Does coffee raise blood sugar? Conclusion.

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

For the last few weeks, I’ve been testing whether or not drinking coffee raises my blood-sugar levels.

The answer may seem obvious as coffee contains no sugar, but some people believe coffee can have an effect, and our first coffee experiment indicated just that.

Since then, I’ve repeated the experiment twice. Here’s what I found.

Planning the coffee experiment

I designed the following experiment: I would drink a cup of coffee and measure my blood-sugar levels two hours prior to and after drinking it. Then I would analyze the data to see if drinking coffee seemed to raise my blood-sugar levels.

To increase the reliability of the experiment, I made sure of four things:

1. I would drink the coffee black – nothing would be added to it.
2. I wouldn’t eat or drink anything else, feel stressed, nor do any form of exercise, 2 hours prior to and after drinking the coffee.
3. I would eat ketogenic.
4. I would go to bed and wake up around the same time as I normally do.

It was coffee time.

Drinking coffee

The house was quiet, but I was up, feeling excited about doing another experiment. Everything was ready – beans, scale, grinder, and coffeemaker. The glucose sensor was safely installed in my body – constantly monitoring any change in blood-sugar levels.

Using the above food scale, I put 0.63 ounces (18 grams) of coffee beans into the grinder (about the same amount as for a double espresso). I turned it on. “Grrrrooooooooooooooooouhhh”, it sounded like an elephant had been let loose in the kitchen!

After 20 seconds, the noise finally stopped. I picked up the coffee-bean container, held it to my nose, and inhaled. “Mmmmmmmm, what a delicious smell!”.

I poured the coffee into the Bialetti coffee maker along with 7 oz (2 dl) of boiling water. A few minutes later the coffee was ready. I immediately took a sip. “Oooh, powerful!”.

The coffee didn’t taste as well as it normally does as I hadn’t added any cream – my favorite coffee is the Creamuccino – coffee with steamed cream – but I couldn’t have that this morning as it would affect the experiment. Still, the coffee was nice and a few minutes later there was no more.

Using the constant-glucose-monitoring app I was watching any change in my blood-sugar levels like a hawk. “Ohhhhhh, interesting!”.

Coffee does NOT seem to raise my blood-sugar levels

The graphs below show my blood-glucose levels over a 4-hour period for three separate experiments. The blue line is the blood glucose on a morning I drank coffee. The red line is on a morning without coffee.

Experiment one

The first experiment indicated that coffee may raise my blood sugar. But only slightly:

blood-sugar-coffee-first-experiment

The graph above could suggest that drinking coffee prevents my blood-sugar levels from dropping to the same levels as the morning when I didn’t drink coffee.

Experiment two

The second experiment indicates that coffee does NOT noticeably raise my blood sugar. There’s a lot of variety in my blood-glucose levels on these two days, but to us it seems that coffee does not raise my blood sugar relative to not drinking coffee (the control).

blood-sugar-levels-experiment-with-coffee-and-no-coffee-2

Experiment three

The third experiment also suggests that coffee does not raise my blood sugar much. My blood-sugar levels are quite flat, both on the morning when I drank coffee and the morning I didn’t, indicating that the coffee didn’t impact my blood-sugar levels much.

blood-sugar-levels-experiment-with-coffee-and-no-coffee-3

Conclusion

The chart below shows my average blood-sugar levels based on the three experiments.

blood-sugar-levels-experiment-with-coffee-and-no-coffee-average

To me, this indicates that drinking coffee does not significantly raise my blood-sugar levels.

But, the chart also indicates that my blood-sugar levels on coffee-drinking mornings are on average slightly higher than on mornings I didn’t drink coffee.

However, this difference is not significant and could be due to random variance in blood-sugar levels (see for example how the blood-sugar levels start out higher on coffee-drinking mornings even though I haven’t had any coffee yet).

No significant change in blood-ketone levels

The below chart shows my average blood-ketone levels on coffee-drinking mornings for experiment two and three (regretfully, I didn’t measure ketones for the first experiment).

blood-ketones-levels-experiment-with-coffee

To us, this graph also seems to indicate that coffee does not raise my blood sugar significantly.

