Ever since I discovered the glycemic index a dozen years ago, I thought that I knew that anything we eat or drink has to have calories for it to raise our blood glucose levels. In fact, those calories have to come from carbohydrates - not protein or fat - to give those levels much of a spike.
Now, however, new studies have found a strange and disturbing exception to the rule.
That exception is coffee. The fact that drinking coffee will increase my blood glucose is particularly disturbing to me, because like most Americans who have diabetes it's what I use to come to my senses every morning.
The amount of the increase can't compare with the glycemic boost that we get from eating a baked russet potato or even a slice of wheat bread. But it can be enough to explain some otherwise inexplicable levels.
Black coffee has no calories. But testing by three scientists at the Duke University Medical Center in Durham, North Carolina, revealed that a moderate dose of caffeine in black coffee (equal to four 8-ounce cups of brewed coffee) increased average levels of daytime glucose from 133 mg/dl (7.4 mmol/L) to 144 mg/dl (8.0 mmol/L) among the 10 habitual coffee drinkers with type 2 diabetes in the study.
In the three hour period after eating a standardized breakfast their usual levels of 144 mg/dl (8.0 mmol/L) at that time went to 157 (8.7 mmol/L). After lunch their levels increased from 122 mg/dl (6.8 mmol/L) to 140 mg/dl (7.8 mmol/L). Likewise, after dinner their levels went from 122 mgl/dl (6.8 mmol/L) to 140 mg/dl (7.8 mmol/L).
The scientists, James Lane, Ph.D., Mark Feinglos, M.D., and Richard Surwit, Ph.D., don't know why coffee has this glycemic effect on us. "The mechanisms of action are uncertain at this time," they write in "Caffeine Increases Ambulatory Glucose and Postprandial Responses in Coffee Drinkers with Type 2 Diabetes."
The professional journal of the American Diabetes Association, Diabetes Care just published the article online ahead of print. Dr. Lane kindly send me an advance full-text copy.
The test subjects were all people who regularly drink coffee. They "had sufficient opportunity to develop tolerance to the drug." This refutes the common belief that we develop a tolerance to the effects of caffeine. They concluded that those of us with diabetes can expect similar increases when we drink our daily coffee.
This study disturbed me so much that I wrote the leading glycemic index researcher about it. "Is what they write sense? Or nonsense?" is the question that I posed to Professor Jennie Brand-Miller, Ph.D., of the Human Nutrition Unit at the University of Sydney. We have worked together for a long time, most closely as co-authors of The New Glucose Revolution: What Makes My Blood Glucose Go Up...And Down? (New York: Marlowe & Co., second American edition, July 2006).
"We have a little study under review at present that does indeed show that coffee consumed one hour ahead of a meal increases the glycemic response to the meal," Jennie replied. She attached the abstract of their study to her email.
The abstract begin by pointing out that habitual coffee consumption has been linked to a lower risk of type 2 diabetes. Nevertheless, drinking just 250 ml (about 8 fluid ounces) of black coffee an hour before a high-carb meal increased the average blood glucose level of eight volunteers by 26 percent compared to when they drank hot water.
These new studies don't refute earlier studies that suggest that people can reduce their risk of type 2 diabetes by drinking coffee. Different factors may be at work. Maybe it's because people who don't have diabetes can produce enough insulin to overcome the insulin resistance that caffeine can temporarily cause.
But once we have diabetes it makes sense to limit our coffee consumption - as painful as even the thought of it is to habitual coffee drinkers like me. Herbal tea, anyone?