The systematic review and meta-analysis was the work of first author Dr Rachel Huxley, of The George Institute for International Health, University of Sydney, Australia and colleagues from the UK, The Netherlands, France and the US, and was published online in the Archives on Internal Medicine on December 14.
Estimates suggest that by 2025 there will be about 380 million people with type 2 diabetes around the world.
The extent to which diet and lifestyle contributes to this remains somewhat uncertain, despite considerable research attention, except perhaps for studies that consistently point to obesity and physical inactivity, wrote the authors.
They also said that a number of studies have reported higher levels of coffee consumption are linked to lower risk of developing type 2 diabetes mellitus, and similar links have been reported for decaffeinated coffee and tea.
And while an earlier meta-analysis suggested there might be a link between coffee drinking and reduced risk of diabetes, since then the number of studies has more than doubled, so they decided to do a new one.
For their investigation, Huxley and colleagues searched for prospective studies published between 1966 and July 2009 that reported an estimate of the link between coffee, decaffeinated coffee, or tea with incident diabetes and found 18 studies covering a total of 457,922 participants.
Among these, six studies covering over 225,000 participants included information about decaffeinated coffee, whereas seven studies with over 286,000 participants reported on tea consumption.
When they pooled and analyzed the data, they found that:
- Drinking coffee was inversely related to risk of diabetes, ie more coffee was linked to lower risk.
- After adjusting for potential confounders, every extra cup of coffee consumed in a day was linked to a 7 per cent reduction in the excess risk of diabetes.
- Drinking 3 to 4 cups per day was linked to a 25 per cent lower risk than drinking none or up to two cups per day.
- In those studies that assessed decaffeinated coffee consumption, drinking more than 3 to 4 cups a day was linked to about one third lower risk of diabetes compared to none.
- Drinking more than 3 to 4 cups of tea per day was linked to a one fifth lower risk of diabetes compared to non-tea drinking.
"High intakes of coffee, decaffeinated coffee, and tea are associated with reduced risk of diabetes. The putative protective effects of these beverages warrant further investigation in randomized trials."
They also said their results should be interpreted with caution since "owing to the presence of small-study bias, our results may represent an overestimate of the true magnitude of the association".
The authors suggested the effect of tea and coffee consumption on diabetes risk could be due to direct biological effects, since their apparent protective effect appears to be independent of potential confounding variables.
The apparent link between decaffeinated coffee and diabetes risk caffeine alone is unlikely to be the reason. It could partly be due to other compounds present in tea and coffee, such as magnesium, antioxidants known as lignans or chlorogenic acids, they suggested.
"If such beneficial effects were observed in interventional trials to be real, the implications for the millions of individuals who have diabetes mellitus, or who are at future risk of developing it, would be substantial," they wrote.
If we could find out what these active compounds are, we might find possible new ways of preventing type 2 diabetes, they added.
"It could also be envisaged that we will advise our patients most at risk for diabetes mellitus to increase their consumption of tea and coffee in addition to increasing their levels of physical activity and weight loss," wrote the authors.
Related reading: Drinking Coffee: More Good Than Harm? (9 July 2012).