Case-control and ecological studies have suggested that eating red meat increases the risk of breast cancer, but any putative association remains far from clear. Previous studies have generally focused on diet in mid-life or later. Conceivably components of red meat such as heterocyclic amines, heme iron, and residual exogenous hormones, which have estrogenic properties, could influence the breast cancer risk through actions on hormone receptors.
A previous analysis of premenopausal women 26 to 46 years of age in the Nurses' Health Study II (NHS-II) disclosed that animal fat intake did correlate with the risk of breast cancer. In addition, there was indication of an association with eating red meat-a major food source of animal fat. Four added years of follow-up now are available for analysis. The NHS-II cohort consists of 116,671 female registered nurses who have been followed up since 1989. A semiquantitative food frequency questionnaire was sent to participants in 1991, 1995, and 1999.
During 12 years of follow-up of 90,659 women through 2003, 1021 invasive breast cancers were documented in premenopausal women. Of 789 cancers with available data, 512 were positive for both estrogen receptor (ER) and progesterone receptor (PR). For all cancers, the highest intake of red meat was only weakly-and not significantly-associated with breast cancer, but a strong association was evident for ER+/PR+ tumors. No such association was found for ER-/PR- breast cancers. Compared with eating 3 or fewer servings of red meat each week, the multivariate risk ratio for ER+/PR+ breast cancer was 1.l4 (95% Confidence Interval, 0.90-1.45) for more than 3 to 5 or fewer servings per week; 1.42 (1.06-1.90) for more than 5 servings per week to 1 or fewer servings per day; 1.20 (0.89-1.63) for more than 1 to 1.5 or fewer servings per day; and 1.97 (1.35-2.88) for more than 1.5 servings per day. The respective relative risk figures for ER-/PR- breast cancer were 1.34, 1.21, 0.69, and 0.89. High intake of several individual red meat items also correlated with an increased risk of hormone-positive breast cancer. The difference in results between hormone-positive and hormone-negative breast cancers was statistically significant. Comparable results were obtained when grams were estimated rather than serving sizes of red meat intake, and also when quintiles of intake were calculated. Neither adjusting for other food groups nor excluding cancers diagnosed during the first 2 years of follow-up altered the findings. The relationship was stronger for cumulative average intake rather than for baseline or most recent intake.
Eating red meat was associated with an increased risk of hormone receptor-positive breast cancer in this large population of relatively young premenopausal women. The findings deserve further evaluation because of their public health implications. Unlike most risk factors for breast cancer, this one can readily be modified.