Dairy and related nutrient intake and risk of uterine leiomyoma: a prospective cohort study

STUDY QUESTION: Is there an association between consumption of dairy foods and related nutrients and risk of uterine leiomyoma? SUMMARY ANSWER: While dairy consumption was not consistently associated with uterine leiomyoma risk, intake of yogurt and calcium from foods may reduce risk of uterine leiomyoma. WHAT IS KNOWN ALREADY: Two studies have examined the association between dairy intake and uterine leiomyoma risk with inconsistent results. Dairy foods have been inversely associated with inflammation and tumorigenesis, suggesting that vitamins and minerals concentrated in these dietary sources may influence uterine leiomyoma risk. STUDY DESIGN, SIZE, DURATION: A prospective cohort study was carried out using data collected from 81 590 premenopausal women from 1991 to 2009 as part of the Nurses' Health Study II cohort. PARTICIPANTS/MATERIALS, SETTING, METHODS: Diet was assessed with a validated food frequency questionnaire every 4 years. Cases were restricted to self-reported ultrasound or hysterectomy-confirmation uterine leiomyoma. Cox proportional hazards models were used to calculate hazard ratios (HRs) and 95% CI. MAIN RESULTS AND THE ROLE OF CHANCE: Eight thousand one hundred and forty-two cases of ultrasound or hysterectomy- confirmed uterine leiomyoma were diagnosed over an 18-year period. When compared to participants who consumed two servings a week of total dairy foods, participants who consumed four or more servings had a borderline significant 8% reduced risk of uterine leiomyoma (HR = 0.92, 95% CI = 0.85, 1.00 P-trend = 0.19). When the association between specific dairy foods and uterine leiomyoma was examined, the relation between dairy-food intake and uterine leiomyoma appeared to be driven primarily by yogurt consumption (HR for 2+ servings/day = 0.76; 95% CI = 0.55, 1.04 compared to <=4 servings/week; P-trend = 0.03); however, there was a small number of cases in the 2+ servings/day group (n = 39). Of the nutrients examined, the association was strongest for calcium from foods (HR fifth quintile = 0.92, 95% CI = 0.86, 0.99; P-trend = 0.04). LIMITATIONS, REASONS FOR CAUTION: Some cases of uterine leiomyoma were likely misclassified, particularly those that were asymptomatic. It is possible that dairy product constituents reduce uterine leiomyoma symptomology rather than development, giving the appearance of a protective effect on leiomyoma development: no data on uterine leiomyoma symptomology were available. We did not have vitamin and mineral concentrations from actual blood levels. Similarly, there is the potential for misclassification of participants based on predicted 25(OH)D, and changes in vitamin D supplementation over time may have impacted prediction models for 25(OH)D. Further, some error in the self-reporting of dietary intake is expected. Given our prospective design, it is likely that these misclassifications were non-differential with respect to the outcome, likely biasing estimates toward the null. WIDER IMPLICATIONS OF THE FINDINGS: While no clear association between overall dairy consumption and uterine leiomyoma risk was observed, our findings suggest that intake of yogurt and calcium from foods may reduce risk of uterine leiomyoma.