Participants were selected from three U.S. prospective cohort studies. NHS was established in 1976, when 121 700 female nurses aged 30–55 years completed a self-administered questionnaire on risk factors for cancer and other diseases. From 1989–1990, 32 826 participants provided blood samples for analysis. NHSII began in 1989 when 116 671 female nurses aged 25–42 years completed and returned a baseline questionnaire. Between 1996 and 1999, 29 611 participants (aged 32–54 years) provided blood samples. HPFS comprises 51 529 male dentists, optometrists, osteopaths, podiatrists, pharmacists, and veterinarians aged 40–75 years at baseline in 1986. Blood samples were provided by 18 225 of these men between 1993 and 1994. Blood samples have been stored in liquid nitrogen freezers (≤ −130°C) since collection. For all three cohorts, biennial questionnaires are sent to participants to update information on risk factors and to identify newly diagnosed diseases. Diet is assessed by a validated semiquantitative food frequency questionnaire approximately every 4 years(35 (link)–38 (link)).
Plasma 25(OH)D measurements were available from men and women who served as controls in previous nested case-control studies of chronic diseases. None of the participants had a history of cancer at the time of blood draw. For each cohort, we selected two independent samples: a “training” sample was used to develop the 25(OH)D prediction model and a “test” sample served as a validation dataset. Training samples comprised controls from all completed and on-going nested case-control studies with 25(OH)D assay results when analyses began. Test samples were drawn from more recently established nested case-control studies as this project unfolded and additional plasma 25(OH)D assay results became available. Prior to exclusions for missing data, the training sets consisted of 2246 women in NHS, 1646 women in NHSII, and 1255 men in HPFS. An additional 818 women in NHS, 479 women in NHSII, and 841 men in HPFS were available for the test sets.
In 2000 and 2001, all women in NHS who gave blood in 1989–1990 and were alive were invited to provide a second blood sample. Of the 18 473 women who participated in the second blood collection, 443 women with no history of cancer had measured 25(OH)D available at both time points. These samples were used to assess within-person variability of plasma 25(OH)D concentrations over 10–11 years.
This study was approved by the institutional review boards of the Harvard School of Public Health and Brigham and Women's Hospital. All participants gave written informed consent at enrollment.
Plasma 25(OH)D measurements were available from men and women who served as controls in previous nested case-control studies of chronic diseases. None of the participants had a history of cancer at the time of blood draw. For each cohort, we selected two independent samples: a “training” sample was used to develop the 25(OH)D prediction model and a “test” sample served as a validation dataset. Training samples comprised controls from all completed and on-going nested case-control studies with 25(OH)D assay results when analyses began. Test samples were drawn from more recently established nested case-control studies as this project unfolded and additional plasma 25(OH)D assay results became available. Prior to exclusions for missing data, the training sets consisted of 2246 women in NHS, 1646 women in NHSII, and 1255 men in HPFS. An additional 818 women in NHS, 479 women in NHSII, and 841 men in HPFS were available for the test sets.
In 2000 and 2001, all women in NHS who gave blood in 1989–1990 and were alive were invited to provide a second blood sample. Of the 18 473 women who participated in the second blood collection, 443 women with no history of cancer had measured 25(OH)D available at both time points. These samples were used to assess within-person variability of plasma 25(OH)D concentrations over 10–11 years.
This study was approved by the institutional review boards of the Harvard School of Public Health and Brigham and Women's Hospital. All participants gave written informed consent at enrollment.
