The investigation was a meta-analysis of three large observational studies: 1) female nurses aged 30-55 (1986-2006), younger female nurses aged 25-42 (1991-2007), and male professionals 40-75 (1986-2006). Detailed information on diet, lifestyle, medical conditions, and other chronic diseases was collected every 2 to 4 years for over 20 years in all three studies.
The authors evaluated 4 year changes in coffee (caffeinated or decaffeinated) and tea intake in relation to the risk of type 2 diabetes in the following 4 years. Information was self-reported.
The results were as follows:
"The authors documented 7,269 incident type 2 diabetes cases, and found that participants who increased their coffee consumption by more than 1 cup/day (median change=1.69 cups/day) over a 4-year period had a 11% lower risk of type 2 diabetes in the subsequent 4-years compared to those who made no changes in consumption. Participants who decreased their coffee intake by 1 cup a day or more (median change = -2 cups/day) had a 17% higher risk for type 2 diabetes. Changes in tea consumption were not associated with type 2 diabetes risk. Those with highest coffee consumption and who maintained that consumption -- referred to as "high-stable consumers" since they consumed 3 cups or more per day -- had the lowest risk of type 2 diabetes, 37% lower than the "low-stable consumers" who consumed 1 cup or less per day."
To summarize, increasing coffee consumption by 1.7 cups a day led to a 11% lower risk of type 2 diabetes, whereas decreasing coffee consumption by 2 cups a day led to a 17% higher risk. People who consumed 3 cups or more per day had a 37% lower risk than those who consumed 1 cup or less per day. Drinking tea had no effect. Decaffeinated coffee consumption had a smaller effect than caffeinated coffee.
What are the possible explanations for this correlation?
One possibility that the authors mention is that there is "reverse causation" in which those at risk for type 2 diabetes (because of high blood pressure, elevated cholesterol, cardiovascular disease, etc.) may reduce their coffee consumption.
Another possibility is that the reduction in diabetes risk arises from some additional factor that correlates with drinking coffee For example, it could be that people who drink more coffee are also more likely to exercise. Ideally these additional factors are controlled for in the study, and indeed the researchers did adjust for exercise.
A third possibility is that some ingredient in coffee other than caffeine is responsible for the correlation. This explanation seems less likely given the reduced impact of decaffeinated coffee, and the fact that tea showed basically no change in diabetes risk. Tea contains about 26 mg of caffeine per cup, whereas coffee contains approximately 95 mg of caffeine per cup.
My personal pet theory (speculation) is that the reduced type 2 diabetes risk has something to do with the stimulant properties of caffeine. Two other more potent stimulants methamphetamine and cocaine are associated with weight loss for long-term users although the exact reasons why are not completely understood. Naively, a stimulant would cause more metabolic activity and perhaps less appetite leading to loss of weight (i.e. becoming less obese) which would reduce the risk of type 2 diabetes.
In the future, it would be interesting to examine whether increased intake of alternative sources of caffeine (i.e. that are not associated with high levels of sugar such as No-Doz tablets) also lead to a lower risk of type 2 diabetes.
Thus, although we don't have a good explanation for the connection, the data is growing stronger that there is a correlation between drinking coffee and reducing risk for type 2 diabetes.
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