diabetes book
Diabetes Diet

Glycemic Coffee

Ever since I discovered the glycemic index a dozen years ago, I thought that I knew that anything we eat or drink has to have calories for it to raise our blood glucose levels. In fact, those calories have to come from carbohydrates – not protein or fat – to give those levels much of a spike.

Now, however, new studies have found a strange and disturbing exception to the rule.

That exception is coffee. The fact that drinking coffee will increase my blood glucose is particularly disturbing to me, because like most Americans who have diabetes it’s what I use to come to my senses every morning.

The amount of the increase can’t compare with the glycemic boost that we get from eating a baked russet potato or even a slice of wheat bread. But it can be enough to explain some otherwise inexplicable levels.

Black coffee has no calories. But testing by three scientists at the Duke University Medical Center in Durham, North Carolina, revealed that a moderate dose of caffeine in black coffee (equal to four 8-ounce cups of brewed coffee) increased average levels of daytime glucose from 133 mg/dl (7.4 mmol/L) to 144 mg/dl (8.0 mmol/L) among the 10 habitual coffee drinkers with type 2 diabetes in the study.

In the three hour period after eating a standardized breakfast their usual levels of 144 mg/dl (8.0 mmol/L) at that time went to 157 (8.7 mmol/L). After lunch their levels increased from 122 mg/dl (6.8 mmol/L) to 140 mg/dl (7.8 mmol/L). Likewise, after dinner their levels went from 122 mgl/dl (6.8 mmol/L) to 140 mg/dl (7.8 mmol/L).

The scientists, James Lane, Ph.D., Mark Feinglos, M.D., and Richard Surwit, Ph.D., don’t know why coffee has this glycemic effect on us. “The mechanisms of action are uncertain at this time,” they write in “Caffeine Increases Ambulatory Glucose and Postprandial Responses in Coffee Drinkers with Type 2 Diabetes.”

The professional journal of the American Diabetes Association, Diabetes Care just published the article online ahead of print. Dr. Lane kindly send me an advance full-text copy.

The test subjects were all people who regularly drink coffee. They “had sufficient opportunity to develop tolerance to the drug.” This refutes the common belief that we develop a tolerance to the effects of caffeine. They concluded that those of us with diabetes can expect similar increases when we drink our daily coffee.

This study disturbed me so much that I wrote the leading glycemic index researcher about it. “Is what they write sense? Or nonsense?” is the question that I posed to Professor Jennie Brand-Miller, Ph.D., of the Human Nutrition Unit at the University of Sydney. We have worked together for a long time, most closely as co-authors of The New Glucose Revolution: What Makes My Blood Glucose Go Up…And Down? (New York: Marlowe & Co., second American edition, July 2006).

“We have a little study under review at present that does indeed show that coffee consumed one hour ahead of a meal increases the glycemic response to the meal,” Jennie replied. She attached the abstract of their study to her email.

The abstract begin by pointing out that habitual coffee consumption has been linked to a lower risk of type 2 diabetes. Nevertheless, drinking just 250 ml (about 8 fluid ounces) of black coffee an hour before a high-carb meal increased the average blood glucose level of eight volunteers by 26 percent compared to when they drank hot water.

These new studies don’t refute earlier studies that suggest that people can reduce their risk of type 2 diabetes by drinking coffee. Different factors may be at work. Maybe it’s because people who don’t have diabetes can produce enough insulin to overcome the insulin resistance that caffeine can temporarily cause.

But once we have diabetes it makes sense to limit our coffee consumption – as painful as even the thought of it is to habitual coffee drinkers like me. Herbal tea, anyone?

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

Never Miss An Update

Subscribe to my free newsletter “Diabetes Update”

I send out my newsletter on first of every month. It covers new articles and columns that I have written and important developments in diabetes generally that you may have missed.

Previous Post Next Post

You Might Also Like These Articles

  • Albert Greer at

    I just want to say I am just all new to weblog and seriously enjoyed your website. Likely I’m going to bookmark your blog . You certainly have terrific articles. With thanks for sharing your web site.

  • Kiana Zeier at

    It’s the small changes that create the largest shift.

  • Clinton at


    I appreciate the reasearch and references you’ve provided. It has helped me in my research as to the validity of the claims made by a coffee that is the “first patented” low glycemic (among other claims) coffee.

