Expert Comment: Can coffee help in type 2 diabetes risk reduction?

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Siamak Bidel MD. PhD, National Institute for Health and Welfare (THL), and Hjelt Institute, University of Helsinki

At first, I admit I was surprised when we found out that simple coffee consumption, a daily pleasant lifestyle habit, may help to prevent diabetes later in the life. It seem so easy and hard to believe! But now we are quite confident about this association – the published results from across the world showed the same results. Moreover, four to five meta-analysis of these studies confirmed the consistency.

Since coffee is one of the main sources of caffeine in the diet, it can be difficult to directly separate an effect of caffeine from, say, coffee or tea. However, since decaffeinated coffee is reported to have a similar size association as regular coffee, it is unlikely that caffeine plays a role in the negative association for development of type 2 diabetes.

A number of mechanistic theories have explored which coffee components may protect against type 2 diabetes. Antioxidants, such as chlorogenic acid and trigonelline, have been found to reduce early glucose and insulin levels at 15 minutes in oral glucose tolerance tests (OGTT). Decaffeinated soluble coffee showed no effect in this test. These observations are in line with the data of a French study reporting the strongest association for coffee consumed at lunchtime.

Coffee could also partly inhibit high blood sugar following meals (postprandial hyperglycemia), and hence prevent the occurrence of type 2 diabetes. According to a cross-sectional multi-ethnic study, the effect of regular coffee is positively related to insulin sensitivity while decaffeinated coffee improves pancreatic beta-cells function.

Among the other mechanisms investigated, a Dutch study did not find that either magnesium, potassium, caffeine or blood pressure were involved in the coffee or tea association with type 2 diabetes. Another case-control study found a correlation between coffee consumption and plasma levels of sex-hormone-binding-globulin, which directly modulates intracellular signalling of sex hormones that play a critical role in type 2 diabetes development. This association was however not found for decaffeinated coffee and tea.

A 2011 randomized controlled trial analysed the effects of caffeinated and decaffeinated coffee on biological risk factors for type 2 diabetes. Participants who consumed either 5 cups per day of soluble caffeinated coffee, decaffeinated coffee, or no coffee for 8 weeks were compared. Caffeinated coffee was found to increase adiponectin and interleukin-6 concentrations, possibly reflecting anti-inflammatory and insulin sensitizing effects, whilst consumption of decaffeinated coffee decreased fetuin-A concentrations, a biomarker for inflammation and liver function. No significant differences were found between treatment groups for measures of glucose tolerance, insulin sensitivity and insulin secretion. The authors therefore concluded that improvements in adipocyte and liver function as indicated by changes in adiponectin and feutin-A concentrations may contribute to the beneficial metabolic effects of long term coffee consumption.

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