Does coffee raise blood sugar? Preliminary findings

A few weeks ago, I started an experiment to better understand how different foods and lifestyle decisions impact blood sugar, using a constant-glucose-monitoring device.

Right now, I’m testing what effect drinking coffee has on blood-sugar levels.

Here’s the preliminary data:

coffe-blood-sugar-premliminary

The blue line is the blood glucose on a day I drank coffee. The red line is on a day without coffee. As you can see, the blood-sugar levels are on average higher when drinking coffee than when not (I added nothing to the coffee).

To reduce the probability that this difference is random, I’ll repeat the experiment this week.

What do you think will happen? On days when drinking coffee will the blood sugar be (on average) higher, lower or the same? Leave your guess in the comments below.

We’ll share our findings in a few days.

Earlier experiments

Are you interested in our earlier tests? Check out the series:

  1. Why You’re Not in Ketosis
  2. How Much Protein Can You Eat in Ketosis?
  3. What to Eat in Ketosis
  4. Do Artificial Sweeteners Affect Blood Sugar?

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50 comments

Top comments

  1. Etai
    Did you eat the same things on both of those days? It looks like your blood sugar on the day you drank the coffee was already higher prior to drinking it and on the non-coffee day your sugar level dropped prior to the time you would have drank the coffee. So it makes me wonder if there is some other factor involved.
    Reply: #12
    Read more →
  2. Karla
    Are you drinking black coffee (really, the ONLY way to drink coffee)? :-)
    Reply: #9
    Read more →

All comments

  1. Karla
    Are you drinking black coffee (really, the ONLY way to drink coffee)? :-)
    Reply: #9
  2. Tracie
    Do you think it's a significant difference or a slight difference?

    Is 60 a good blood sugar number or is that low?

    Reply: #11
  3. Nicole
    I believe its very individualized on how coffee impacts a person. For instance, I can't drink coffee or anything with caffeine in it. It literally feels like my intestines are rotting on the inside when I drink it. It took years to discover it was coffee and tea that caused the painful inflammation and I don't know anyone else who experiences this. Due to this - I know it would raise my blood glucose response. But I see so many other people, and it does nothing to them, just like ingesting sweeteners (like stevia, xylitol etc.) has no impact on my blood glucose.

    But interesting work!

  4. Marketa
    I tested my blood glucose for coffee for a few weeks and I saw no difference. Only I slept worse.
  5. Elwin
    I am a retired Paramedic who has drank coffee in excess all my life
    Like a lot of Information that was preached on weight loss we were told that coffee was not a problem, WELL, I joined the Diet Doctor and I am following the LCHF diet I do not cheat I am a T2 Diabetic on Insulin Injections 4 times a day
    I have lowered my A1C from 8 to 6 and reduced my Insulin Injections by 2 a day
    I get very discouraged when I get a High reading now. So I look back to see what went wrong What did I do to have this Increase and the CULPRIT IS COFFEE
    This site has great Information on all aspects of weight loss But I need more information on a T2 trying to reduce Insulin Injections another Culprit in weight control: My weight problem ballooned when I was put on Lantus Long Acting Insulin
    My weight problem ballooned when I was put on Lantus Long acting Insulin
    Replies: #10, #19
  6. Etai
    Did you eat the same things on both of those days? It looks like your blood sugar on the day you drank the coffee was already higher prior to drinking it and on the non-coffee day your sugar level dropped prior to the time you would have drank the coffee. So it makes me wonder if there is some other factor involved.
    Reply: #12
  7. Shaun
    Hi, there, are you testing for differences, if any, between effects of regular vs. decaf? Is it coffee affecting blood glucose, or is it caffeine? I read of one study that found that caffeine, and not just coffee, impacts lipid metabolism: http://www.sciencedirect.com/science/article/pii/0026049568900541

    Would be interesting to know if there are similar differences between regular and decaf w.r.t. blood glucose levels.

