women is testosterone. you have denser bones. you pack in more muscle fibers, longer proportional limbs, more red blood cells, so it's good for endurance athletes. they can absolutely complete change your athleticism. >> before i let you go, i have to ask you one more difficult, personal question. as you know, you were all east honors in varsity track. congratulations. what portion of that was your genes? >> you know, i think a huge portion of it was my genes. i worked out so hard in college that i was, you know, throwing up many times in practice and people just said, oh, such a hard worker. for this book, i got my genes tested. they showed that while i did have a low sort of aerobic baseline, an ability to use oxygen while exercising, i had genes that made me respond well to training. >> maybe you're gist genetically disposed to throw up a lot. >> that could be too. >> david epstein, thanks for hanging out with us. i want to tell you about a hot new workout play list. that's why you watch this show.