Table 1.

Characteristics of the 391 participants at baseline by study arm

Vitamin D3
Characteristics of participantsPlacebo (n = 96)1,000 IU/day (n = 96)2,000 IU/day (n = 99)3,000 IU/day (n = 100)
Age (y) (mean ± SD)42.9 ± 6.242.8 ± 5.242.7 ± 5.242.7 ± 5.5
White race (%)95.899.0100.097.0
Body mass index (kg/m2) (mean ± SD)24.4 ± 4.124.7 ± 5.124.3 ± 4.124.0 ± 3.6
Number of full-term pregnancies (mean ± SD)1.4 ± 1.11.7 ± 1.01.7 ± 1.21.7 ± 1.0
Hormonal contraceptive ever use (%)91.791.795.093.0
Natural menstrual cycles (%)64.674.068.775.0
Personal history of breast biopsy (%)9.45.29.18.0
Dietary vitamin D intakea (IU/d) (mean ± SD)200 ± 169188 ± 113177 ± 98179 ± 113
Use of supplements with vitamin Db (%)33.340.638.443.0
Vitamin D from supplements (IU/d) (mean ± SD)406 ± 587302 ± 318444 ± 433425 ± 424
Total vitamin D intakea (IU/d) (mean ± SD)337 ± 417306 ± 260354 ± 349366 ± 373
Dietary calcium intakea (mg/d) (mean ± SD)1,024 ± 6031,028 ± 479969 ± 389997 ± 507
Use of supplements with calciumb (%)34.439.641.438.0
Calcium from supplements (mg/d) (mean ± SD)86 ± 141116 ± 211137 ± 200163 ± 256
Total calcium intakea (mg/d) (mean ± SD)1,054 ± 6031,075 ± 4841,027 ± 3971,061 ± 515
Food energy intakea (kcal/d) (mean ± SD)1,746 ± 6641,693 ± 5061,714 ± 5841,764 ± 639
Minutes/week outdoorsc (mean ± SD)470 ± 362366 ± 237403 ± 287417 ± 270
Season at time of baseline mammography (%)
 Winter31.325.028.327.0
 Spring31.337.535.438.0
 Summer13.511.512.113.0
 Fall24.026.024.222.0
Serum 25(OH)D3 (nmol/L) (mean ± SD)64.4 ± 23.166.3 ± 26.865.5 ± 24.960.4 ± 20.6
Screening mammographyd (%)90.689.685.987.0
Digital direct radiographye (%)44.847.950.548.0
Radiology clinicsf (%)
 Site 156.354.252.556.0
 Site 212.512.512.112.0
 Site 310.49.410.110.0
 Site 46.37.36.16.0
 Site 53.13.14.02.0
 Site 611.513.515.214.0
  • aIn the last year, among the 380 women who completed the food frequency questionnaire at baseline (placebo, n = 95; 1,000 IU/day, n = 93; 2,000 IU/day, n = 95; 3,000 IU/day, n = 97).

  • bIn the last year, based on face-to-face interview at recruitment. Mean supplemental intake is provided among those having supplemental intake.

  • cBetween 10 am and 4 pm, in the last year.

  • dVersus mammography for symptoms.

  • eVersus digital computed radiography.

  • fRadiology clinics (sites) where mammograms were done correspond to randomization strata.