    Reply: #8
  8. Ann Whitfield
    I'd love to know this too
  9. Bjarte Bakke Team Diet Doctor
    Normally I drink my coffee with cream, but for this experiment I drink it black.
  10. BobM
    Elwin, go to Dr. Fung's website: https://intensivedietarymanagement.com/blog/

    Insulin causes insulin resistance.

    Reply: #17
  11. Bjarte Bakke Team Diet Doctor
    It's a little low compared to the average person, but I'm in ketosis - and get most of my energy from fat - so lowish blood sugar isn't an issue for me.
  12. Bjarte Bakke Team Diet Doctor
    I didn't eat anything during the experiment.
  13. BobM
    There are many issues with this. Your blood sugar was higher before you drank the coffee.

    I've tried to test this myself, but there are too many variables, not the least of which that the blood sugar monitors have a lot of errors. If coffee has an effect, it's subtle. I've taken my blood sugar in the morning, then had coffee, then taken my blood sugar about an hour afterwards. Sometimes it does up slightly, but most times, it goes down.

    What would be interesting to see was if there was an insulin effect. That would also affect blood sugar. Maybe coffee reduces insulin? That would yield a higher blood sugar, but would cause less hunger.

    Reply: #15
  14. John K
    Very cool. Thanks for posting this.

    This is one day's data for each case? It would be good to have a bunch of days of data to work with since there can be significant variability in glucose on any given day. And we see that here in blue which is bouncing around all over the place even before you have your coffee. Really, both of them are moving around a fair amount.

    Reply: #16
  15. Bjarte Bakke Team Diet Doctor
    I agree BobM - it's not possible conclude based on this experiment (and it's a n=1).

    I repeated the experiment yesterday and today - will probably share the results this week.

    Do you know of a simple and reliable way of measuring insulin levels several times during the day?

  16. Bjarte Bakke Team Diet Doctor
    Agree John.
  17. Elwin
    Thank You, Bob M
    I have been to Dr. Fung's Site I find the Information more confusing
    He does not give out Individual help to Insulin Problems Which is understandable, He refers you to your Doctor and I have one who is not in agreement with LCHF diets or reversing T2 Diabetes
    Like many, We go it alone
  18. Lorraine
    Elwin -

    These people can help you:

    https://www.virtahealth.com/

    Best of luck!

  19. Bjarte Bakke Team Diet Doctor
    Reply: #26
  20. Chris
    It looks to me as though one could interpret the above results as coffee assisting in keeping the blood sugar stable, which usually is beneficial, especially if you were doing a daily intermittent fast since it would seem to lower feelings of hunger.
  21. LowCarbArcher
    Wait, was your diet exactly the same for the two days? You day nothing about what else you consumed.
    Reply: #39
  22. Tobias
    If you look at the graph, it shows that raising and lowering of blood sugar starts already before you consume or not consume the coffee. I wouldn't be surprised if the results next time can be just the opposite.
    Reply: #40
  23. Dr. Kenneth Solomon
    Actually if you haven't consumed any food during this time your blood glucose is at a healthier level with the caffeine (coffee). Your levels without it actual border on hypoglycemia. There has been a great deal of data testing caffeine (black coffee) and its effects on blood sugar. The results have been either neutral or a positive effect, meaning more often in the normal range.
    Reply: #41
  24. Wie Pan
    This is very unscientific and as such quite a useless set of data. As a minimum you want to share what other variables that you control or don't. There are many factors that can contribute to glucose level. Also, like you said n=1 is very limited data. I wish Diet Doctor can be a more credible site by publishing scientific based data.
  25. John s
    It looks to me that coffee ha no effect whatsoever on the blood sugar level. The starting blood sugar level was 80, rose very modestly to 85 and ed up at 80. The no coffee day the blood sugar started at 57.5, rose to 77.5 and ended up at 65. WTF???
  26. Elwin
    Bjarte I am a Member
  27. B. D.
    I have found that coffee does raise my back most of the time.. but not always. I make strong coffee in a French press and most of the time it does raise my bs. I never add things to my coffee , I just drink it black. If you add milk or cream you have the dates and natural sugars that need to be counted and factored in.. I would imagine milk would prolong the high blood sugar levels. Though not drinking coffee is better for my bs, drinking it makes me happy and much nicer to be around.. so I try to drink more water before my cup of coffee, which I found helps some.
  28. Karen Stewart
    I've been drinking coffee since I was 5 year's old and much blood sugar level is normal who ever believe in that needs to get there head examine
  29. Keith
    Not enough information about whether the two readings were taken under exactly the same conditions ie time of day, activity, food eaten before t=0 etc.

