Green tea and coffee polyphenols may improve insulin sensitivity Original paper

Observational studies generally report that the consumption of coffee^[1] and/or green tea is associated with a reduced risk of type 2 diabetes.^[2] Both beverages are rich in polyphenols. Green tea is rich in catechins, such as epigallocatechin gallate, and coffee is most notably rich in chlorogenic acid. Does drinking a beverage containing green tea catechins and coffee chlorogenic acid improve insulin sensitivity?

In this 3-week randomized crossover trial, 11 healthy men (average age of 41; average BMI of 23) drank a beverage enriched in green tea catechins and chlorogenic acid (620 mg of green tea catechins, 373 mg of chlorogenic acid, and 119 mg of caffeine) or a placebo beverage (119 mg of caffeine) daily. After a washout period of 2 weeks, the participants crossed over to the other intervention. They were also instructed to limit their intake of green tea and coffee to once daily, and dietary intake was assessed during the last 3 days of each intervention using a questionnaire.

At the end of each intervention, blood was collected under fasting conditions and at 30, 60, 90, 120, 180, and 240 minutes after consuming a test meal (a high-fat cookie; 592 kcal, 43% of energy from fat, 50% of energy from carbohydrate) and the assigned beverage. The primary outcome was glucose area under the curve (AUC; the total rise in glucose during a given time) over the 4-hour period following consumption of the test meal. The secondary outcomes included insulin AUC, glucagon AUC, C-peptide AUC, GLP-1 AUC, glucose-dependent insulinotropic polypeptide (GIP) AUC, and insulin sensitivity (measured via the Matsuda index).

There was no difference between groups for glucose AUC, but the peak glucose level was lower in the intervention group (118.5 vs. 105.5 mg/dL). Insulin AUC, C-peptide AUC, and GIP AUC decreased, and glucagon AUC, GLP-1 AUC, and insulin sensitivity increased in the intervention group compared to the placebo group. The change in GLP-1 was positively correlated with the change in insulin sensitivity.

Because the participants were healthy, the results may not be generalizable to people with insulin resistance or type 2 diabetes.