Obesity is associated with increased breast cancer risk, especially among postmenopausal women who do not use HT. The WHI observational study observed 85,917 women aged 50 to 79 years and collected information on weight history as well as known risk factors for breast cancer. Height, weight, and waist and hip circumferences were measured. With a median follow-up of 34.8 months, 1,030 of the women developed invasive breast cancer. Among the women who never used HT, increased breast cancer risk was associated with weight at entry, body mass index (BMI) at entry, BMI at age 50 years, maximum BMI, adult and postmenopausal weight change, and waist and hip circumferences. Weight was the strongest predictor, with a RR of 2.85 (95% CI, 1.81–4.49) for women weighing more than 82.2 kg, compared with those weighing less than 58.7 kg.
Many epidemiologic studies have shown an increased risk of breast cancer associated with alcohol consumption. Individual data from 53 case-control and cohort studies were included in a British meta-analysis. Compared with women who reported no alcohol consumption, the RR of breast cancer was 1.32 (95% CI, 1.19–1.45; P < .001) for women consuming 35 g to 44 g of alcohol per day and 1.46 (95% CI, 1.33–1.61; P < .001) for those consuming at least 45 g of alcohol per day. The RR of breast cancer increases by about 7% (95% CI, 5.5%–8.7%; P < .001) for each 10 g of alcohol (i.e., one drink) consumed per day. The same result was obtained, even after additional stratification for race, education, family history, age at menarche, height, weight, BMI, breast-feeding, oral contraceptive use, menopausal hormone use and type, and age at menopause.
Factors Associated With Decreased Risk of Breast Cancer
Active exercise may reduce breast cancer risk, particularly in young parous women.Numerous observational studies have examined the relationship between physical activity and breast cancer risk. Most of these studies have shown an inverse relationship between level of physical activity and breast cancer incidence. The average RR reduction is reportedly 30% to 40%. However, it is not known to what degree, if at all, the observed association is to the result of confounding variables, such as diet or a genetic predisposition to breast cancer. A prospective study of more than 25,000 women in Norway suggests that doing heavy manual labor or exercising 4 or more hours per week is associated with a decrease in breast cancer risk. This decrease is more pronounced in premenopausal women and in women of normal or lower-than-normal body weight. In a case-control study of African American women, strenuous recreational physical activity (>7 hours per week) was associated with decreased breast cancer incidence.