Weight and weight changes in early adulthood and later breast cancer risk.

Obesity is a well-established cause of postmenopausal breast cancer. However, early life adiposity is inversely associated with breast cancer incidence. To understand these conflicting relations, we use validated measures to assess adiposity in childhood and late adolescence, as well as weight change, in relation to total invasive breast cancer incidence and receptor subtypes. We conducted a prospective observational study among 74,177 women from the Nurses' Health Study from 1980-2012, with updated risk factors every 2 years during which 4,965 incident invasive breast cancers occurred. Overall, weight at age 18 was inversely associated with both premenopausal (HR per 30 kg = 0.52, 95% CI = 0.39-0.71) and postmenopausal (HR per 30 kg = 0.81, 95% CI = 0.72-0.92) breast cancer which was largely explained by adiposity at age 10. Long-term weight gain from age 18 both during premenopause and postmenopause were positively associated with postmenopausal breast cancer risk. However, premenopausal weight gain was not related to premenopausal breast cancer risk. Furthermore, weight gain since age 18 was positively associated with ER+/PR+ postmenopausal breast cancer (HR per 30 kg = 1.50, 95% CI = 1.36-1.65) but not ER+/PR- (HR per 30 kg = 0.96, 95% CI = 0.78-1.19) or ER-/PR- (HR per 30 kg = 1.16, 95% CI = 0.95-1.42) postmenopausal breast cancer. Overall, 17% of ER+/PR+ postmenopausal breast cancer and 14% of total postmenopausal breast cancer are attributable to weight gain of > 5 kg since age 18.