Tea, coffee and diabetes

“Tea and coffee drinkers have a lower risk of developing type 2 diabetes,” reported the BBC, adding that the protection may not be down to caffeine since decaffeinated coffee has the greatest effect.

This story is based on a systematic review and meta-analysis that pooled data from studies of the association between tea and coffee consumption and the risk of developing type 2 diabetes. It found the more tea, coffee or decaffeinated coffee was drunk, the lower the risk of developing diabetes.

People should not drink more tea or coffee on the strength of this evidence. The review did not account for diet, exercise and lifestyle, and the studies included were varied. The results do, however, suggest that further research is warranted. Maintaining a healthy weight, choosing a sensible diet and participating in physical activity remain the best ways to protect against type 2 diabetes.

Where did the story come from?

This research was carried out by Dr Rachel Huxley and colleagues from the University of Sydney. It was funded by the National Heart Foundation of Australia. The study was published in the peer-reviewed medical journal Archives of Internal Medicine .

The newspapers highlighted the benefits of tea and coffee based on this research, but generally also reported that other factors such as diet and exercise also play a role.

What kind of research was this?

This was a systematic review and meta-analysis of a number of studies. The researchers say it has been suggested that coffee may be able to reduce the risk of developing type 2 diabetes. To investigate this, they searched scientific databases to look for prospective studies on coffee, decaffeinated coffee and tea that estimated the effect of these drinks on diabetes over time.

What did the research involve?

The researchers searched several scientific databases for relevant articles using the keywords ‘tea’, ‘coffee’, ‘decaffeinated coffee’ and ‘diabetes’. These search results were then condensed further, using the abstracts of the articles as guidance. Studies in animals, cross-sectional studies and case series (which did not include a time component to the analysis) were excluded, as were studies of only type 1 diabetes.

To be included, studies had to be prospective cohort studies giving an estimate of the risk of type 2 diabetes over time and the number of cups of tea, coffee and decaffeinated coffee consumed. They also had to take into account age and body mass index (BMI), which are major risk factors for type 2 diabetes.

This resulted in the pooling of data from 18 studies. As there was some variability in what each study tested, the researchers grouped the participants into two groups for analysis, those who drank three to four cups versus those who drank less than two cups or no cups. The researchers also classified tea drinkers as people who drank green, black or oolong tea rather than analysing each type of tea separately.

The study populations were predominantly white with 21% of the data derived from Asian cohorts.

The majority of the studies asked the participants to keep a drinks diary to record their tea or coffee consumption. In one study, participants were asked to estimate their tea and coffee consumption over the past 24 hours. Type 2 diabetes was either determined by the participants reporting their diagnosis or with an oral glucose-tolerance test.

What were the basic results?

The 18 studies had a total of 457,922 participants, in which coffee drinkers had a 25% lower risk of developing diabetes than people who drank no coffee or up to two cups of tea a day (relative risk [RR] 0.76, 95% confidence interval [CI] 0.69 to 0.82). As these studies were varied, the authors refined their analysis to 11 studies, which they adjusted for age, sex other confounders and found the same association. The researchers also found that smaller studies tended to show larger effects so decided to include only the six largest studies. This resulted in a smaller, 15% reduced risk of diabetes in people who drank three to four cups of coffee a day.

In six studies that looked at decaffeinated coffee (a total of 225,516 participants) individuals that drank three to four cups of decaffeinated coffee a day had approximately a third less chance of developing diabetes (RR 0.64, 95% CI 0.54 to 0.77).

The effect of tea on diabetes was investigated by seven studies with a total of 286,701 participants. The pooled results showed the risk of diabetes was about a fifth lower in people who drank more than three to four cups of tea per day compared to people who drank no tea (RR 0.82, 95% CI 0.73 to 0.94).

How did the researchers interpret the results?

The researchers concluded that “high intake of coffee, decaffeinated coffee, and/or tea is associated with a reduction in the risk of new-onset type 2 diabetes”.

They also say that although they would advise their patients most at risk of diabetes to increase their levels of physical activity and to lose weight, they might also advise patients to increase their tea and coffee consumption.

Conclusion

This research pooled and analysed data from studies of the association between drinking tea and coffee and the risk of developing type 2 diabetes, and found that these drinks reduced the risk. However, though this type of systematic review is stronger evidence than that from each of its individual components, it still has several limitations that the researchers highlight:

• The included studies varied greatly in the length of time the participants were followed up and their ages. Although age and BMI were taken into account, other factors that can increase the risk of diabetes, such as smoking, diet, lifestyle and exercise, were not.
• The studies varied greatly in the types of coffee and tea consumed and their preparation (for example filtered versus unfiltered coffee, cup size, cup strength, addition of milk or sugar and other variations). However, estimates of the protective effect did not vary greatly despite this meaning that it is difficult to know which component of tea or coffee is responsible for the effect.
• Most of the studies did not give data on the effects of these beverages or their components on measures of hyperglycaemia and insulin sensitivity (physical signs of diabetes). As such, the research does not provide further evidence on the biological mechanisms involved that may underlie the effects.
• Only 20% of the cohorts included in the review were from non-white populations. This is an important consideration given that the pattern of beverage consumption and background risk of diabetes may differ across ethnic groups. It may not be possible to generalise these findings to other populations.

People should not increase their tea or coffee consumption based on the findings of this review. However, the findings do indicate that further research into the effect of these beverages on diabetes is warranted.