What Matters

Coffee, caffeine, and glucose tolerance


Some of us are at least peripherally aware of the proposed link between coffee consumption and the development of diabetes. Fewer of us are aware of the apparent contradiction in this relationship: Caffeinated and decaffeinated coffee are associated with a lower risk of type 2 diabetes mellitus, but ingestion of caffeine and caffeinated coffee results in worsening of glucose tolerance prior to a glucose load or a meal. Compounds in coffee other than caffeine have beneficial effects on glucose metabolism.

So what are we telling our patients about coffee?

Certainly we should not be recommending coffee to many of our patients who already have issues with insulin sensitivity. Perhaps more data are needed to make us feel comfortable about the impact of acute ingestion of coffee on serum glucose concentrations.

Keizo Ohnaka, Ph.D., and colleagues conducted a randomized controlled clinical trial in Japan that may put us more at ease on this issue (J. Nutr. Metab. 2012 Nov. 5 [doi:10.1155/2012/207426]). In this study, overweight men with an average fasting glucose of 108 mg/dL were randomized to consumption of five cups of caffeinated or decaffeinated instant coffee per day or no coffee for 16 weeks. The primary outcome measure was the 75-g oral glucose tolerance test.

Men in the coffee group were instructed to prepare a cup or glass of coffee using one spoonful of instant coffee without sugar, milk, or any other additives.

The men who drank caffeinated coffee demonstrated a statistically significant decrease in the 2-hour concentrations and area under the curve of glucose. The decaffeinated-coffee and no-coffee groups demonstrated no significant changes.

Interestingly, waist circumference decreased in the caffeinated coffee group, increased in the decaffeinated group, and did not change in the no-coffee group. After adjusting for waist size changes, the authors concluded that both caffeinated and decaffeinated coffee appear to be protective against deteriorating glucose tolerance.

The study seems to support the general notion that compounds in coffee are the likely agents exerting positive effects on serum glucose. These compounds are substances with names that are challenging to remember, such as chlorogenic acids and polyphenols.

This study showed clearly that consumption of coffee – both caffeinated and decaffeinated – did not worsen glucose tolerance. The data remain consistent with evidence that caffeine and sugar result in worsening of blood glucose.

So, when counseling our patients about the benefits of coffee, tell them it needs to be "straight up" rather than "fancy" (mocha or latte) to work its magic.

Dr. Ebbert is professor of medicine and primary care clinician at the Mayo Clinic in Rochester, Minn. The opinions expressed are solely those of the author.

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