Caffeine does not cause an immediate spike in blood glucose levels. Instead, it encourages a "dump" of insulin right after you drink the caffeinated beverage (typically at the same time as you eat a high-carbohydrate food), so that your body does not continue to clear sugar out of the blood stream 3, 4, and 5 hours after you eat. Caffeine interferes with what is called Phase 2 secretion of insulin, the longer-term action of insulin as your meal is more completely digested.
Light-roast coffee, on the other hand, may protect against developing diabetes. The research study is in Japanese, and although I do read some Japanese, I just skimmed the article to the part where the scientist notes that the protective compounds found in light-roast coffee that aren't found in dark-roast coffee are chlorogenic lactones and 5-HMF.
You'd get even more of the chlorogenic acid that's believed to be diabetes-protective if you drank coffee from raw beans, but this is not necessary. Light-roast coffee doesn't contain the sugars that raw beans do, but it does have the trigonelline that dark-roast doesn't. Dr. Oka, author of the Japanese study, believes that these compounds may protect against diabetes.
There is one possible exception to the general rule that caffeinated coffee is bad for diabetics, and this comes from a study of coffee drinkers in Greece.
Among Greeks aged 65 to 100 who:
- Did not drink tea but
- Did drink boiled coffee (that is, coffee with the grounds in the bottom of the cup)
Rates of diabetes in old age were 53 per cent lower.
This could mean, however, that people drank boiled coffee just didn't tend to live to be 65, but it's also possible that there is something in boiled coffee that protects against diabetes, too. The protective effect of coffee was not found in people who also drank tea.
You may also be interested in:
Panagiotakos DB, Lionis C, Zeimbekis A, Makri K, Bountziouka V, Economou M, Vlachou I, Micheli M, Tsakountakis N, Metallinos G, Polychronopoulos E. Long-term, moderate coffee consumption is associated with lower prevalence of diabetes mellitus among elderly non-tea drinkers from the Mediterranean Islands (MEDIS Study). Rev Diabet Stud. 2007 Summer;4(2):105-11. Epub 2007 Aug 10.
Oka K. [Pharmacological bases of coffee nutrients for diabetes prevention] Article in Japanese. 1: Yakugaku Zasshi. 2007 Nov;127(11):1825-36.