If coffee had raised my blood-sugar levels significantly, we would have expected insulin levels to increase as well, causing a subsequent drop in ketone levels. But there was no significant decrease in ketone levels after drinking coffee.

What happens to your blood-sugar levels when you drink coffee?

Please note: This is a n=1 self experiment and my findings may not apply to you. I am a 36-year old insulin-sensitive male, weigh 152 pounds (69 kg), exercise for 10-15 minutes five times a week, with no history of obesity or diabetes.

Although coffee doesn’t seem to raise my blood-sugar levels, the same may not be the case for you.

What happens to your blood sugar when you drink coffee? Let us know in the comments below.

How to test

To test what happens to your blood-sugar levels after drinking coffee, do this:

1. Eat and drink nothing for 2 hours.
2. Measure your blood glucose – this is your baseline.
3. Drink a cup of coffee with nothing added to it (no sugar, no sweeteners, no milk, no cream etc.). Don’t eat or drink anything else for the next two hours, and avoid exercise and stress (these can impact blood sugar).
4. Measure your blood-sugar levels every 30 minutes up to two hours after drinking the coffee (five times in total including the baseline measurement).
5. Evaluate your results. Is there a significant increase or reduction in your blood-sugar levels? *

* It’s normal for blood-sugar levels to fluctuate during the day even when you don’t eat. Therefore, smaller changes – up or down 0.5 mmol/L (10 mg/dl) for example – could be random variation and not caused by drinking coffee.

What do you want us to test next?

We’ve received lots of suggestions for what else to test, for example sugar alcohols, more artificial sweeteners, fasting, alcohol, and resistant starch. What else would you like us to test?

Thanks for suggesting ways to improve these experiments. As per your request, we’ll measure ketones in addition to blood sugar. However, for the experiments we’ve already done but not yet published, ketones data may lack.

Measuring insulin levels is tricker. Do you know of a reliable and simple way to measure it several times per hour? Let us know in the comments below.

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?
  5. Does Coffee Raise Blood Sugar? Preliminary Findings

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70 comments

Top comments

  1. Chris
    I am an insulin sensitive person with diabetes. I am female. I've had Type 1 diabetes 44 yrs. Coffee, black, DOES increase my blood sugar! Quite significantly. Everyone's body is different.
    Read more →
  2. Erica
    I've been doing intermittent fasting for a year now, skipping breakfast. I drink black coffee (3 cups) every day. I haven't done the specific experiment you did, but I notice that just prior to eating my lunch (anywhere from 11:30am to 1:30pm) my blood sugar is at its lowest (90 or below). I am a T2D (dx 2008) and was very insulin resistant. In the past year I have lost 50 lbs and my A1c went from 12.6 to 5.8. So for me, coffee helps me fast and does not raise my blood sugar.
    Reply: #48
    Read more →
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All comments

