Dairy consumption and incident risk of thyroid cancer in Japan: a pooled analysis of the Miyagi Cohort Study and the Ohsaki Cohort Study

Purpose: The impact of dairy consumption on thyroid cancer is unclear. The purpose of this study was to elucidate the association between dairy consumption and the risk of thyroid cancer in Japanese people. Methods: The association between dairy consumption and the risk of thyroid cancer in Japanese people was examined by conducting a pooled analysis of two prospective studies of residents in Miyagi Prefecture, Japan. Data from 64,340 men and women aged 40–79 years registered in the Miyagi Cohort Study in 1990 and in the Ohsaki Cohort Study in 1994 were analyzed. Dairy consumption was assessed at baseline using a self-administered food frequency questionnaire and was divided into quartiles based on the weight (in grams) of total dairy consumption per day. Results: During 1,075,018 person-years of follow-up, there were 190 incident cases of thyroid cancer (29 men and 161 women). The hazard ratios (HRs) and 95% confidence intervals (CIs) for thyroid cancer incidence in the highest quartile of dairy consumption compared with the lowest quartile were 0.83 (95% CIs 0.28–2.43, P-trend = 0.823) for men and 0.67 (95% CIs 0.42–1.06, P-trend = 0.056) for women. After stratification for BMI, a decreased risk was observed in women with BMI ≥ 25 kg/m (HRs: 0.37, 95% CIs 0.18–0.79, P-trend = 0.010). Conclusion: Dairy consumption is inversely associated with the risk of thyroid cancer in women with BMI ≥ 25 kg/m.