Adolescent Diet and Risk of Breast Cancer
Background: Early life exposures, including diet, have been implicated in the etiology of breast cancer.
Methods: A nested case-control study was conducted among participants in the Nurses' Health Study who completed a 24-item questionnaire about diet during high school. There were 843 eligible cases diagnosed between onset of study (1976) and before the return of the high school diet questionnaire (1986), who were matched 10:1 with controls on the basis of age.
Results: Women who had, during adolescence, a higher consumption of eggs, vegetable fat and fiber had a lower risk of breast cancer, whereas risk of breast cancer was increased among women who consumed more butter.
Conclusions: A possible association of elements of adolescent diet with risk of breast cancer is reported, but the findings require confirmation in prospective study.

The evidence that adolescent diet may affect the risk of breast cancer derives from several lines of evidence. Rates of breast cancer among Asian immigrants to the United States do not approach those of US white women until the second or third generation, suggesting that exposures during childhood and adolescence are important in establishing a higher risk of breast cancer. Norwegian women who were adolescents during World War II, when average caloric intake decreased by 22%, have a reduced incidence of breast cancer, suggesting that energy restriction might affect risk. Similarly, in animal models, energy restriction in the peripubertal period inhibits mammary tissue proliferation and reduces the subsequent risk of mammary tumors. Exposure of rats to carcinogens before first pregnancy increases the incidence of mammary tumors compared with exposure after first pregnancy. After differentiation of the mammary gland at the time of first full-term pregnancy of the rat, the rate of cell division decreases and length of the cell cycle increases, allowing more time for DNA repair. This biologic phenomenon might explain the apparent vulnerability of the adolescent breast tissue to carcinogenic exposures. Among atomic bomb survivors and women exposed to ionizing radiation as part of their treatment for Hodgkin's disease, the risk of breast cancer increases with younger age at exposure.

Although this analysis was principally an exploratory analysis to guide prospective studies, the authors had previously proposed that certain elements of adolescent diet might be protective against future risk of breast cancer. For instance, we and others have proposed that fiber might be protective by lowering endogenous estrogen levels and that folate might be protective owing to its role in DNA methylation.