  1. Warner Dubicki
    Coffee raises my sugar for sure. I have type 2diabetes.
  2. Travie Wells
    I am a type 1 diabetic and consuming plain, black coffee does in fact raise my blood sugar level significantly! So much so, that I no longer consume it.
  3. Yasser
    Great experiment. The only part i suggest is that you may need consider rise/drop instead of absolute values. I see comparing average is misleading as the starting point for coffee experiment is consistently higher than non coffee. Ideally same time in the day, same conditions, and same starting point would do. But we all know very difficult to achieve so i suggest recording rise/drop referenced to the starting point
  4. Dee
    What about it's influence on insulin sensitivity? Do you feel coffee/caffeine reduces insulin sensitivity? If so, do you feel it would affect blood sugar if consumed with a meal containing carbohydrates?
  5. Heaven Tuite
    For type 2, this experiment has nothing to do with me. I would love to see you try this on someone with type 2 diabetes and see how they score. I had to give up coffee because it was the cause of major issues with my blood sugar. I drink black coffee but I definitely see changes immediately with my blood sugar just by coffee. If I eat a pastry or a donut with a coffee it’s allllll bad.
  6. James Close
    I am eating breakfast right now and drinking black coffee as usual. I tend to keep sugar out of my diet. I felt a sudden sluggish feeling and realized I'm low on sugar. Looked at you're experiments and notice a huge drop in blood sugar right at the time you drank coffee. Argument that one cup temporarily lowers blood sugar?
  7. Scott T.
    another variable to consider is this experiment for a pre-diabetic or diabetic person, but with a defined 'meal' eaten, say, 2 hours after the coffee. black coffee doesn't affect my bg directly by itself as far as I can tell, but if I eat within ~4 hours of drinking it, I get spikes far beyond what the same meal does later in the day.
  8. 1 comment removed
  9. Penny
    Exactly! Assuming you have normal insulin response all you might be proving here is that you are producing enough insulin to correct for the BS increase the coffee "might" be causing. I'm a TD1 and inject rapid acting insulin just before drinking coffee and see a similar pattern. If I don't, I see a significant increase in BS. Be interesting to test a few TD1s. Also mornings are a bit mysterious as to blood sugar compared to other times of the day because of the dawn phenomenon many people exhibit. Trying the coffee experiment at different times of the day might also produce different results.
  10. William Ward
    I'm a type 2 diabetic on insulin and tablets. I had been out shopping this afternoon, and before returning home I have to test my sugars before driving (a legal requirement in the UK for insulin users every 2 hours) and tested at 5 mmo/L just on the limit. I got home put some coffee on whilst my evening meal was cooking, drank half the mug of coffee and tested my bloods before eating and they had increased to 8 mmo/L and this is quite a regular occurrence for me. I have to test before meals as I am on a clinical trial and have to keep a diary 2 in fact as I have one for the hospital clinic I attend.
    Reply: #65
  11. john perkins
    doctors are out on this one my ex wife husband doctor swears it effects the pancraes which raises blood sugar my doctor said she been drinking for years and to keep drinking as it has no effect wow cant get a straight answer
  12. Jae
    Thank you so much for posting this experiment! Great info, and a great point of reference for those of us watching our sugar levels.
  13. Jonathan Payne
    You're one person bro. N=1 can't be used to make conclusions.
  14. Marion C Holt
    I don't feel your experiment can be a true conclusion! Everyone is different!
    I am a type 1 diabetic....
    I take insulin 2x a day,
    I take fast acting insulin before each meal
    I also have celiac disease
    I also have many other medical issues
    Drinking coffee, black or other wise strongly effects my glucose levels
    As does stress, illness etc.
    So unfortunately, thank you for your efforts, but not something to be used for health care.
    Get to know YOUR body and it will let you know what helps you function
  15. Marion C Holt
    I am a type 1, diabetic. I was told by the diabetic clinic not to take the rapid acting insulin at no interval less than 4 hours apart. As this can cause what the call an insulin load, that can cause the insulin to not be effective. I live in Canada and we have a similar test for driving. I need to have my eyes checked as well every 2 years, and sign a document confirming I will not drive if my levels are above or below a certain level. Makes life very restrictive. Had to quit my job as I drove daily to appointments.

    I'm a type 2 diabetic on insulin and tablets. I had been out shopping this afternoon, and before returning home I have to test my sugars before driving (a legal requirement in the UK for insulin users every 2 hours) and tested at 5 mmo/L just on the limit. I got home put some coffee on whilst my evening meal was cooking, drank half the mug of coffee and tested my bloods before eating and they had increased to 8 mmo/L and this is quite a regular occurrence for me. I have to test before meals as I am on a clinical trial and have to keep a diary 2 in fact as I have one for the hospital clinic I attend.

    I'm a type 2 diabetic on insulin and tablets. I had been out shopping this afternoon, and before returning home I have to test my sugars before driving (a legal requirement in the UK for insulin users every 2 hours) and tested at 5 mmo/L just on the limit. I got home put some coffee on whilst my evening meal was cooking, drank half the mug of coffee and tested my bloods before eating and they had increased to 8 mmo/L and this is quite a regular occurrence for me. I have to test before meals as I am on a clinical trial and have to keep a diary 2 in fact as I have one for the hospital clinic I attend.

