the Journal of Clinical Endocrinology and Metabolism.