The designs of the Health Professionals Follow up Study (HPFS), Nurses' Health Study (NHS) and Nurses' Health Study-II (NHS II) have been described previously (Ascherio et al. 2001; Colditz and Hankinson 2005). The NHS is a prospective cohort study comprising 121,700 female U.S. registered nurses aged 30 to 55 years in 1976. The HPFS is a prospective cohort study comprising 51,529 male U.S. health professionals aged 40 to 75 years in 1986. The NHS II is a prospective cohort study comprising 116,671 female U.S. registered nurses aged 25 to 42 years in 1989. Participants in all cohorts were followed with biennial questionnaires on lifestyle (including diet every 4 years), medication use, and disease incidence.