  16. Andrew
    I'd really like to see the coffee - BGL experiment repeated with 50mL of milk included in the coffee.

    I have tried a couple of goes measuring BGL before and 1 hr after drinking instant coffee with 50mL of milk (whilst doing 16:8 and 20:4 intermittent fasting) - unsure if the coffee + milk causes a rise in BGL, or just "random" stress events causing BGL to rise slightly.

    Reply: #67
  17. Crystal Pullen Team Diet Doctor

    I'd really like to see the coffee - BGL experiment repeated with 50mL of milk included in the coffee.
    I have tried a couple of goes measuring BGL before and 1 hr after drinking instant coffee with 50mL of milk (whilst doing 16:8 and 20:4 intermittent fasting) - unsure if the coffee + milk causes a rise in BGL, or just "random" stress events causing BGL to rise slightly.

    As milk is high in lactose (milk sugar), you should expect an increase in BGL. If this is a concern for you, give heavy cream a try as it has less lactose and more fat.

  18. VL
    I have reactive hypoglycemia and insulin resistance from Polycystic ovarian syndrome. Drinking coffee, even straight black coffee will cause my sugar to drop to dangerous levels in 20 minutes, especially when my insulin levels are too high.
  19. Brian Lucido
    I'm always fascinated by blood sugar experiments in people who have a functioning pancreas. The issue is: If you eat something and your blood sugar doesn't go up, is that because the test food/beverage doesn't have a glycemic effect, or because your body is able to release enough insulin to counteract those effects? Without testing blood insulin levels, you never know. I'm a type 1 diabetic, and since my body does not produce insulin, I can answer the question: I, and most (but not all) type 1 diabetics need to inject insulin for black coffee. I'm a member of several T1d Facebook groups, and that is definitely the consensus. This doesn't mean that coffee is bad, but if you're trying to avoid an insulin rise as a non-diabetic, you MIGHT wish to limit your coffee consumption. For reference, it seems like most people need about 1 or maybe 2 units of insulin to counteract the blood sugar rise from coffee. This comes from poking around in a lot of Facebook comments, and is by no means scientific. For those who don't know, that could be the equivalent of 10g to 20g of carbohydrate. Coffee does have a minimal amount of carbohydrate - but it is possible that the blood sugar increase comes from the liver converting stored glycogen to glucose (there are papers on this). Another possibility is that caffeine causes an increase in lipid metabolism (there are also papers on this).... I haven't seen research on the following, but here is my theory: If lipids are being favored as fuel for a period of time, that means that glucose utilization would temporarily decrease. If that is the case, the glucose, which is being released at a steady state from the liver of a diabetic might start to accumulate in the blood while the lipids are being favored. This would be easily mitigated in a non diabetic, because a very small amount of insulin released from the beta cells (which are right next to the glucagon releasing alpha cells) could quickly reduce the amount of glucose being released by the liver. Unfortunately, the insulin injected by T1d's does not have that proximity to the alpha cells, and hence the need to inject additional insulin.
  20. Brenda
    It does raise my blood sugar. But it seems to go back to normal in about an hour. There’s contradictory information on diabetics drinking coffee. I wonder if maybe coffee flushes sugar out of a fatty liver (which a lot of diabetics have) into the blood stream where it can be disposed.
    Reply: #71
  21. Crystal Pullen Team Diet Doctor

    It does raise my blood sugar. But it seems to go back to normal in about an hour. There’s contradictory information on diabetics drinking coffee. I wonder if maybe coffee flushes sugar out of a fatty liver (which a lot of diabetics have) into the blood stream where it can be disposed.

    Anyone who suffers from diabetes should test their blood glucose with any food or drink of concern.

  22. Lenka
    Diabetes type 1, there is no effect of black coffee (seen many times). And yes, this was test done by non-diabetic person producing enough insulin to correct for possibly increasing BS, but he is ketogenic, if he produced some insulin, his ketones would be affected, but they were not. :)
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