Table 4.
The association between body mass index and breast cancer risk according to birth year and country
| n | Body mass index at baseline (kg/m2) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| < 18.5 | 18.5– < 21 | 21– < 23 | 23– < 25 | 25– < 27.5 | 27.5– < 30 | 30- | Trend (per 5 kg/m2) | p for interaction | |||
| HR (95% CI) | HR (95% CI) | HR | HR (95% CI) | HR (95% CI) | HR (95% CI) | HR (95% CI) | HR (95% CI) | p | |||
| Birth year | |||||||||||
| Premenopausal women at baseline | |||||||||||
| Breast cancer that developed before 50 years of age | |||||||||||
| 1944 or before | 32,654 | 1.03 (0.29–3.66) | 1.35 (0.66–2.76) | 1 (ref.) | 1.22 (0.57–2.60) | 1.96 (0.92–4.19) | 0.43 (0.06–3.31) | 4.53 (1.47–13.96) | 1.25 (0.85–1.84) | 0.27 | 0.69 |
| 1945–1952 | 39,693 | 0.88 (0.44–1.76) | 0.99 (0.69–1.42) | 1 (ref.) | 0.84 (0.57–1.23) | 1.16 (0.77–1.75) | 1.13 (0.62–2.05) | 1.18 (0.51–2.73) | 1.12 (0.90–1.38) | 0.32 | |
| 1953 or after | 34,446 | 0.82 (0.44–1.51) | 1.23 (0.91–1.67) | 1 (ref.) | 1.07 (0.77–1.49) | 1.11 (0.76–1.62) | 0.85 (0.46–1.56) | 0.75 (0.30–1.86) | 0.92 (0.76–1.12) | 0.40 | |
| Breast cancer that developed after 50 years of age | |||||||||||
| 1944 or before | 38,518 | 0.85 (0.58–1.24) | 0.94 (0.74–1.20) | 1 (ref.) | 1.03 (0.81–1.31) | 1.26 (0.97–1.64) | 1.33 (0.93–1.90) | 1.18 (0.69–2.01) | 1.15 (1.02–1.30) | 0.03 | 0.98 |
| 1945–1952 | 41,962 | 1.02 (0.65–1.60) | 0.81 (0.63–1.05) | 1 (ref.) | 1.10 (0.88–1.36) | 1.11 (0.88–1.41) | 1.09 (0.79–1.51) | 1.05 (0.67–1.64) | 1.10 (0.97–1.24) | 0.13 | |
| 1953 or after | 30,060 | 0.74 (0.40–1.38) | 0.92 (0.68–1.23) | 1 (ref.) | 0.94 (0.70–1.24) | 1.03 (0.75–1.41) | 0.99 (0.63–1.57) | 0.98 (0.49–1.93) | 1.08 (0.91–1.28) | 0.39 | |
| Postmenopausal women at baseline | |||||||||||
| 1928 or earlier | 68,886 | 0.66 (0.48–0.92) | 0.68 (0.53–0.87) | 1 (ref.) | 1.17 (0.94–1.46) | 1.23 (0.97–1.55) | 1.69 (1.27–2.24) | 1.81(1.24-.62) | 1.44(1.31–1.58) | < 0.001 | 0.24 |
| 1929–1936 | 71,600 | 0.99 (0.67–1.46) | 0.79 (0.62–1.01) | 1 (ref.) | 1.32 (1.10–1.59) | 1.26 (1.04–1.53) | 1.23 (0.96–1.56) | 1.45 (1.07- | 1.23 (1.12–1.35) | < 0.001 | |
| 1937 or later | 59,917 | 0.80 (0.51–1.27) | 1.17 (0.93–1.46) | 1 (ref.) | 1.31 (1.08–1.59) | 1.42 (1.16–1.73) | 1.55 (1.21–1.99) | 2.03 (1.52–2.72) | 1.30 (1.19–1.43) | < 0.001 | |