    What is apparent is that after not eating or drinking for 4 hours your BG drops to 65 as opposed to drinking coffee approx 77. So as mentioned drinking coffee at first sight appears to keep your BG stable but there is not enough information and the reasons why blood sugar levels rise and fall can involve a different pathway each time. The body is always using energy the best way possible.

    My guess is that the sudden drop in BG ten minutes after drinking coffee could be caused by adrenaline or perhaps the incretin hormones from the gut are rising which will increase insulin production that results in the momentary drop in BG.

    I have also heard that it is well known that coffee assists with fat burning. If this is the case then since Bjarte is keto adapted he may be approaching fasting BG level after 2 hours but by drinking coffee he further stimulates fat burning (lypolysis) which increases the output of Free Fatty Acids and Glucose from the liver as it breaks down the flow of triglycerides coming from the adipose tissue. The increase in glucose production from the liver would explain the BG 77 being above the fasting 65.

  30. kate
    My body feelings tell me your findings could be possibly true, coffee does boost my performabce. But what I am more interested is a whole day cycle. Because a few more hours later I will feel more tired compared to I don't drink coffee. I guess when blood sugar decreases, it is our body to send signal to our brain to eat and rest, while we exhaust us by drinking coffeez
  31. PulsarH
    My guess this is insignificant change due to many variables... by the way your activity level can affect blood sugar too.
    However there may be a slight effect on either insulin or Glucagon secretion; thus might decrease or increase blood sugar.

    Vjarte Bakke, Did you established the base line of your fasting levels for many days or you just picked a random one day fasting?

    Thanks for the post ... very helpful

  32. Sam zayn
    It's well known that carbs are diabetics worst nightmare let's be realistic all this other info.that says otherwise is a bunch of hog wash and most so called diet doctors are scamms that prey on people's vaurnability ...for money or statues
  33. Sam zayn
    It's all in the food we eat ...everything is injected with hormones for bigger bottom line
  34. YukonBlend
    Unless done with proper scientific methods, your data is garbage & any conclusions drawn from it are garbage as well. You are doing your readership a huge disservice by publishing this. Shame on you.??
  35. Dwight
    Qustion; did you drink anything else on those days you when you weren't drinking coffee? A raise in glucose, although it was mofest, might just be a response to food despite it having little carbs in it. Kind of a Pavlovian response.
  36. Subramanian Ramajayam
    I do not think drinking coffee will upset the blood sugar control,if taken without sugar.
  37. Richie
    The problem is the testing will never be exact because your body changes day to day for a diabetic it's even worse even if you ate and drank the exact same thing in the same amount ever day it would never match everything impacts your body including time of day amount of sleep sunshine
  38. Joyce E Liberty-Clayton
    I'm a diabetic and prior to being diagnosed with the disease in 2003, I had been a pot-a-day caffeinated black coffee drinker, until I quit drinking the beverage around that time. I recall mentioning to my husband that as long as I was drinking coffee I had no issues with blood sugar. Anyway, there are so many variables involved in your sugar spikes and plunges on days when you drink coffee; what you've eaten plays a large role, so ...
  39. Bjarte Bakke Team Diet Doctor

    Wait, was your diet exactly the same for the two days? You day nothing about what else you consumed.

