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Does Caffeine Cause Blood Sugar Changes?

David Mendosa Health Guide April 14, 2011
  • The first thing I did after reading the new study that caffeine can increase our blood glucose level was to switch to drinking green tea. The second thing I did was to switch again -- this time to decaffeinated green tea.

    I overreacted. Several years ago I had switched from coffee to Darjeeling tea, which has about half the caffeine per cup. While green tea has even less, I don’t like it much and only drink it rarely. I disliked the decaffeinated green tea so much that I threw out the package after taking the first sip.

    Now, I’m almost entirely back to Darjeeling tea. People call it a black tea, although it is light-colored and is technically more oolong than black and is therefore lower in caffeine than true black teas.

    While I control my blood glucose level, I’m not a purist. I have to enjoy everything that I eat or drink. While my diet includes no starch, sugar (no sucrose or high-fructose corn syrup), salt, or alcohol, except occassionally when I eat out, I don’t miss any part of this standard American, or SAD, diet. My recent A1C result was 5.3 percent.

    Caffeine may well raise my blood glucose level. But after carefully reading the new study, I doubt it.

    The new study is a review in the April 7 issue of the Journal of Caffeine Research by James D. Lane, associate research professor in the department of psychiatry and behavioral sciences at Duke University Medical Center. Dr. Lane sent me the full-text of the study, which is now free online.

    Dr. Lane reviewed 17 studies that showed that caffeine increased insulin resistance or impaired glucose tolerance of  people who don’t have diabetes. Using accepted study design -- double-blind and placebo-controlled -- and moderate doses of caffeine equivalent to two or three cups of brewed coffee, these studies consistently showed a transient increase in insulin resistance.

    Yet many other studies of people who don’t have diabetes show apparently contradictory results. Consistently, these studies have found that when people without diabetes drink a lot of coffee they are much less likely to get diabetes.

    So, who knows whether coffee is good or bad for them. But for us who have diabetes, other studies found that caffeine exaggerates the rise in glucose when people with diabetes ate carbohydrates. Dr. Lane thinks that the effect is clinically significant. He compares the effect -- an increase of 18 to 26 percent in blood glucose -- to a similar, albeit somewhat less, decrease in blood glucose when we take a sulfonylurea, Starlix, or Precose.
    “Caffeine in coffee, tea, or soft drinks,” Dr. Lane says, “causes a transient insulin resistance that can produce exaggerated glucose and insulin responses when carbohydrate is consumed.” This seems to me to be the key.
    It’s really the same old story. “Diabetes,” as Dr. Lane points out, results “from defects in carbohydrate metabolism.” Carbs, more than caffeine, is the story.

    Since I follow a very low-carb paleo diet, eliminating caffeine entirely might well increase my insulin sensitivity. A bit. I will try to drink fewer cups of tea, but no way will I switch to decaffeinated tea.

    But coffee drinkers, particularly those who still consume starch and sugar, may well want to test for themselves. My friend Gretchen Becker, who also writes here at HealthCentral, has done just that.

    Gretchen’s testing shows that for her “caffeine does seem to have an effect.” But while her blood glucose level rose by 15 points, that was because of the carbs she ate. The caffeine just delayed the peaks and didn't seem to increase them.

    She recommends this procedure:

    Get up and have caffeine or no caffeine. Wait an hour. Eat breakfast. Measure every hour until lunch. Then measure for a few hours after lunch, as the effect is supposed to persist.

    Now, what effect does caffeine have on your blood glucose levels?