Other coffee constituents as potential mechanism

Caffeine

Since coffee and tea are the main sources of caffeine in the diet in most countries, it is difficult to separate the potential effect of caffeine from coffee per se. However, since decaffeinated coffee is reported to have a similar size association as regular coffee, it is unlikely that caffeine plays a major role in the inverse association for development of type 2 diabetes^3,6-8.

Paradoxically, research has suggested that an acute (typically short-term, high-level) amount of caffeine intake leads to glucose intolerance and insulin insensitivity, while chronic (typically longer-term, low-level) caffeine intake has only a small effect on glucose metabolism^31,32.

• One 2009 study looked more closely at the effect of acute caffeine amounts on glucose tolerance in women with or without gestational diabetes. Caffeine did not affect glucose and insulin levels in the group without gestational diabetes, but it did impair insulin sensitivity in women with gestational diabetes³³

Constituents within coffee

Coffee contains many other constituents, such as polyphenols which include chlorogenic acids (CGAs). Research suggests that these components may have an effect on glucose metabolism, and may be associated with a reduced risk of developing type 2 diabetes.

Polyphenols in coffee

• A 2016 analytical study of key biomarkers for diabetes suggested that coffee polyphenols may have anti-diabetic effects, but further replications with large samples of both genders were recommended³⁴

• A 2018 preliminary human study suggested that individual polyphenols in coffee may have specific effects, including improvement of glycemic control in subjects with insulin resistance, reduction of blood glucose and HbA1c levels and the improvement of insulin secretion and resistance. However, further work in this area is required to clarify these effects³⁵

Research suggests that the chlorogenic acids (CGAs), a type of polyphenol, may downregulate fasting glucose and plasma glucose peak in glucose tolerance tests by attenuating intestinal glucose absorption³⁶

• One 2008 study showed that CGA may reduce early glucose and insulin levels at 15 minutes in oral glucose tolerance tests (OGTT), although decaffeinated soluble coffee showed no effect in this test⁹‍

• A further 2018 study suggested that coffee intake was positively associated with fasting and the first hour insulin levels and inversely associated with 2^nd and 3^rd hour plasma glucose levels during the OGTT³⁷

• Literature reviews in 2009 and 2017 respectively, suggested that CGA may have a significant impact on glucose metabolism regulation and, therefore, on related disorders including diabetes^10,11

• Additionally, a 2018 study suggested that CGA may be involved in both lipid and glucose metabolism regulation and may be associated with a lower risk of hepatic steatosis, cardiovascular disease, and diabetes. It may also have hepato-protective effects by protecting from chemical or lipopolysaccharide-induced injuries³⁸‍

• A 2020 review of the role of chlorogenic acid and metabolites on glycaemic responses suggested that there is no direct effect of CGAs on post-prandial blood glucose or insulin, but some effects are observed on glucose absorption and utilisation. The study concluded that although chlorogenic acids do not seem to affect carbohydrate digestion directly, they may affect glucose absorption and subsequent utilisation³⁹

• A further 2020 review of dietary and metabolomic data from the Nurses’ Health Study suggested that coffee consumption is associated with widespread metabolic changes in women, among which lipid metabolites maybe critical. Trigonelline and both polyphenol and caffeine metabolites were the main metabolites associated with a reduced risk of type 2 diabetes in this study⁴⁰

Antioxidants

It has been suggested that coffee may also contribute to the total antioxidant load of the diet, and in turn may help to limit oxidative stress and consequently, the development of type 2 diabetes^{41, 42}. This potential role for coffee is interesting, but the hypothesis has not yet been verified.