Coffee and Caffeine Consumption in Relation to Sex Hormone–Binding Globulin and Risk of Type 2 Diabetes in Postmenopausal Women

OBJECTIVE: Coffee consumption has been inversely associated with type 2 diabetes risk, but its mechanisms are largely unknown. We aimed to examine whether plasma levels of sex hormones and sex hormone–binding globulin (SHBG) may account for the inverse association between coffee consumption and type 2 diabetes risk. RESEARCH DESIGN AND METHODS: We conducted a case-control study nested in the prospective Women's Health Study (WHS). During a median follow-up of 10 years, 359 postmenopausal women with newly diagnosed type 2 diabetes were matched with 359 control subjects by age, race, duration of follow-up, and time of blood draw. RESULTS: Caffeinated coffee was positively associated with SHBG but not with sex hormones. Multivariable-adjusted geometric mean levels of SHBG were 26.6 nmol/l among women consuming ≥4 cups/day of caffeinated coffee and 23.0 nmol/l among nondrinkers (P for trend = 0.01). In contrast, neither decaffeinated coffee nor tea was associated with SHBG or sex hormones. The multivariable-adjusted odds ratio (OR) of type 2 diabetes for women consuming ≥4 cups/day of caffeinated coffee compared with nondrinkers was 0.47 (95% CI 0.23–0.94; P for trend = 0.047). The association was largely attenuated after further adjusting for SHBG (OR 0.71 [95% CI 0.31–1.61]; P for trend = 0.47). In addition, carriers of rs6259 minor allele and noncarriers of rs6257 minor allele of SHBG gene consuming ≥2 cups/day of caffeinated coffee had lower risk of type 2 diabetes in directions corresponding to their associated SHBG. CONCLUSIONS: Our findings suggest that SHBG may account for the inverse association between coffee consumption and type 2 diabetes risk among postmenopausal women.