Does Caffeine Raise Blood Sugar? The Dose Math, Per Can

Our can says 0g sugar. That is true, and it is on the Nutrition Facts panel. It is also not the whole blood-sugar story, and I would rather you hear that from the brand than figure it out on your own later.

The reason is the other number on the can: 320mg of caffeine. Caffeine is not sugar and it contains no calories, but in controlled studies it has a measurable short-term effect on how the body handles glucose. Most coffee brands leave that out, because "zero sugar" is a cleaner sentence. This post covers what the research found, how the doses in those studies compare to a single 12 fl oz can, and what the studies did not show.

This is general information about coffee and caffeine research. It is not medical advice. If you manage blood sugar or take medication for it, your clinician is the right person to talk to.

The short answer

In small, controlled studies, caffeine taken before a carbohydrate load was associated with a higher blood-glucose and insulin response than placebo, and with lower measured insulin sensitivity. The effect was dose-dependent. Over the long term, though, large population studies associate habitual coffee drinking with a lower risk of type 2 diabetes. Both findings hold up, and they measure different things. The rest of this post works through why.

What the controlled studies actually found

Four studies come up again and again. Here they are, with the design details that matter.

1. Keijzers et al., Diabetes Care, 2002

Twelve healthy volunteers, randomized, double-blind, crossover. Caffeine or placebo was given intravenously, and insulin sensitivity was measured with a hyperinsulinemic-euglycemic clamp, the gold-standard method. Caffeine lowered insulin sensitivity by about 15%, from 0.46 to 0.39 μmol/kg per min/mU/l. Because the caffeine went into a vein, this tells you what caffeine does. It does not tell you what a cup of coffee does.

2. Moisey et al., American Journal of Clinical Nutrition, 2008

Ten healthy men drank caffeinated coffee (dosed at 5 mg of caffeine per kg of body weight) or the same volume of decaf. An hour later they ate a cereal providing 75g of carbohydrate, either high or low glycemic index. With the high-GI meal, caffeinated coffee produced a 147% greater area under the curve for glucose, 29% for insulin and 40% for C-peptide, compared with decaf. Those are big percentages from a small sample: ten men.

3. Beaudoin et al., Applied Physiology, Nutrition, and Metabolism, 2013

Twenty-four healthy volunteers, 12 men and 12 women. Each did four trials: caffeine at 1, 3 or 5 mg/kg, or placebo, followed an hour later by a two-hour oral glucose tolerance test. The authors found that insulin sensitivity got worse as the dose went up, starting at the lowest dose, with no difference between men and women. Every additional 1 mg/kg raised the insulin area under the curve by 5.8% and C-peptide by 8.7%.

4. Lane et al., Diabetes Care, 2004

Fourteen habitual coffee drinkers with type 2 diabetes who did not use insulin. Double-blind, placebo-controlled, crossover, using caffeine capsules. Caffeine did not change fasting glucose. After a liquid mixed meal, the caffeine condition showed a 21% higher glucose response and a 48% higher insulin response than placebo, according to Duke University's summary of the study.

The pattern across all four: caffeine taken before carbohydrates was followed by a higher glucose and insulin response in the short term. That is a statement about caffeine in a lab protocol, not a diagnosis of anyone's morning.

The dose math: where one 320mg can lands

Studies dose caffeine per kilogram of body weight. A can is a fixed 320mg. So I converted one can into mg/kg for a range of body weights. This is plain arithmetic: 320 divided by body weight in kilograms (pounds × 0.4536).

Three things stand out:

• For most adults, one can falls between 3 and 5 mg/kg. That is the same range as the middle and top doses in Beaudoin and the dose in Moisey. For a 150 lb person, Moisey's 5 mg/kg protocol works out to 340mg, which is about one can plus a sip.
• The lowest dose that showed an effect in Beaudoin, 1 mg/kg, is well under one standard cup. The FDA puts an 8 oz cup of coffee at 80 to 100mg. For a 150 lb person, 1 mg/kg is 68mg.
• One can is 80% of the FDA's 400mg daily reference. The FDA describes 400mg a day as an amount not generally associated with dangerous, negative effects in healthy adults. That is not the same thing as "no effect on glucose."

