“Consuming as little as 1/4 to 1/3 cup of dairy milk per day was associated with an increased risk of breast cancer of 30%,” said first paper author Gary E. Fraser, MBChB, PhD, in a press release. “By drinking up to one cup per day, the associated risk went up to 50%, and for those drinking two to three cups per day, the risk increased further to 70% to 80%.”
The study included 52,795 women (mean age, 57.1 years; 29.7% were black) who were cancer free at the start of the study. Women were followed for nearly eight years and assessed for dietary intakes using food frequency questionnaires. In 1,011 calibration study subjects, six structured 24-hour dietary calls were implemented. Matched cancer registry data were used to identify incident invasive breast cancers.
The researchers did not observe a clear relationship between soy intake and breast cancer, independent of dairy, but a correlation was observed when evaluating higher intakes of dairy calories (hazard ratio [HR]=1.22; 95% confidence interval [CI], 1.05–1.40) and dairy milk (HR=1.50; 95% CI, 1.22–1.84) when comparing 90th to 10th percentiles of intakes; outcomes did not largely differ between full and reduced fat milks. There were no significant associations between cheese and yogurt and breast cancer. The authors further noted that when replacing median intakes of dairy milk users with those of soy milk drinkers, the correlated HR was 0.68 (95% CI, 0.55–0.85).
“Similar-sized associations were found among pre- and post-menopausal cases, with CIs also excluding the null in estrogen receptor (ER+, ER-), and progesterone receptor (PR+) cancers. Less biased calibrated measurement-error adjusted regressions demonstrated yet stronger, but less precise, HRs and CIs that still excluded the null,” the study authors further wrote.
U.S. dietary guidelines currently call for a daily consumption of three cups of milk, which has been shown by previous studies to have some health benefits. However, according to Dr. Frasier, the outcomes of the current study suggest those recommendations be taken with caution.
The study was published in the International Journal of Epidemiology.