The Mystery of Coffee and Diabetes

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Is coffee good or bad for diabetes? Some studies show that coffee is protective, while others say it’s harmful. Some say decaf is better; others say it’s worse. Let’s try to sort this out.

For years, various studies have reported that coffee drinkers are less likely to develop Type 2 diabetes. A recent UCLA study found that “women who drink at least four cups of coffee a day are less than half as likely to develop diabetes as non-coffee drinkers.” Lead scientist Simin Liu said that coffee may improve the body’s tolerance to glucose by increasing metabolism or lowering insulin resistance.

In 2008, Diabetes Self-Management blogger Amy Campbell reported on several other studies showing benefits for coffee. A study published in Diabetes Care in 2006 followed about 900 adults, roughly 300 of whom had prediabetes, for eight years. The people who drank caffeinated coffee had a 60% lower risk of getting diabetes than those who didn’t drink coffee.

Another Diabetes Care study published the same year looked at more than 88,000 women in the Nurses’ Health Study II. It found that women who drank two or more cups of coffee daily had a lower risk (slightly more than half the risk) of getting diabetes than those who drank just one cup of coffee daily, or no coffee at all. And it didn’t matter whether the coffee was regular or decaf.

So right there you have a disagreement about caffeine. Meanwhile, other studies have shown that coffee, or the caffeine in coffee, raises after-meal (postprandial) blood glucose levels up to 20% in people with diabetes. These studies have not been large but have received a lot of attention. In the most-reported study, from Duke University, ten subjects, all with diabetes, were tested — given either caffeine capsules or a placebo (inactive treatment), then switched to the other type of capsules. All showed higher blood glucose levels on the days they took caffeine.

Lead researcher James Lane, PhD, wrote that “In contrast to nondiabetic subjects, our subjects demonstrated exaggerations of both glucose and insulin responses when caffeine was ingested with carbohydrates.”

In a survey on a Mayo Clinic diabetes blog, most readers reported that coffee raised their glucose, even if they had no cream, sugar, or any other food at the time. A reader named Sherri posted, “I drink one large cup in the morning (with flavored cream) before breakfast. [When I monitor], I find that my glucose level is already at high normal for two hours after a meal, and I didn’t even eat yet!”

So how can coffee protect against diabetes if it raises blood sugar? Is coffee good for preventing diabetes, but bad once you already have it? How could this be?

How Coffee Might Protect
First of all, what is going on in people’s bodies when they drink coffee? The UCLA researchers have found that a protein called sex hormone-binding globulin (SHBG) somehow lowers insulin resistance. Most coffee drinkers greatly increased levels of SHBG in the UCLA study. The people with higher SHBG were the ones who didn’t develop diabetes.

A 2009 study in The New England Journal of Medicine of over 800 women found that “higher plasma levels of SHBG were… associated with a lower risk of Type 2 diabetes.” The lowest SHBG group had twenty times the risk for Type 2 of the highest SHBG group.

This is pretty convincing evidence for SHBG, and since coffee is the first way anyone has found to raise SHBG, it might be very helpful stuff. But coffee is a complex substance, with hundreds of natural chemicals in it. How does it work, and what kind of coffee is best?

Is It the Caffeine?
If coffee is good for you, but caffeine is bad, then maybe decaf coffee is the answer? But the UCLA study found that decaf drinkers did not get the protective benefits of coffee. In the two Diabetes Care studies cited by Amy Campbell, one found that decaf or regular worked equally well, while the other found benefits only for caffeinated coffee.

Perhaps the mice have the answer. Reporting in the Journal of Agricultural and Food Chemistry, Fumihiko Horio and colleagues at Nagoya University in Japan fed either water or coffee to a group of laboratory mice engineered to be susceptible to diabetes. According to Science Daily, “Coffee consumption prevented the development of high blood sugar and also improved insulin sensitivity in the mice, thereby reducing the risk of diabetes. Coffee also caused a cascade of other beneficial changes in the…liver and inflammatory [chemicals] related to a reduced diabetes risk. Additional lab studies showed that caffeine may be ‘one of the most effective anti-diabetic compounds in coffee,’ the scientists say.”

So what’s the answer? It certainly seems that coffee protects against Type 2 diabetes, with more evidence for the caffeinated version than for decaf. For people with Type 2 diabetes, going with decaf would be safer, but it’s possible that regular might be more effective. Maybe the temporary glucose spikes, associated with drinking caffeinated coffee with meals, may be worth it in the long run. Or maybe not. People with Type 1 might be better off with decaf. We just don’t know, but with or without caffeine, coffee seems to be one of nature’s wonder drugs for many people. Of course, you should check your own blood glucose levels and see how it affects you personally.

I’d like us to conduct our own study here. Does coffee raise your glucose levels or lower them? What have you noticed about the effects of coffee drinking on your diabetes?

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