A new Canadian study just appeared in the American Journal of Clinical Nutrition and is getting a bit of press. Participants were non-diabetic men, aged 18-50 (average age was 23) with a BMI between 19-29 (average was 25).
The men were either given caffeinated coffee or decaf coffee 1 hour before their breakfast. The ones that received caffeinated coffee were given an amount of Maxwell House Original Roast that provided an equivalent of 5 mg caffeine/kg. The average weight of the participants was 78.5kg so they got an average of 390mg caffeine* or about 2.5 cups of this coffee (which had 62g caffeine/100mL). The decaf group received an equivalent volume of decaf.
The participants were then given either a high glycemic index bowl of cereal – Crispex- or a low glycemic index breakfast- All Bran- with the same amount of milk. The amount of cereal was calculated so that both meals provided 75g carbohydrates.The subjects were randomly placed in all 4 groups with 1-2 weeks between them. 2 days before each trial, subjects were asked to not drink any caffeinated or alcoholic beverages or do any strenuous exercise.
As expected, blood glucose rose after they ate either meal. What was surprising, however, was that blood glucose rose significantly higher when the participants had the caffeinated coffee versus decaf- regardless of whether they had the low GI or high GI meal. At the same time, the caffeine made the people more resistant to insulin, causing their blood glucose to rise higher.
According to the researchers, this is no big deal for healthy, non-diabetic people since their blood sugar will get back to normal automatically. However, they point out that “caffeine should be considered a dietary risk factor for blood sugar control” for people with diabetes and, as a result, they would benefit from drinking decaf coffee.
In my opinion, more research has to be done. There were only 10 participants, all men, enrolled in this study and none of them had diabetes. What would be the effect in people that were actually insulin resistant? Moreover, we all know caffeine affects everybody differently, especially if you're not used to having it. 2 of the 10 participants didn't normally drink coffee and the others were light to moderate caffeine consumers. This should be controlled in future studies.
If you do have diabetes, test your blood sugar before your meal and 2 hours after and check to see if there's a difference whether you drink decaf versus caffeinated coffee.
*Health Canada recommends an intake of less than 400mg caffeine/day, but <300mg class="MsoNormal">Sources: Moisey LL, Kacker S, Bickerton AC, Robinson LE, Graham T. Caffeinated coffee consumption impairs blood glucose homeostasis in response to high and low glycemic index meals in healthy men. Am J Clin Nutr 2008; 87:1254-61.