It's Genetic

Or is it?:

"...In other words, certain stretches of DNA related to energy metabolism were "switched off" by methyl groups; a single hard workout removed these methyl groups to allow the genes to be expressed. This rapid change in DNA methylation is highly surprising -- scientists are only now beginning to figure out how these epigenetic mechanisms work. A further part of the study showed a dose-response effect: harder exercise produced more demethylation...."

The science of genetics is still a young one, and we're far from understanding how genes work.  So if someone tells you "it's genetic", with the implication that there's nothing you can do about it, odds are they're likely wrong.  There are many factors at work on our genes.