Potential mechanisms

Researchers have yet to discover a plausible mechanism to explain the associations between consumption of regular coffee, decaffeinated coffee or tea and a lower risk of developing type 2 diabetes.

A role for caffeine?

Since coffee and tea are the main sources of caffeine in the diet in most countries, it is difficult to directly separate an effect of caffeine from either 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.

Paradoxically, acute caffeine intake leads to glucose intolerance and insulin insensitivity, while chronic caffeine has only a small effect on glucose metabolism²³. An acute study tested for the effects of caffeine in women with or without gestational diabetes. Caffeine did not affect glucose and insulin levels in the group without gestational diabetes. However caffeine impaired insulin sensitivity in women with gestational diabetes²⁴.

Other constituents

Other coffee constituents, in particular antioxidants like chlorogenic acid and trigonelline, reduce early glucose and insulin levels at 15 minutes in oral glucose tolerance tests (OGTT). Decaffeinated soluble coffee shows no effect in this test²⁵. These observations are in good accordance with the data of a French study reporting the strongest association for coffee consumed at lunchtime⁹.

Coffee could also partly inhibit postprandial hyperglycemia and hence prevent the occurrence of type 2 diabetes²⁶. According to a cross-sectional multi-ethnic study on 954 non-diabetic adults, the effect of caffeinated coffee is positively related to insulin sensitivity while decaffeinated coffee improves pancreatic beta-cells function²⁷.

Effect on subclinical inflammation – a new hypothesis

A Finnish study tested the effects of progressively increasing coffee consumption in obese volunteers in a medium term intervention trial (in the first month, participants abstained from coffee, for the second month 4 cups of coffee were consumed per day and in the third month, participants had 8 cups per day). No effects in the OGTT were seen. Coffee consumption appeared to have beneficial effects on some markers of subclinical inflammation, considered to be risk factors for type 2 diabetes²⁸.

A Greek study, which controlled for oxidative stress and inflammatory biomarkers, suggested that the inverse association between habitual coffee drinking and diabetes was found to be mediated by serum amyloid A levels (serum amyloid A is a lipoprotein that may be associated with chronic inflammation in the body)²¹.

Because of its high content of antioxidant compounds, coffee could contribute to the total antioxidant capacity of the diet that is necessary to reduce oxidative stress. Oxidative stress can lead to favorable conditions for the development of type 2 diabetes^29,30. This hypothesis has not yet been verified.

Other areas under investigation

Among the other mechanisms studied, a Dutch study did not find that either magnesium, potassium, caffeine or blood pressure were involved in coffee or tea’s association with type 2 diabetes⁸.

One Japanese study reported a link with psychological factors, perceived mental stress in men and Type-A behaviour in women³¹. Interestingly, further research has suggested that coffee consumption was inversely associated with depressive symptoms amongst a group of patients with type 2 diabetes. The authors suggested this might be due to biologically active compounds in coffee in addition to caffeine³².

A US case-control study found a correlation between coffee consumption and plasma levels of sex-hormone-binding-globulin, which directly modulate intracellular signalling of sex hormones that, themselves, play a critical role in type 2 diabetes development. This association was not found for decaffeinated coffee and tea³³.

In 2011, researchers at Harvard conducted a randomized controlled trial looking at the effects of caffeinated and decaffeinated coffee on biological risk factors for type 2 diabetes. They compared participants who consumed either 5 cups per day of soluble caffeinated coffee, decaffeinated coffee, or no coffee for 8 weeks. Compared with consuming no coffee, consumption of caffeinated coffee increased 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 concluded that improvements in adipocyte and liver function, as indicated by changes in adiponectin and feutin-A concentrations, may contribute to beneficial metabolic effects of long-term coffee consumption³⁴.

Coffee consumption has been related to a decreased risk of type 2 diabetes mellitus among those with high levels of serum γ-glutamyltransferase. A 2012 study in a Japanese population examined the association between coffee and glucose tolerance and the effect modification of serum γ-glutamyltransferase on this association. These authors found coffee drinking to be protective against glucose intolerance. Furthermore, they suggested that the observed possible effect modification of serum γ-glutamyltransferase on the protective association between coffee and type 2 diabetes warrants further research³⁵.