ST. LOUIS — Convincing your diabetic patients to stop consuming caffeine could significantly reduce their postprandial glucose levels and possibly improve their overall metabolic control, Richard S. Surwit, Ph.D., said at the annual meeting of the American Association of Diabetes Educators.
It has long been known that caffeine increases blood pressure, heart rate, and levels of stress (also known as “counterregulatory”) hormones, which in turn are associated with reduced insulin sensitivity. In two placebo-controlled studies designed to look specifically at the impact of oral caffeine on carbohydrate metabolism in regular coffee drinkers with type 2 diabetes, Dr. Surwit and his associates at Duke University, Durham, N.C., have shown that although caffeine does not appear to affect fasting blood glucose levels, it has a major impact on both 2-hour postprandial glucose values and insulin levels. The lead author of both studies was James D. Lane, Ph.D., professor of medical psychology at Duke.
Indeed, the magnitude of the effect is in the range of glucose-lowering medications that are taken before meals, such as nateglinide and acarbose, said Dr. Surwit, professor and chief of the division of medical psychology, and vice chairman of the department of psychiatry and behavioral sciences at Duke.
“If you get your patients off caffeine, you can have a 20% improvement in postprandial glucose, for free. … You can't get that effect without spending a few dollars a day for a pill. Here, you're getting it without adding anything to their regimen, just taking something away,” Dr. Surwit remarked.
In the first study, 14 habitual coffee drinkers with type 2 diabetes were given gelatin capsules of either 125 mg of anhydrous caffeine plus dextrose filler or the filler alone on 2 days within a 2-week period. After fasting blood was drawn, they ingested 250 mg of caffeine or placebo in two capsules with water, and another fasting blood sample was taken an hour later. The patients then consumed a liquid meal containing 75 g of carbohydrates (Boost), and additional blood samples were taken at 1 and 2 hours after the meal (Diabetes Care 2004;27:2047–8).
Caffeine did not affect the fasting levels of plasma glucose or insulin, compared with placebo. After the liquid meal, however, glucose levels were 21% higher and insulin levels were 48% higher when the patients had consumed caffeine before the meal, compared with when they hadn't.
The second study was designed to overcome the first study's limitation of using caffeine-containing capsules rather than real coffee or tea, both of which contain numerous organic compounds that might independently affect glucose tolerance positively or negatively. This time, another group of 20 patients with type 2 diabetes who were also regular coffee drinkers were given decaffeinated coffee with or without 250 mg of anhydrous caffeine dissolved into it, roughly equivalent to a 16-ounce mug of regular brewed coffee. This method allowed for precise control of caffeine content and equivalence of other chemical compounds present in coffee—such as magnesium and roasted quinides—that might influence blood sugar levels, said Dr. Surwit, who is also codirector of Dukes' Behavioral Endocrinology Clinic.
Again, there was a significant postprandial effect: The mean glucose value following caffeine consumption was 28% higher than it was without caffeine, and the mean insulin values were 19% higher than they were without caffeine (Endocr. Pract. 2007;13:239–43).
The magnitude of the effect was not related to age, body weight, body mass index, hemoglobin A1c, fasting plasma glucose, or the usual amount of caffeine consumed. The only correlation was with duration of diabetes: The difference between caffeine and placebo grew by 0.17 mmol/L every 2 hours for each year of diabetes history among the patients. The authors speculated that this could be because patients with a longer duration of diabetes would have less available insulin reserve, which would result in a reduced capacity to overcome the insulin resistance caused by the caffeine.
These findings do not conflict with highly publicized studies suggesting that coffee drinking might reduce the incidence of type 2 diabetes (JAMA 2005;294:97–104), because those data are correlational and not causal—people who drink more coffee might eat less, for example, Dr. Surwit pointed out.
It has not been shown whether or not cutting caffeine can result in a significant improvement in overall metabolic control, but increasingly, data suggest that postprandial glucose values may influence HbA1c to a greater extent than do fasting levels. Dr. Surwit's group hopes to do that study next.
Getting patients to quit drinking caffeine may be tricky, but it's not impossible. “The idea that people need caffeine to stay alert and be productive and be active is nonsense,” said Dr. Surwit. Patients will experience headaches and irritability for a few days, after which those symptoms go away. “It doesn't take more than 3 or 4 days to get people completely off caffeine.”