[1] Univ Calif Berkeley, Sch Publ Hlth, Div Epidemiol, Berkeley, CA 94720 USA
[2] Univ Calif Berkeley, Coll Letters & Sci, Dept Mol & Cell Biol, Berkeley, CA 94720 USA
[3] Stanford Univ, Sch Med, Div Gen Med Disciplines, Stanford, CA 94305 USA
[4] Univ Texas Austin, Dept Kinesiol & Hlth Educ, Austin, TX 78712 USA
[5] Univ Calif Berkeley, Sch Publ Hlth, Div Community Hlth & Human Dev, Berkeley, CA 94720 USA