In the latest of these studies, Duke University researcher James D. Lane, PhD, and colleagues put continuous blood-sugar monitors on 10 people with type 2 diabetes. All were regular coffee drinkers averaging four cups a day, but they stopped drinking coffee during the experiment.
On one day, each patient took a 250 mg caffeine capsule at breakfast and another 250 mg caffeine capsule at lunch. That's roughly the same as having them drink two cups of coffee at each meal. On another day, the same people got placebo pills with no caffeine in them.
The result: On the days the patients took caffeine, their blood-sugar levels were 8% higher. And after every meal -- including dinner -- their blood sugar spiked higher than it did on the day they had no caffeine.
"These are clinically significant blood-sugar elevations due to caffeine," Lane tells WebMD. "Caffeine increases blood glucose by as much as oral diabetes medications decrease it. ... It seems the detrimental effects of caffeine are as bad as the beneficial effects of oral diabetes drugs are good."
Lane warns against reading too much into this small, 10-patient study. But he says it does show that caffeine has real effects on the everyday lives of people with diabetes.
"For people with diabetes, drinking coffee or consuming caffeine in other beverages may make it harder for them to control their glucose," he says.
(If you have diabetes, how much caffeine do you consume on a regular basis? Talk with others on WebMD's Type 2 Diabetes Support Group message board.)
Diabetes, Coffee, and Caffeine
Several studies have found that coffee drinkers -- especially those who drink a lot of coffee -- have a lower risk of diabetes than do other people. So how can coffee both protect against diabetes and worsen diabetes?
WebMD took this question to Harvard researcher Rob van Dam, PhD, who recently analyzed all of these studies.
"In 2002, we thought this did not make any sense," van Dam says. "This is quite a consistent observation, that coffee has a positive effect on diabetes. But it is becoming increasingly clear it is not the caffeine that is beneficial. The picture is now evolving where we see that some other components of coffee besides caffeine may be beneficial in long-term in reduction of diabetes risk."
In fact, van Dam says, it appears that decaf coffee may actually help people keep their blood sugar under control, whereas regular coffee has a detrimental effect on blood sugar. Caffeine unbalanced by other coffee compounds, he says, may be even worse.
Lane says that if there are anti-diabetes compounds in coffee, they don't offset the harmful effects of caffeine.
"We did do one study where we put caffeine in decaf coffee, and still we saw the same exaggeration of glucose after meals in people with diabetes," he says. "So it seems those other compounds in coffee certainly don't eliminate the caffeine effect we have seen."
So what should people do if they have diabetes or are at high risk of diabetes?
"We take a more nuanced posture -- not that coffee is good for you or bad for you, but that maybe it's better to switch to decaf coffee if you have diabetes or the metabolic syndrome," van Dam says.
Lane says people with diabetes are likely to get different effects from coffee.
"I am not going to say that everyone with diabetes has to quit drinking coffee, but I think those who are concerned about their blood sugar not being as low as they'd like it to be should try quitting coffee," he says. "They will be able to tell right away if it improves their glucose control. And it may help reduce their risk of complications of diabetes or reduce their need for additional diabetes medications."
For regular coffee drinkers, Lane says, quitting caffeine may mean three or four days of headache, sleepiness, or mental grogginess. But it does not mean interminable morning misery.
"Every morning we wake up in withdrawal, thinking we need coffee to wake us up -- but it really is just helping us get rid of withdrawal symptoms from not drinking coffee overnight," he says. "Once we get off, well, I wake up feeling better than I did when I was drinking coffee. I don't feel groggy or tired. Everyone I talk to has had a similar experience -- it is not that we become zombies after quitting drinking coffee."
Lane and colleagues report their findings in the February issue of Diabetes Care.