    I’ve had the opportunity to meet (personally while in Denver) one individual who was on an insulin pump (no longer due to weight loss) and able to consume the coffee prior to ditching the pump without issue. He is now an advocate, sharing his experience with others.

    I’ve read the patent (#5,480,657 (you can google it)) information as well as those referenced looking for details on what exactly it is in the coffe that inhibits the response. http://soeasy.bfreesystem.com/products/b_skinny.html

    I’m not diabetic myself and have limited knowledge (other than what I’ve read and talking with the nurses I work with). I’m interested in becoming an advocate of the product but not before I’m fully confident that it is safe for diabetics and the claims are legit.

    Thank You!
    (Just drove through Colorado visiting my parents last week after all the rain and it’s beautiful up there!)

  • JoAnn at

    I’m looking for GI and GL of plain, raw, whole cranberries. I can’t find it in any lists I have searched. ???

    • David Mendosa at

      Dear JoAnn,

      No reputable testing of raw cranberries have been done yet. My guess is that this is because no one could get volunteers to eat at last 25g of them. Raw cranberries are not sweet!


  • Alexandra at

    I have a theory for ya…

    My theory is that the increase in BG levels has to do with what else we consume after the coffee for the duration the coffee stays in our system, not the coffee itself. Because coffee is a stimulant and raises heart rate, speeds up metabolism, etc. then theoretically speaking eating carbs of any kind with or following coffee may cause us to process those sugar quicker and with more dramatic effect observable by us.

    I personally notice no problems in myself when I drink coffee in moderation (ie: 3 cups a day; not 4 cups in a two hour span…) I do however notice that if I have any carbohydrates within reasonable time of consuming the coffee that it does effect me dramatically.

    Also speaking in terms of Ayurveda…

    Coffee increases Vata and Pitta dosha. While this is good for people with Kapha imbalances (overweight, etc), like many people with BG sensitivities, Vata dosha cannot handle white sugar. Increasing Vata with coffee or any Vata-inducing food (lettuce, etc.) would then further increase sugar/carb sensitivities. However if these theories based on Ayurveda are correct that would mean that it doesn’t matter if you drink green tea or other herbal teas. It would mean any herbal tea that increases Vata dosha would have the same effect as coffee.

    I don’t know if that made sense to anyone else, but just a few thoughts…


    • David Mendosa at

      Dear Alexandra,

      Thanks for sharing your thoughts. Your theory makes a lot of sense to me.


  • Mary at

    Ok, I do NOT like tea and never have liked it. Even as a kid I wouldn’t drink tea BUT I know a person can develop a taste for anything and after reading the article below I have decided to learn to drink Green Tea in place of coffee. I will probably have less trouble with the caffeine in Green Tea because I am not fond of Green Tea and won’t drink more than 1 cup most likely.


  • Mary at

    Hmmmm coffee with sugar???? ok, I now want to know who funded this study? Coffee industry or Sugar industry? But seriously, I do think it matters when we hear something like this. We should check out the funding of the study.

  • Mary at

    OMG I had gotten off coffee because it was supposed to be bad for us. Then they changed it and said it was actually good for us so I started drinking it again and am addicted again! So now I get to get off coffee again.

    Crap! I will just stay off it this time and if they say it will increase lifespan, insure good looks and put money in your pocket I will not get on it again!

    Alternatively, is there a recommended safe daily amount? Maybe I should just cut it to 8 oz a day? Or would it be better to just get off it?

    I hate it when they tell us something is bad for us then comeback a year or two later and say it is healthy. When I quit smoking I half expected them to discover tobacco had unknown health properties !



  • Scott C at

    Coffee with sugar is best? A surprising result of a scientific study showed drinking black coffee before a meal raised post-meal glucose a ton, but coffee with sugar actually lowered post-meal glucose by 20% versus not drinking anything.


    The results seem to match the Nurses Health Study that showed people that drank coffee, regardless of whether with or without sugar, particularly before lunch, had lower diabetes risk.

    These studies however were with healthy adults, has anyone seen effect on those with diabetes? Has anyone taken their reading after coffee with sugar?

    I always thought coffee was bitter and tasteless when I was younger, and instead drank a lot of sugary beverages. Perhaps coffee’s presumed anti-diabetic effect is not cause and effect, but just an indicator of taste preferences?

  • Good Levels at

    Hello everyone
    My experience shows that cinnamon can increase the heart rate.So anyone thinking of adding cinnamon to coffee should consider this side affect of cinnamon which will be further augmented by caffeine!!!

