How much lean body mass a woman has appears to have a lot to do with her genes, according to a new twin study published online in the Journal of Clinical Endocrinology and Metabolism.

Lean body mass, fat mass and bone mass are the three major components of body composition, Dr. Gregory Livshits of Tel Aviv University in Israel and colleagues note. But while much is known about the genetics of fat mass and bone mass, as well as their effects on health and fitness, less data are available on lean mass.

Understanding lean mass is important, Livshits and his team add, because it can dwindle with age, leading to reduced fitness and greater frailty.

To understand the role of genes in lean mass, the researchers evaluated 3,180 UK women, including 509 identical twin pairs and 1,081 fraternal twin pairs. Such twin studies allow scientists to separate out the effects of environment and heredity; identical twins share 100 percent of their genetic code while fraternal pairs share 50 percent.

Height was strongly linked to lean mass, the researchers found, while total fat mass and bone mineral density made a weaker but still significant contribution. After adjusting the data to account for the influence of other factors, the researchers found that genetic factors accounted for 65.2 percent of the variation in total lean body mass.

The researchers also performed genome scans on all of the fraternal twins to identify genes that might play a role in determining lean mass. They identified candidates on chromosomes 12 and 14.

"Replication studies from other populations followed by fine mapping should be the next step in identifying the individual genes involved which could have important physiologic effects on bone and fat as well as muscle," the researchers conclude.