According to Rachel Huxley, PhD, of the George Institute for International Health in Sydney, Australia, in a meta-analysis of 18 studies, drinking three to four cups of coffee per day was associated with a 25% lower risk of diabetes than drinking two cups or less per day (RR 0.76, 95% CI 0.69 to 0.82). There were similar results for decaf coffee and tea.
"If such beneficial effects were observed in interventional trials to be real, the implications for the millions of individuals who have diabetes, or who are at future risk of developing it, would be substantial," the researchers concluded in the Dec. 14/28, 2009 Archives of Internal Medicine.
Over the years, a variety of investigators have reported that coffee and tea consumption are inversely associated with Type 2 diabetes. To sort out the data, Huxley and colleagues conducted a meta-analysis of 18 prospective studies between 1966 and July 2009 with information on 457,922 patients.
The researchers found a significant inverse relationship between coffee consumption and subsequent risk of diabetes. Each additional cup of coffee consumed in a day was associated with a 7% reduction in the excess risk of diabetes (95% CI 0.91 to 0.95, P<0.001).
The researchers said the heterogeneity across studies was independent of effects involving gender, geographic region, or the method of diagnosis vs. self-report.
Six of the studies reported on the association between drinking decaffeinated coffee and subsequent risk of diabetes.
A pooled summary estimated that those who drank more than three to four cups of decaf coffee per day had about a third lower risk of diabetes than those who didn't drink any decaf (RR 0.64, 95% CI 0.54 to 0.77). Seven studies also looked at the association between tea and diabetes risk. Again, pooled summaries showed that patients who drank more than three to four cups of tea per day had about a 20% lower risk of diabetes than those who drank no tea (RR 0.82, 95% CI 0.73 to 0.94).
The researchers noted that the coffee findings may be an overestimate due to "small-study bias," and cautioned that any possibility that the association between coffee and diabetes risk is age-dependent warrants further investigation.
The findings suggest that the protective effects of tea and coffee may not be solely related to the effects of caffeine, but rather involve a broader range of chemical constituents including magnesium, lignans, and chlorogenic acids, the researchers wrote.
Tea catechins, for example, may decrease glucose production in the gastrointestinal system, leading to lower levels of glucose and insulin, and green tea in particular may prevent damage to pancreatic beta cells.
The study was limited by the potential for uncontrolled confounding, and because it precludes a more detailed analysis of the effect of adjustment for confounders at an individual level. Also, it may be limited in its generalizability because only 20% of cohorts were from nonwhite populations.
Lars Rydén, MD, of the Karolinska Institute in Sweden, a spokesperson for the European Society of Cardiology, called the study a "cautiously and carefully conducted meta-analysis."
"There are sometimes claims that coffee may do harm, that it may increase the propensity to cardiovascular disease, but there is no evidence for this," Dr. Rydén said. "The message is that people may drink coffee safely. Coffee from this point of view may actually be of benefit, as well as reducing the risk of getting diabetes -- although the reduction is small."
But Rydén noted that other lifestyle issues are far more important than coffee intake. "Coffee helps, but other things are even more important," he said. "Those who are overweight should reduce their bodyweight by 5% to 10% -- not too much -- and include physical activity, such as a brisk walk for 30 minutes a day. Then those people who are at risk of developing diabetes will reduce this risk by 40 to 50%."
- Explain that in a meta-analysis of 18 studies, tea, coffee, and decaf coffee were associated with a significantly reduced risk of Type 2 diabetes.
Huxley R, et al "Coffee, decaffeinated coffee, and tea consumption in relation to incident Type 2 diabetes mellitus" Arch Intern Med 2009; 169(22): 2053-63.