Caffeine, whether consumed as coffee or other sources, is popular with many people today; it gives us the little extra burst of energy we think we need to get through the day. Sources of caffeine include not only coffee but also soft drinks, tea and chocolate.

Scientific studies of the effects of coffee and/or caffeine in people with diabetes have given contradictory results, and whether it's reasonable for someone with diabetes to continue to drink coffee, or switch to decaffeinated coffee, or quit, is a matter of opinion. Major authorities, such as the American Diabetes Association, have not issued recommendations; the ADA states "The few small clinical studies involving diabetes and functional foods thought to have high antioxidant potential (e.g., tea, cocoa, coffee) are inconclusive." Some of the more important issues about use of caffeine by people with diabetes (or at risk of developing diabetes) are discussed below.

People who drink coffee regularly may lower their risk of developing type 2 diabetes

A review of 9 studies of coffee consumption and risk of type 2 diabetes (T2DM), including 193,473 participants and 8,394 new cases of type 2 diabetes, compared relative risks of developing T2DM for highest (≥6 or ≥7 cups per day) and second highest (4-6 cups per day) categories of coffee consumption compared with the lowest consumption category (0 or ≤2 cups per day). In studies conducted in both Europe and Japan, higher coffee consumption was consistently associated with a lower prevalence of newly detected hyperglycemia, particularly postprandial hyperglycemia.The authors conclude that habitual coffee consumption is associated with a substantially lower risk of T2DM. (1)

Coffee may make blood glucose rise in people with T2DM

Several studies make this point. An example: 10 habitual coffee drinkers with T2DM who drank brewed coffee daily were switched to either caffeine-containing pills, or placebos. The authors found that caffeine had adverse effects on glucose metabolism, producing higher average daytime glucose concentrations and exaggerated postprandial glucose responses in these patients. (2)

Regular coffee consumption is not associated with increased risk for cardiovascular events or mortality in T2DM

Researchers conducted a prospective study including 3,497 men with T2DM but without cardiovascular disease at baseline. After adjustment for age, smoking, and other cardiovascular risk factors, they compared the relative risks for cardiovascular events and for all-cause mortality for patients consuming ≥4 cups/day of caffeinated coffee compared with those for non-coffee drinkers. These data indicate that regular coffee consumption is not associated with increased risk for cardiovascular events or mortality in men with T2DM. (3)

Caffeine cuts nighttime hypoglycemia in type 1 diabetes

In a double-blinded randomized study, researchers investigated the effect of caffeine given in capsules versus placebo in 19 patients with long-standing type 1 diabetes (T1DM) using continuous glucose sensing technology and simultaneous assessment of autonomic function using Holter monitoring. They found that caffeine reduced the duration of nocturnal hypoglycemia with a mean duration of 49 minutes (range 0-235) versus 132 (0-468) minutes. The reduction in duration of nighttime hypoglycemia was due to a decline in the number of episodes of moderate hypoglycemia at the expense of mild hypoglycemic episodes. There was no overall correlation between reduced heart rate variability (a marker of autonomic nervous system malfunction) and hypoglycemic events. They concluded that caffeine is associated with a significant reduction in nocturnal hypoglycemia. The reduction in nocturnal hypoglycemia was not linked to the concomitant rise in parasympathetic activity that is associated with caffeine. (4)

Is coffee safe?

With all these studies, the issue basically is safety. It remains unclear how much of the effects that have been seen are related to caffeine (in which case the data should be similar for tea and soft drink consumers) or to some non-caffeine constituent in coffee -- in which case decaffeinated coffee might show the same results as regular coffee. To summarize the data presented above, what to do seems to depend upon whether there's T1DM, T2DM, or a risk of future diabetes. If there's a risk of developing diabetes in the future, coffee may lower the risk of getting diabetes. If there is T2DM, coffee may make the blood glucose rise, but there may be no increased risk of cardiovascular events. If there's T1DM, coffee drinkers may have less problems with nocturnal hypoglycemia.

What should a coffee drinker do? Realistically, at this time, there's insufficient data to make any recommendations to change patterns of coffee consumption for people with diabetes. (5)

Also see:

(1) Coffee Consumption and Risk of Type 2 Diabetes
(2) Caffeine Increases Ambulatory Glucose and Postprandial Responses in Coffee Drinkers With Type 2 Diabetes
(3) Coffee Consumption and Risk of Cardiovascular Diseases and All-Cause Mortality Among Men With Type 2 Diabetes
(4) Influence of Caffeine on Frequency of Hypoglycemia Detected by Continuous Interstitial Glucose Monitoring System in Patients With Long-Standing Type 1 Diabetes
(5) Caffeine: A Cause of Insulin Resistance?