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Does Caffeine Cause Blood Sugar Changes?

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?

This article is based on an earlier version of my article published by HealthCentral.

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  • Gavin at

    Nope, it’s definitely not sugar related-I don’t have sugar in my coffee or anything else, due to being diabetic.

  • Gavin at

    I drink a double espresso before cycling as it raises my blood sugar levels without need for eating.
    Caffeine increases blood sugars by stimulating the “flight or fight” response. This stimulates the adrenal glands to release adrenalin, which in turn stimulates the liver to release glucose into the bloodstream to provide the energy. Diabetics are unable to use this increased sugar effectively, unless they exercise, allowing the muscles to use the glucose.

    • David Mendosa at

      Are you sure that it’s not the sugar in your double espressos that raise your levels, Gavin?

  • jeff at

    I’ve been experiencing rising fbg from high nineties to slightly over one hundred and four over the past few years as a Kaiser patient. They were never concerned about it. I’m also a Life Extention member and had learned that optimal levels are 70-85. Last year my employer gave us a PPO and I found a good osteopath who uncovered my fatty liver and started me on metformin for the pre diabetic state that’s been hounding me despite low carb paleo diet and daily exercise and successful stress mgt. I had the impression from Kaiser that because type 2 runs in my family, my father, brother, grandfather, that there’s really nothing to be done unless it hits the diagnosis level etc. No thanks. Well, employer switched us back to KP this year and I continued educating myself. I have since discovered that all the caffeine I’ve been using via cups of coffee and even energy drinks like low carb Monster and sugar fee RockStar had been inhibiting my insulin which my c peptide lab results always showed enough insulin being made. Since quitting the caffeine entirely my fbg is around 85 via use of 500 mg er metformin, gtf chromium, zinc, daily use of decaffeinated green tea with ginseng and my liver enzymes are back to normal levels. I’m 49 and also on thyroxine for hypo and have finally lost more than ten pounds due to the changes despite several year of cardio and steady weight gain 🙂

    • David Mendosa at

      You are doing great, Jeff! Your fasting blood glucose levels are indeed just where they need to be. You also need to consider your post-prandial levels, of course. That is even more important. The best way is to get a regular A1C test. What is your current A1C level?

  • Steve at

    I am a type 2 diabetic currently taking Lantus once a day. and humalog on a sliding scale before meals. Fasting BSG 137 after consuming 2 cups of black coffee (strong) my BSG rises roughly 50 points. How can this happen? No sugar no carbs no foods of any kind

    • David Mendosa at

      Dear Steve,

      This sounds like the Dawn Phenomenon. One study that I reported on here says that it can last until 11 a.m. Actually, however, I would be much more concerned with you high fasting blood sugar level.

      Best regards,

  • Prabodh Kumar at

    Whether coffee is suitable to diabetes ?

    • David Mendosa at

      Dear Prabodh,

      Yes, coffee is perfectly fine for us (although personally I prefer black tea). But do drink your coffee black. The sugar and milk or half and half have too many carbs. Full cream, however, doesn´t have carbs.



  • Jennifer at

    After 2 cups of decaf coffee in the morning my blood sugar increases by 70+ points. I only use coffee mate as a creamer and no sugar. I don’t understand because just about everything I read says that coffee doesn’t raise your blood sugar.

    • David Mendosa at

      Dear Jennifer,

      I suggest that you might drink your coffee without Coffeemate and use heavy cream instead. Coffeemate comes in dozens of different flavors and formulations, and some, if not all of them, have several grams of carbohydrate. Heavy cream and coffee itself doesn’t.



  • Maia at


    Do you have an update of how your caffeine-free test went?

    • David Mendosa at

      Dear Maia,

      That’s the lastest info I’ve seen.



  • Ron at

    I am not diabetic but became worried about being prediabetic after I had switched to a low carb diet. My readings prior to this had been normal, at least the last time I had checked which was probably a year ago. So, I decided I had better check to see where I am at after being about 7 weeks into this new diet. fbg 104. I thought this can’t be. So I did the research and found info this was normal for some low carbers. I have been doing a lot of exercise to drop weight along with this new diet. I completed an almost two hour workout (still had not eaten anything since the night before) checked by bg it was 136 (within 5 minutes of workout). 1 hr it was back down to 104. My number during the day post meal 1 and 2 hours were good and always back down to the 104. I did an A1c test it was 5.1 However, this still troubled me because the 5.1 must have been higher than what it would have otherwise been because if the 104 was my new base that would be up from before beginning the diet. In the last couple of years I began drinking coffee throughout the day. A few days back I only had one cup and then switched to decafinated green tea and drank that through the day. I check bg and it was 95 then again 85. Next morning fbg 85, I drank no caffeine just decaf tea. It stayed in this lower range. Did that for a couple of days. Then thought I would try regular green tea. That morninf it went from and 89 fbg to 104 after drinking it and still had not eat. Dropped the caffeine and it went back down. I don’t get it >s< but I thought I would share my story with you.