I'm not showing you this table to scare you off the product. I'm showing it because if you're going to pick a high-caffeine coffee, you should know what dose you're taking, measured the same way the research measures it. A 320mg can is a real dose. We print it on the front for that reason.

Coffee is not the same as caffeine

This is where most articles stop, and it is where the story starts to get interesting.

Battram et al. (Journal of Nutrition, 2006) gave men pure caffeine, caffeinated coffee, or placebo before a glucose tolerance test. Pure caffeine produced a 50% greater glucose area under the curve than placebo. Caffeinated coffee did not differ from placebo on glucose, although insulin was elevated in both caffeine conditions. The authors concluded that compounds in coffee other than caffeine seem to offset part of caffeine's acute effect on glucose.

The research does not all point one way. A 2011 study in the European Journal of Nutrition, titled "Coffee does not modify postprandial glycaemic and insulinaemic responses induced by carbohydrates," found no effect under its conditions. Results depend on dose, timing, the meal, whether the caffeine came from coffee or a capsule, and who was in the study.

For anyone reading labels, the takeaway is simple. Findings about pure caffeine, like the IV caffeine in Keijzers, do not transfer directly to brewed coffee. Findings about brewed coffee are mixed.

The paradox: short-term response vs long-term association

Now the other side. Ding et al. (Diabetes Care, 2014) pooled 28 prospective studies covering 1,109,272 participants and 45,335 cases of type 2 diabetes. Compared with no or rare coffee drinking, the relative risk was 0.92 at one cup a day and fell steadily to 0.67 at six cups a day. Each extra daily cup was associated with a 9% lower risk for caffeinated coffee and 6% lower for decaf.

Two cautions keep this honest:

• It is association, not causation. These are observational cohorts. Coffee drinkers can differ from non-drinkers in many ways the researchers can only partly adjust for.
• Decaf showed an association too. That suggests whatever is going on is not only about caffeine, which fits the Battram finding that coffee and pure caffeine behave differently.

So a short-term study can show caffeine raising the glucose response to a meal, and a 20-year cohort can show coffee drinkers developing type 2 diabetes less often. Both can be true. They measure different things, over different time frames, in different ways.

What these studies did not test

• Tiny samples. 10, 12, 14 and 24 people. Useful for mechanisms, weak for predicting any one person's response.
• One-off lab protocols. Most gave caffeine on its own, an hour before a large carbohydrate load, after an overnight fast. That is not how most people drink coffee.
• Not cold brew specifically. None of these studies used ready-to-drink cold brew. I'm applying their dosing to our can by arithmetic, not claiming anyone measured our product.
• No individual prediction. Nothing here can tell you what your glucose will do. Only measuring it can.

What's actually in the can

Straight from the label of a 12 fl oz Bare Brew cold brew: two ingredients (Honduran Arabica coffee and water), 320mg caffeine, 0g sugar, 5g total carbohydrate, 20 calories, 15mg sodium, 350mg potassium.

Note the 5g of carbohydrate. It is small, it is not sugar, and it is on the panel. I'm including it because a blood-sugar article that skips its own carb line has no business lecturing anyone about labels. For how "0g" gets rounded on a Nutrition Facts panel in the first place, see Zero Sugar vs Sugar Free: What Coffee Labels Really Mean.

Choosing black coffee with no added sugar removes the sugar variable. The research above covers the caffeine variable. They are separate questions, and you should weigh both.

If you manage your blood sugar

I won't tell you what to do. That's your clinician's call. The research does suggest some questions worth bringing to the appointment:

• Does the timing of caffeine relative to carbohydrate-heavy meals matter for me? Most of the studies gave caffeine an hour before carbohydrates.
• Does my total daily caffeine matter, in mg/kg? Use the table above to find your number.
• If I wear a continuous glucose monitor, what would a fair self-comparison look like? For example, the same breakfast with and without caffeine on different days.
• Does anything I take interact with caffeine? Your prescriber or pharmacist is the one to ask.

