High- and Low-fat Dairy Intake, Recurrence, and Mortality After Breast Cancer Diagnosis

Candyce H. Kroenke; Marilyn L. Kwan; Carol Sweeney; Adrienne Castillo; Bette J. Caan

Disclosures

J Natl Cancer Inst. 2013;105(9):616-623. 

In This Article

Discussion

Consistent with much previous literature, we found no overall association between dairy intake and recurrence or breast cancer– specific survival. However, and as expected, high-fat dairy intake was related to poorer breast cancer survival in long-term breast cancer survivors. We were unable to evaluate associations between high-fat dairy consumption and levels of estrogens. However, the suggestive association of high-fat dairy with breast cancer recurrence, the statistically significant association with breast cancer mortality, and the lack of association of low-fat dairy with breast cancer outcomes were consistent with the hypothesis that dairy fat intake may increase the risk of adverse postdiagnosis breast cancer outcomes through effects on estrogens. To the authors' knowledge, this is the first study of high- and low-fat dairy intake and breast cancer survival.

Dairy foods have been positively associated with hormonal cancers, including prostate,[31–33] postmenopausal endometrial cancer,[34] and ovarian[35] cancer, although not in all studies.[36,37] Previous studies have generally not analyzed high- and low-fat dairy separately, although one study that did[8] report findings with breast cancer risk was consistent with this study's findings. Additionally, in another study, high-fat dairy was positively and low-fat dairy inversely associated with prostate cancer survival.[25] In agreement with our findings, butter was also positively associated with risk of breast cancer in the EPIC study.[3] Similar to many previous studies of dairy and breast cancer incidence,[1,2,9,14,17] we found little evidence of an overall association between dairy and breast cancer outcomes.

Several studies have found inverse associations of dairy and breast cancer risk,[4,6,7,10,11] primarily in premenopausal women, suggesting that associations may differ by menopausal status. We had insufficient power to stratify by menopausal status because women in this cohort were primarily postmenopausal (75%) and there were few adverse outcomes in premenopausal women. Our null associations for low-fat dairy may also be because we were often unable to distinguish nonfat from low-fat items and many "low-fat" items contained considerable fat. Future research should attempt to evaluate whether associations differ for nonfat vs low-fat dairy items and whether associations differ by menopausal status.

Mechanisms proposed to influence breast cancer include calcium and vitamin D, given their potential to decrease cell proliferation and increase cell differentiation.[38] In this study, we found these nutrients were unrelated to breast cancer outcomes (data not shown). It was difficult to disentangle the effects of saturated fat intake and high-fat dairy given the large correlation between the two. Despite this correlation, when we examined associations of saturated fat alone, associations for saturated fat were weaker than for dairy and breast cancer outcomes (data not shown), suggesting that saturated fat intake consumption per se, including saturated fat from nondairy foods (eg, palm kernel, coconut, and cottonseed oils used commonly in processed foods or meat intake), may not be related to outcomes. These findings suggest that saturated fats in dairy foods specifically augment risk.

If high-fat dairy is associated with higher levels of estrogenic hormones, consuming plant-based milks or nonfat dairy products may be a reasonable approach for limiting risk of adverse outcomes, particularly when breast cancer treatment has been completed. A study in mice found that dairy fat augmented effectiveness of chemotherapy against tumor metastasis while protecting against its side effects,[39] suggesting possible benefits during early treatment. However, decreasing intake in favor of low-fat intake after treatment may be advisable and is consistent with general nutritional guidelines, which recommend nonfat or low-fat dairy instead of high-fat dairy, to minimize cardiovascular risk.[40]

A study strength was the ability to adjust for variables related to breast cancer severity, including stage, tumor size, nodal status, hormone receptor status, and HER-2 status as well as breast cancer treatment. A second strength was the ability to adjust carefully for reproductive history and lifestyle, demographic, and socioeconomic variables. Furthermore, we were able to assess dietary intake at two time points, which may better approximate habitual diet than diet measured at one time point.

One limitation, if women didn't respond, indicated they didn't know, or indicated multiple choices for the type of milk they consumed, whether high- or low-fat, we were not able to assign milk consumption to either the high- or low-fat category, which resulted in an underestimation of intake. In fact, studies of diet employing food frequency questionnaires underestimate intake generally.[41,42] We considered employing regression calibration methods to correct for measurement error. However, we lacked dietary validation data in LACE, and the use of data from other studies has been criticized.[43] Therefore, unless systematic bias influenced dietary estimates, effect estimates generated here likely underestimate the true effect of high-fat dairy on post–breast cancer outcomes, and we were not able to correct for this. We also had limited power to carefully examine specific dairy foods or associations stratified by variables such as estrogen receptor/progesterone receptor status that might help shed additional light on associations. Future studies should replicate these findings in a larger cohort, with data to enable correction for measurement error.

To summarize, greater intake of high-fat dairy was related to higher risk of breast cancer–specific and non–breast cancer mortality in this cohort of long-term, early-stage breast cancer survivors. High-fat dairy consumption may increase levels of estrogens, which may augment the risk of breast cancer recurrence and mortality.

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