  • David Mendosa at

    Dear Jackie,

    The article doesn’t say! I wish that I knew too, because I’ve switched to decaf.

    Best regards,


  • jackie at

    Any idea whether DECAF coffee has the same glycemic effect????

  • Marcus at

    John, the first thing the doctor should have done is give you a sheet with with foods to eat more of and foods to eat in moderation or avoid. I would be shocked if they didn’t say how to control your diabetes. The more refined the carbs, (sugar or fructose bieng the most refined) the more/quicker your blood glucose will spike. With type 2 diabetes for one specific reason or another your body can’t utilize carbohydrates as efficiently as before, and when it gets backed up it can’t finish taking them out of your blood as quickly so it leaves some there which can lead to complications if it is consistently like that. Good luck and God bless.

  • John at

    I recently was told that I have type 2 diabetes, and just amazed how lowering my carbs contributes to a lower BG reading. The doctor and nutritionist nerver mention this. Your website has been a great help Thank You

  • marcus at

    Homeschool Mom,

    Even if both your parents have diabetes you might not have the same tendency to get it. If my genetics memory is right you may still have up to a 1 in 2 chance that you aren’t susceptible to it. Keep aware about your numbers though.

  • Ron at

    In my experiments with eating, I am finding that fats play a huge part in how my numbers read. I wonder if that oily stuff on the coffee beans is considered fat and effects (however small) how much glucose actually gets through the door to nourishing our cells.

  • David Mendosa at

    Dear Eric,

    What great advice! Thanks for sharing. I will continue to drink a lot of water when I drink coffee (and otherwise). I am addicted to both!

    Best regards,


  • Eric at

    Hi All,
    I have experienced the spikes associated with caffeine. I also have seen some of the studies re: insulin sensitivity and coffee consumption. However, for me at least, I have come to the conclusion that the effects of coffee are really related to the effects of dehydration resulting from coffee, which doesn’t seem to be considered in studies.

    I would typically drink a 4 shot espresso on a weekly driving trip on Mondays in which I had to get up quite early, and my sugar would spike weekly. Since I have started drinking a large bottle of water with it, the effects have been mitigated. Try “overhydrating” with the caffeine and see if it helps. The only issue is, if you’re driving, plan to stop often!

  • David Mendosa at

    Dear Lou,

    I’m sorry, but I haven’t read either that paper or that book.

    Best regards,


  • LOU at



  • LOU at

    I have noticed and read the diabetic breakthough paper by MARK ANASTASI and the $47 book entitled “DIABETIC MEDICAL BREAKTHOUGH’,
    what do you think, Dave, have you read it ?

  • David Mendosa at

    Dear Lou,

    Hang in there! I really don’t think that coffee can possibly raise our BG levels very much. Diabetes has not taken coffee away from me — please see my more recent post at http://www.mendosa.com/blog/?p=331

    Best regards,


  • LOU at

    thanks for the input on coffee, but it broke my heart. I have always said…with my diabetes, at leasr they have not taken away my coffee ? ouch !

  • David Mendosa at

    Dear Jennifer,

    Glad that the cream works so much better than the creamer!

    The best guide for very low-carb diet is Dr. Bernstein’s Diabetes Solution, third edition.

    But I generally have two poached egg whites for breakfast (no carbs). I snack on Vital Choice mackerel or sardines. For lunch I usually have a salad or bratwurst and sauerkraut. For dinner I will have fish or chicken with maybe some yogurt and a vegetable like winter squash or okra (my two favorites). Avocados are a mainstay of my diet too.

    Also see: http://www.healthcentral.com/diabetes/c/17/20167/good-veggies/

    Best regards,


  • Jennifer at

    I tried the cream in my coffee today with GREAT results. My blood sugar increased by 1.

    Now another question. I was reading your article on low carb diets. The 6g for breakfast and only 12 for lunch and dinner….wow. Can you give me a sample meal. I am 57, 5’2″ and 125 lbs. My a1c is 6.5.

  • David Mendosa at

    Dear Jennifer,

    100 points! That’s huge! Yes, definitely one other suggestion. It’s got to have mostly to do with the powder creamer. Either stop using cream in your coffee (and if it is good coffee, I guarantee that it will taste better, at least after the first few cups), for switch to pure cream — not half and half. Pure cream has no carbs, but half and half and creamers do.

    Let me know!


