[1] Univ N Carolina, Coll Arts & Sci, Chapel Hill, NC 27515 USA
[2] Univ N Carolina, Gillings Sch Global Publ Hlth, Nutr Sci Program, Chapel Hill, NC 27515 USA
[3] Univ N Carolina, Div Gen Pediat & Adolescent Med, Chapel Hill, NC 27515 USA
[4] Univ N Carolina, Cecil G Sheps Ctr Hlth Serv Res, Chapel Hill, NC 27515 USA
[5] Univ N Carolina, Sch Med, Div Gen Med & Clin Epidemiol, Dept Med, 5034 Old Clin Bldg,CB 7110, Chapel Hill, NC 27599 USA