    I ate ketogenic both days and similar foods but not exactly the same. I didn't eat anything 10 hours before the experiment, and nothing during the experiment.

    Sleep, stress, and activity levels were all similar both days.

  40. Bjarte Bakke Team Diet Doctor

    I wouldn't be surprised if the results next time can be just the opposite.

    Fair point, the results could be random variability.

  41. Bjarte Bakke Team Diet Doctor

    Actually if you haven't consumed any food during this time your blood glucose is at a healthier level with the caffeine (coffee). Your levels without it actual border on hypoglycemia. There has been a great deal of data testing caffeine (black coffee) and its effects on blood sugar. The results have been either neutral or a positive effect, meaning more often in the normal range.

    I'm in ketosis and therefore less sensitive to lower levels of blood glucose.

  42. Dindo
    Your experiment is illogical..everybody knows that coffee has zero sugar content and I think it also has no carb component..so trace back what you ate previous or prior to taking that experiment. .like low glycemic index food takes sometime to become sugar.
    Replies: #43, #48
  43. Victor
    Caffeine is a stimulant and as such it is a reasonable hypothesis that perhaps it triggers the body to release glucose.

    Your experiment is illogical..everybody knows that coffee has zero sugar content and I think it also has no carb component..so trace back what you ate previous or prior to taking that experiment. .like low glycemic index food takes sometime to become sugar.

  44. Sina matata
    Am glad to see this testing or research, whatever we can call it. I am a type 2 dietetic on metformin 1000 (bid). I love to take a cup of coffee every morning. However, to get a helpful research on the effects of coffee on dietetics will depend on many factors, what you are before, either active or not, what was in the coffee, was it decaffeinated, or caffeinated, and many other things. Please consider those, and it will help many of us. Thanks for what you are doing.
  45. Gerry Richards
    I drink coffee all day everyday as I own a coffee shop also type 2 diabetes and There is no way sugar levels raise drinking coffee Gerry Richards
  46. Rita Gomez
    How do you make your coffee, with sugar, flavored syrup? Do you put creamer, skinny or whole milk? I believe those elements matter unless you take your coffee black and no sweetener whatsoever.
  47. Ginger
    While yes your drinking black coffee (As it was clearly stated people) did you eat the exact same foods and portions both days, that is truly the only way to know considering different foods have different levels of sugars and different portion sizes are going to change that as well I would love to know the results on two separate days of eating the exact same thing drinking the exact same amount while consuming the exact same amount of coffee. I'm a coffee drinker myself and I have type 2 diabetes I can't even imagine having to give up my coffee but if it makes that much of a difference I think I would just do it but I don't have it in me to try to do the research myself LOL
  48. Rainday
    It sounds illogical, but I can tell you that caffeine affects blood sugar. Type one diabetic here. Whenever I drink coffee or energy drinks, my blood sugar spikes. So there.
    (And no, I do not take sugar in my coffee and I only drink sugar-free energy drinks)
  49. Michelle
    Coffee can absolutely raise insulin levels. Caffeine causes a response from the adrenals, adrenaline well known, but also cortisol. Cortisol induces pancreatic release of insulin. Which may be why you saw the dip initially, then if you wind up hypoglycemic after the cortisol/insulin surge then the liver starts dumping glucose from its stores to bring it back up...and when this happens and body is reacting to hypoglycemia it usually overshoots the target and the. Lois sugar is higher than if no coffee and a low carb breakfast.
  50. Kyle
    So, comparing the glucose values between the "tests," the only conclusion is that there is no difference with respect to acceptable 2 hour fasting values (<140mg/dL). Variations in the range of your comparison values have no biological significance (especially with no established baseline), in addition to the numerous other flaws with even considering this an experiment, pointed out by others. When you do things like this, please share with your readers the normal clinical range expected for these values so they understand your graph doesn't show anything.

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