A cohort study that followed nearly 5,000 patients for approximately five years found a dose-response relationship for both beverages. Drinking one cup of green tea every day was associated with a 15% lower mortality risk compared with those who drank no green tea; having two to three cups daily was associated with a 27% reduction; and drinking four or more cups was associated with a 40% drop in risk (P=0.001 for trend).  Similarly, as shown in the study online in BMJ Open Diabetes Research & Care, among coffee drinkers, one cup a day was associated with a 12% lower mortality risk, and two or more cups with a 41% reduction compared with those who drank no coffee (P=0.001 for trend). Furthermore, the risk of death was even lower for those who drank both beverages daily: 51% lower for two to three cups of green tea plus two or more cups of coffee; 58% lower for four or more cups of green tea plus one cup of coffee every day; and 63% lower for a combination of four or more cups of green tea and two or more cups of coffee daily (HR 0.37, 95% CI 0.18-0.77, P for trend not given). “To date, no study has investigated the combined effect of green tea and coffee consumption on all-cause mortality,” the researchers wrote. “The present study determined that combined higher green tea and coffee consumption markedly reduced mortality. Further, this cohort study included potential confounders, such as sleep duration, diabetic complications, lifestyle, physical activity, laboratory data, and medications. 

“In conclusion, this prospective cohort study demonstrated that greater consumption of green tea and coffee was significantly associated with reduced all-cause mortality: the effects may be additive,” the study authors wrote. “Our results suggest that consuming green tea and coffee may have beneficial effects on the longevity of people with type 2 diabetes.” 

BMJ, Open Diab Res Care 2020; Additive effects of Green Tea and Coffee on mortality in T2D