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Table 1 Characteristics of the study population by level of total activity

From: Background risk of breast cancer and the association between physical activity and mammographic density

  All women Total activity
Characteristic   <40.0 MET-hours/day 40.0 to 44.9 MET-hours/day 45.0 to 49.9 MET-hours/day ≥50.0 MET-hours/day
Number (%) of participants 38,913 (100) 12,180 (31.3) 15,273 (39.2) 7,591 (19.5) 3,869 (9.9)
Age at mammography screening (years), mean (SD) 55.5 (9.8) 56.6 (10.0) 56.4 (10.1) 53.6 (9.1) 51.8 (8.0)
Body mass index (kg/m2), mean (SD) 25.4 (4.2) 26.1 (4.5) 25.2 (4.0) 24.9 (4.0) 25.2 (4.2)
Age at menarche (years), mean (SD)a 13.1 (1.5) 13.1 (1.5) 13.1 (1.5) 13.1 (1.5) 13.1 (1.5)
Absolute dense volume (cm3), mean (95% CI) 64.2 (63.9 to 64.5) 66.1 (65.6 to 66.7) 63.4 (62.9 to 63.9) 63.0 (62.3 to 63.7) 63.8 (62.8 to 64.7)
Nondense volume (cm3), mean (95% CI) 793.2 (788.6 to 797.7) 865.3 (856.8 to 873.7) 778.4 (771.4 to 785.4) 726.8 (717.1 to 736.6) 754.6 (740.0 to 769.1)
Total breast volume (cm3), mean (95% CI) 857.4 (852.7 to 862.0) 931.3 (922.7 to 940.0) 841.8 (834.6 to 849.0) 789.8 (779.9 to 799.8) 818.3 (803.3 to 833.3)
Percent dense volume (%), mean (95% CI) 9.0 (9.0 to 9.1) 8.5 (8.4 to 8.6) 9.0 (8.9 to 9.1) 9.6 (9.5 to 9.7) 9.5 (9.3 to 9.7)
Nulliparous (%)a 12.2 15.2 11.4 9.8 11.1
Parous women only (n) 34,093 10,307 13,516 6,837 3,433
 Age at first birth (years), mean (SD)a 27.0 (5.2) 27.0 (5.2) 27.2 (5.2) 27.2 (5.1) 26.5 (5.0)
 Number of childbirth, mean (SD) 2.2 (0.8) 2.1 (0.8) 2.2 (0.8) 2.3 (0.8) 2.3 (0.9)
Family history of breast cancer (%)a 13.6 14.2 13.7 12.9 13.0
Education level (%)a      
 Secondary school 13.7 14.9 14.2 11.6 12.0
 High school 30.6 30.4 27.9 29.9 43.2
 University or higher 52.5 51.6 54.4 55.9 41.1
 Other 2.9 2.8 3.2 2.5 3.3
OC use, ever (%)a 80.4 80.9 79.4 81.0 81.4
Premenopausal (%) 43.4 39.4 40.3 49.7 56.2
Postmenopausal women only (n) 22,009 7,377 9,121 3,817 1,694
 Age at menopause (years), mean (SD) 49.9 (5.3) 49.9 (5.3) 50.0 (5.4) 49.9 (5.1) 49.5 (5.4)
 HRT use (%)a      
  Never 62.2 60.8 60.7 65.3 69.0
  Past 22.4 23.3 23.9 19.9 16.5
  Current 5.1 5.2 4.8 5.7 5.6
Smoking status (%)a      
 Never 47.8 45.1 48.5 50.3 48.5
 Past 39.9 41.4 40.1 38.1 38.0
 Current 12.1 13.3 11.1 11.4 13.3
Alcohol consumption (g/day)a      
 None (%) 17.9 17.8 16.6 17.7 23.7
 0.1 to 9.9 (%) 61.2 58.2 62.4 63.9 60.7
 10.0 to 29.9 (%) 16.3 18.1 16.8 14.4 11.8
  ≥ 30.0 (%) 2.7 3.7 2.4 2.3 1.6
  1. aPercentage of women with missing values on age at menarche (1.9%), parity status (0.1%), age at first birth (5.8%), family history of breast cancer (2.7%), education level (0.3%), OC use (0.5%), HRT use (10.2%), smoking status (0.2%) and alcohol intake (1.9%). CI, confidence interval; HRT, hormone replacement therapy; MET, metabolic equivalent of task; OC, oral contraceptives; SD, standard deviation.
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