Re: High- and low-fat dairy intake, recurrence, and mortality after breast cancer diagnosis.

Kroenke et al. (1) recently reported positive associations between high-fat, but not lowfat, dairy consumption and all-cause and breast-cancer mortality in the Life After Cancer Epidemiology (LACE) cohort of women with early-stage breast cancer. The authors suggested that high-fat dairy intake may worsen breast cancer prognosis possibly through an increase in circulating estrogen levels. To further investigate the issue, we used follow-up information until 2006 (median = 12.6 years) on 1453 women (aged 23–74 years) diagnosed with breast cancer between 1991 and 1994 who were previously enrolled in an Italian case–control study (2). Women were interviewed on their dietary habits before diagnosis using a validated food-frequency questionnaire (3). We considered whole milk, cheese, yogurt, ice cream, and desserts as high-fat dairy and part-skim/skim milk and ricotta/mozzarella cheese as low-fat dairy. Among 503 observed deaths, 398 were due to breast cancer. Hazard ratios (HRs) of death with 95% confidence intervals (CIs) were estimated using Cox proportional hazard models adjusted for age at diagnosis, geographical area, year of diagnosis, tumor stage, estrogen/progesterone receptor status, and potential confounders (2). No association emerged between dairy consumption and all-cause (HR for ≥2 servings/day vs <1 serving/day = 1.11, 95% CI = 0.84 to 1.47) or breast-cancer mortality (HR for ≥2 servings/day vs <1 serving/day = 1.06, 95% CI = 0.77 to 1.46) (Table 1). Null results were also found for high(all-cause HR for ≥1 servings/day vs <0.5 serving/day = 0.95, 95% CI = 0.68 to 1.34) or low-fat dairy intake (all-cause HR for ≥1 servings/day vs <0.5 serving/ day = 1.05, 95% CI = 0.84 to 1.33). The analyses on major contributors of dairy intake in our study showed a borderline statistically significant increased mortality for milk (all-cause HR for ≥1 servings/day vs <0.5 serving/day = 1.19, 95% CI = 0.97 to 1.46) and a reduced mortality for cheese (all-cause HR for ≥1 servings/day vs <0.5 serving/day = 0.74, 95% CI = 0.56 to 0.99). Similar patterns were found when considering only breast cancers at stages I and II. The associations between mortality and other dietary factors, except for a negative association with fruit and vegetables, were not statistically significant (2). Compared with the LACE cohort (1), our study had similar magnitude and length of follow-up, but dietary habits were assessed only before diagnosis. Women in the LACE study had early-stage cancers and were primarily postmenopausal. Our results did not show statistically significant changes in strata of tumor stage or menopausal status (data not shown). High-fat dairy was more frequently consumed in our study population— accounting for 79% of total dairy— than in the LACE cohort (median intakes = 1.4 and 0.5 servings/day for our study vs the LACE cohort, respectively), and low-fat dairy was far less common (median intakes = 0.2 and 0.7 servings/ day for our study vs the LACE cohort, respectively). Zero-fat dairy products are rarely consumed in Italy. Notably, approximately 70% of high-fat dairy in the LACE study derived from butter, whereas in Italy butter is not widely used (4,5). Butter consumption could be a proxy of an unhealthy diet (5), as also suggested by the high correlation between high-fat dairy and saturated fat intake reported in the LACE study (1). In conclusion, dairy intake was not associated with breast cancer prognosis in this Italian population, and high cheese intake, in our study, was instead associated with lower mortality. These findings remind us that different bias and confounding may influence the associations of dietary habits with cancer survival in different populations.