Purpose: We utilized the large, prospective NIH-AARP Diet and Health Study to further explore the hypothesis, suggested by two recent prospective cohort studies, that increased intake of coffee, tea, soda, and/or caffeine is associated with reduced adult glioma risk.
Methods: At baseline in 1995-1996, dietary intake, including coffee, tea, and soda, was assessed with a food frequency questionnaire. We used Cox proportional hazards models to calculate adjusted hazard ratios (HR) and 95 % confidence intervals (CI) for glioma risk in relation to beverage intake.
Results: During follow-up of 545,771 participants through 2006, 904 participants were diagnosed with glioma. We found no trends of decreasing glioma risk with increasing intake of specific beverages or total caffeine. HR patterns for consumption of the caffeinated versus decaffeinated form of each beverage were inconsistent with a specific caffeine effect. HR patterns of reduced glioma risk for most categories of beverage intake greater than "none" prompted a post hoc analysis that revealed borderline-significant inverse associations for any versus no intake of tea (HR = 0.84; 95 % CI, 0.69-1.03), total coffee plus tea (HR = 0.70; 95 % CI, 0.48-1.03), and soda (HR = 0.82; 95 % CI, 0.67-1.01).
Conclusions: The borderline-significant inverse associations could be explained by a threshold effect in which any beverage intake above a low level confers a beneficial effect, most likely due to beverage constituents other than caffeine. They could also be explained by non-drinkers of these beverages sharing unknown extraneous characteristics associated with increased glioma risk, or by chance.