Does coffee raise blood sugar?
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A little, for a while. In 7 short trials in healthy people, caffeine on its own made them less sensitive to insulin. But over years, each daily cup of coffee went with 9% less type 2 diabetes, and decaf went with less too. Watch the syrup, sugar and sweet creamer, not the coffee.
What we’d do
Keep your coffee and drop the sweet add-ons. If you wear a sensor and want to know for yourself, eat the same breakfast two mornings with coffee and two without, and compare the hills. Leave coffee until after a fasting blood test.
Why people get mixed messages
Both headlines are true. One kind of study gives people caffeine and measures how well their insulin works over the next few hours, and it works a bit less well. Another kind follows hundreds of thousands of people for years, and the coffee drinkers get less diabetes. They’re answering different questions.
What caffeine does in the short term
A 2016 review pooled 7 randomized trials in healthy people where caffeine was the only thing that changed. Against a placebo, caffeine lowered insulin sensitivity by a wide margin. The authors wrote that “in the short term, caffeine might shift glycemic homeostasis toward hyperglycemia”, meaning it nudges blood sugar up. That’s caffeine on its own, without everything else that’s in a cup of coffee.
What coffee does over years
A 2014 review pooled 28 long-term studies with 1,109,272 people, of whom 45,335 got type 2 diabetes. The more coffee people drank, the lower their risk.
| Cups a day | Risk of type 2 against no coffee |
|---|---|
| 1 | 8% lower |
| 2 | 15% lower |
| 3 | 21% lower |
| 4 | 25% lower |
| 6 | 33% lower |
Each extra daily cup of caffeinated coffee went with 9% lower risk, and each cup of decaf with 6% lower. Decaf doing almost as well points at something in coffee besides the caffeine.
These were observational studies, so coffee drinkers may differ from non-drinkers in other ways. It’s a strong pattern across 28 studies, but studies like these can’t prove coffee is the cause.
What actually raises it
Blaming coffee for a sweet latte is like blaming the cup for what’s poured into it. Two teaspoons of sugar is 8 grams, twice the amount of sugar in all the blood of a 154 pound adult right now, which is about 4 grams. A sweet coffee drink can carry a lot more than that. In a pooled study of 17 groups of people, each daily serving of a sugary drink went with 13% more type 2 diabetes, even after adjusting for weight.
If you’re getting a fasting blood test, leave coffee until after the draw, black or not, since caffeine can nudge the number. Caffeine affects blood pressure and HRV too, covered on bloodpressure.guide and hrv.guide.
5 sources
- Shi X and others, Acute caffeine ingestion reduces insulin sensitivity in healthy subjects: a systematic review and meta-analysis, Nutrition Journal, 2016pmc.ncbi.nlm.nih.gov
- Ding M and others, Caffeinated and decaffeinated coffee consumption and risk of type 2 diabetes: a systematic review and a dose-response meta-analysis, Diabetes Care, 2014pmc.ncbi.nlm.nih.gov
- Harvard T.H. Chan School of Public Health, Added sugar in the diet, The Nutrition Sourcenutritionsource.hsph.harvard.edu
- Wasserman DH, Four grams of glucose, American Journal of Physiology, Endocrinology and Metabolism, 2009pmc.ncbi.nlm.nih.gov
- Imamura F and others, Consumption of sugar sweetened beverages, artificially sweetened beverages, and fruit juice and incidence of type 2 diabetes, BMJ, 2015pmc.ncbi.nlm.nih.gov