| Country | |||||||||||
| Premenopausal women at baseline | |||||||||||
| Breast cancer that developed before 50 years of age | |||||||||||
| Japan | 63,588 | 0.86 (0.48–1.55) | 1.10 (0.81–1.50) | 1 (ref.) | 0.97 (0.70–1.35) | 1.36 (0.95–1.94) | 1.08 (0.61–1.91) | 1.81 (0.94–3.49) | 1.17 (0.97–1.41) | 0.09 | 0.04 |
| Korea | 10,229 | 1.14 (0.46–2.80) | 1.43 (0.85–2.39) | 1 (ref.) | 1.04 (0.57–1.88) | 0.78 (0.36–1.67) | 0.21 (0.03–1.58) | 0.74 (0.17–3.13) | 0.63 (0.43–0.92) | 0.02 | |
| China | 32,976 | 0.67 (0.27–1.69) | 1.09 (0.73–1.63) | 1 (ref.) | 0.95 (0.63–1.42) | 1.16 (0.75–1.80) | 1.01 (0.53–1.94) | 0.67 (0.21–2.13) | 1.02 (0.80–1.29) | 0.88 | |
| Breast cancer that developed after 50 years of age | |||||||||||
| Japan | 67,282 | 0.96 (0.70–1.32) | 0.93 (0.76–1.12) | 1 (ref.) | 1.12 (0.92–1.35) | 1.19 (0.96–1.47) | 1.45 (1.10–1.92) | 1.19 (0.77–1.83) | 1.16 (1.05–1.28) | 0.005 | 0.82 |
| Korea | 6794 | 0.61 (0.08–4.60) | 0.63 (0.28–1.45) | 1 (ref.) | 0.92 (0.46–1.83) | 0.61 (0.25–1.47) | 0.45 (0.10–1.99) | 0.40 (0.05–3.05) | 0.86 (0.53–1.39) | 0.53 | |
| China | 36,464 | 0.77 (0.47–1.27) | 0.89 (0.70–1.13) | 1 (ref.) | 0.96 (0.77–1.19) | 1.13 (0.90–1.42) | 0.90 (0.64–1.26) | 1.04 (0.66–1.62) | 1.08 (0.95–1.22) | 0.25 | |
| Postmenopausal women at baseline | |||||||||||
| Japan | 141,822 | 0.74 (0.57–0.94) | 0.76 (0.64–0.90) | 1 (ref.) | 1.27 (1.11–1.45) | 1.22 (1.05–1.41) | 1.42 (1.18–1.71) | 1.45 (1.11–1.88) | 1.33 (1.24–1.42) | < 0.001 | 0.15 |
| Korea | 22,050 | 0.59 (0.14–2.45) | 1.43 (0.89–2.31) | 1 (ref.) | 1.30 (0.85–1.99) | 0.98 (0.60–1.59) | 1.10 (0.58–2.09) | 1.95 (0.96–3.94) | 1.05 (0.82–1.34) | 0.70 | |
| China | 36,531 | 1.11 (0.69–1.80) | 1.12 (0.83–1.51) | 1 (ref.) | 1.31 (1.03–1.67) | 1.60 (1.27–2.03) | 1.61 (1.23–2.12) | 2.17 (1.61–2.92) | 1.33 (1.21–1.47) | < 0.001 | |
HR hazard ratio, CI confident interval
Estimated hazard ratio after adjustments for age at enrollment (continuous), study (13 cohorts), smoking status (never, former, current smokers), alcohol consumption (non-drinkers or current drinkers), age at menarche (≤ 10 y, 11–12 y, 13–14 y, 15–16 y, ≥ 17 y), nulliparity (yes, no), age at first delivery (≤ 20 y, 21–25 y, 26–30 y, ≥ 31 y), and age at menopause (≤ 44 y, 45–49 y, 50–54 y, ≥ 55 y)(postmenopausal women only)