For caffeine content across the category, normalized per fluid ounce, see our label-transcribed cold brew caffeine statistics for 18 products and how much caffeine is in cold brew. For another look at what coffee research can and can't prove, read our breakdown of the 2026 Finnish coffee and body composition study.

Founder's Take

When I started Bare Brew, "zero sugar" was the whole pitch, and it's still the claim I lead with, because it's true and most canned coffee can't say it. But I've read enough of the research to know "zero sugar" and "does nothing to your blood sugar" are different sentences. I only get to say the first one.

I also chose to make a 320mg can. That's a lot of caffeine, and I'd rather you know exactly how much, in the same units the studies use, than find out from a glucose monitor and feel misled. If the table says one can is 4.7 mg/kg for you, that's useful. Maybe half a can is your number. Maybe it's the whole thing at a different time of day. Maybe it's a conversation with your doctor first. Any of those is a better outcome than buying a can because "zero sugar" made you assume the rest.

Two ingredients and a big printed number is the whole idea. Nothing to hide includes the parts that don't make great ad copy.

— Miles, founder, Bare Brew

FAQ

Does caffeine raise blood sugar?

In small controlled studies, caffeine taken before a carbohydrate load was followed by a higher short-term glucose and insulin response than placebo, and lower measured insulin sensitivity, in a dose-dependent way. Studies of brewed coffee are more mixed, and long-term observational research associates habitual coffee drinking with lower type 2 diabetes risk.

Does black coffee with zero sugar affect blood sugar?

Zero sugar removes added sugar from the equation. Caffeine is a separate variable. Some studies found caffeinated coffee raised the glucose response to a meal, and others found no effect. Individual responses vary, so people who manage blood sugar should talk to their clinician.

How many mg/kg of caffeine is a 320mg can?

Divide 320 by your body weight in kilograms. At 150 lb (68 kg) it is about 4.7 mg/kg; at 190 lb (86 kg) about 3.7 mg/kg; at 250 lb (113 kg) about 2.8 mg/kg.

What is on the Bare Brew label?

Per 12 fl oz can: 2 ingredients (Honduran Arabica coffee and water), 320mg caffeine, 0g sugar, 5g total carbohydrate, 20 calories, 15mg sodium, 350mg potassium.

Sources

• Keijzers GB, De Galan BE, Tack CJ, Smits P. Caffeine can decrease insulin sensitivity in humans. Diabetes Care. 2002;25(2):364-369. PubMed 11815511
• Moisey LL, et al. Caffeinated coffee consumption impairs blood glucose homeostasis in response to high and low glycemic index meals in healthy men. Am J Clin Nutr. 2008;87:1254-1261. AJCN
• Beaudoin MS, et al. Caffeine ingestion impairs insulin sensitivity in a dose-dependent manner in both men and women. Appl Physiol Nutr Metab. 2013. PubMed 23438224
• Lane JD, Barkauskas CE, Surwit RS, Feinglos MN. Caffeine impairs glucose metabolism in type 2 diabetes. Diabetes Care. 2004;27(8):2047. PubMed 15277438; Duke Health summary
• Battram DS, et al. The glucose intolerance induced by caffeinated coffee ingestion is less pronounced than that due to alkaloid caffeine in men. J Nutr. 2006. PubMed 16614416
• Coffee does not modify postprandial glycaemic and insulinaemic responses induced by carbohydrates. Eur J Nutr. 2011. Springer
• Ding M, et al. Caffeinated and decaffeinated coffee consumption and risk of type 2 diabetes: a systematic review and a dose-response meta-analysis. Diabetes Care. 2014;37(2):569-586. PubMed 24459154
• U.S. Food and Drug Administration. Spilling the Beans: How Much Caffeine Is Too Much? (consumer update).