Prospective study of physical activity and risk of postmenopausal breast cancer

Breast Cancer Research

Table 4 Relative risk of postmenopausal breast cancer in relation to total, vigorous and non-vigorous physical activity in subgroups defined by body mass index among US women in the Breast Cancer Detection Demonstration Project (BCDDP; 1987 to 1998)*

Variable	No. of cases	Category of physical activity					P-value, test for trend	P-value, test for interaction
		1	2	3	4	5
		Total physical activity
Body mass index
< 25.0 kg/m²	876	1.0	0.88 (0.71 to 1.08)	0.92 (0.75 to 1.12)	0.91 (0.74 to 1.11)	0.76 (0.61 to 0.94)	0.03
≥ 25.0 kg/m²	630	1.0	1.01 (0.79 to 1.29)	1.07 (0.84 to 1.37)	1.12 (0.88 to 1.43)	1.06 (0.82 to 1.36)	0.48	0.19
		Vigorous physical activity
Body mass index
< 25.0 kg/m²	876	1.0	0.87 (0.72 to 1.07)	0.81 (0.67 to 0.99)	0.90 (0.73 to 1.10)	0.68 (0.54 to 0.85)	0.002
≥ 25.0 kg/m²	630	1.0	1.15 (0.91 to 1.46)	1.00 (0.79 to 1.27)	0.93 (0.72 to 1.21)	1.18 (0.93 to 1.49)	0.33	0.008
		Non-vigorous physical activity
Body mass index
< 25.0 kg/m²	876	1.0	0.97 (0.79 to 1.20)	1.05 (0.85 to 1.31)	0.97 (0.78 to 1.20)	0.98 (0.79 to 1.21)	0.85
≥ 25.0 kg/m²	630	1.0	1.01 (0.79 to 1.28)	0.90 (0.69 to 1.16)	0.98 (0.77 to 1.27)	1.09 (0.85 to 1.39)	0.59	0.47

CI = confidence interval; RR = relative risk. The multivariate model included the following: age at baseline (continuous), family history of breast cancer (any first- or second-degree relative; yes, no or unknown), history of benign breast disease (any benign breast disease prior to study baseline, including during the screening stages; yes or no), breast cancer screening history (women who had undergone a breast biopsy as part of the BCDDP screening but lacked evidence of malignant disease, women who were recommended for surgical consultation as part of the BCDDP but for whom the procedure was not performed, or women who represented a random sample of BCDDP participants who had no surgery or recommendation for surgical consultation during BCDDP screening), height (continuous), age at menarche (≤ 11, 12, 13 or 14+ years of age), age at menopause (age at which menstrual period stopped for at least three continuous months; <47 or 47+ years of age), age at first live birth (continuous), history of oral contraceptive use (yes or no), menopausal hormone therapy (ever or never), education attainment (less than or equal to high school or greater than high school), cigarette smoking (ever or never), and intakes of energy-adjusted dietary fat (continuous) and alcohol (continuous). Within each stratum, the group with lowest vigorous physical activity served as the reference group.
† Body mass index is the weight in kilograms divided by height in metres squared.

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