  • Jennifer at

    I have switched to decaf coffee and still my numbers go up about 100 points after two cups…I use powder creamer and no sugar. Any suggestions other than quitting the coffee?

  • David Mendosa at

    Dear Eunice,

    Aloe may help control diabetes. But we really don’t have good enough proof yet.


  • Eunice at

    YOur web site is very interesting, cabbage, carrots, cinnamon, peanuts can be use to help lower the blood sugar.

    Is aloe bitters okay to also help lower the blood sugar?

  • David Mendosa at


    I really don’t know what gives. But if your readings are in the 90s two hours after the first bite of a meal (or snack) you are doing very well!


  • Ted B. Hale, Jr. at

    I’m a type 2 diabetic of five years duration who has been able to reduce oral medication to about 1/3 the prescribed dosage, a fact I attibute principally to flaxseed oil.

    On infrequent occasion I attend a gathering of locals where cornbread is served. After two (or three) muffins liberally coated with real butter with a hint of added honey, and after 2-3 hours, my reading will be in the nineties. What gives?

    This has happend before with small non-cornbread products, again with liberal butter. I’ve prepared cornbread muffins at home but not with the same results.

  • Norm Reid at

    Anybody have evidence about decaf? My informal observations suggest that decaf, too, accounts for a slight rise in blood glucose levels.


  • David Mendosa at


    You bet that unsweetened green tea is fine for you. The SugarBusters! book was a pretty good one, but it could be even better for you to follow a low-carb diet instead.

    Best regards,


  • tom at

    Is unsweetened, bottled green tea OK for a person in the early stages of diabetis? I got the news last night and I’m desperate to learn what’s good for me. I have an old copy (1998) of SugarBusters! and I’m hoping the info will help me get everything back in check. My cholesterol is also high (145) right now, as well as my blood pressure (I’m on meds for that thought)
    Help, thanks!
    Tom Great

  • Ned Widener at

    I strongly suspect that the glycemic response is related to the adrenal function, that gives much more dramatic effects on blood sugar readings when the blood sugar is already elevated. Try an aerobic workout if your blood sugar is above 150 mg/dl and tell me what the blood sugar is after. I am Type I, insulin dependent for 40 years.

  • David Mendosa at

    Dear Frank,

    I wonder too. The effects of cinnamon are certainly minimal, and those of coffee may well also be minimal. My guess is that they balance each other out.

    Best regards,


  • Frank at

    I filter drip each cup of coffee individually and add a 1/4 teaspoon of cinnamon to the fresh ground coffee before pouring the water. Since cinnamon increases insulin sensitivity, I wonder what the competing effects would be?

  • Elissa at

    Any suggestions about green tea? I’ve not seen it on any GI scales….and I never sweeten it. Thanks!

  • admin at

    I am pretty sure that there haven’t been any tests of what you suggest. Those are good ideas though! But if you do include cream in your coffee — and it is real cream and not the faux stuff that restaurants serve that has HFCS in it or the half-and-half that has carbs in it — my guess is that it might moderate the BG effect somewhat.


  • pennilessneuropathic at

    How would or might black coffee’s effects on blood glucose levels differ
    if the diabetic person were to consume the coffee “after”
    a meal (that has some protein and fibre), rather than “before” a meal?

    And how would or might those effects on blood glucose differ if the “after”-meal coffee (say, a tall Blenz or Starbucks),
    instead of being black, included 1 or 2 Tablespoons of cream?
    If no credible studies have included either of these 2 scenarios yet, what would be your “educated guess,” from your experience? Thank you for any reflections.

  • Homeschool Mom at

    That’s really interesting. Thanks David. I enjoy reading your site. I’m not diabetic – yet. I have two diabetic parents and one sister who is also, and plenty of other relatives. Thus the -yet. I found your site while researching the glycemic index. Thank you for your site. I’ve been reading it for many many months.

  • admin at

    Good question! Real tea (not herbal tea) does have caffeine. But not as much. Not hardly as much. So the effect would be a lot less, I guess.


  • Homeschool Mom at

    Is it coffee or caffeine – and would tea have a similar effect I wonder?

  • Jeetha at

    I recently decided to give up caffeine and found a great substitute to satisfy the need for a warm comfort drink.

    Cafix, made of malted barley, barley, chicory, figs, and beet roots, is caffeine free and tastes just like coffee. I found it at Whole Foods but it is available online.