Daily consumption of caffeine in coffee, tea or soft drinks increases blood sugar levels for people with type 2 diabetes, research suggests.
Caffeine content of coffee can vary with brands and brewing methods
Caffeine pills equivalent to four cups of coffee a day increased blood sugar levels by 8% over the day, US researchers report in Diabetes Care.
Cutting caffeine out of the diet may help diabetics control their blood sugar levels, the team said.
But UK experts said more research was needed before advice could be given.
The ten people who took part in the study were monitored with a tiny glucose monitor embedded under the skin.
The device meant that the researchers could track the effects of caffeine over 72-hours as the patients with type 2 diabetes went about their normal lives.
Previous studies had shown that caffeine increases the body's resistance to insulin, the hormone responsible for managing the response to glucose levels in the blood.
But in healthy people this is not really a problem, said study leader Dr James Lane from Duke University Medical School.
In the diabetic patients, who took caffeine pills on one day and a placebo the next, caffeine caused blood sugar levels to rise.
The effect was particularly strong after meals with a rise of 9% after breakfast, 15% after lunch and 26% after dinner.
Dr Lane is planning to do another trial in larger number of patients to see if cutting caffeine from the diet can help patients control their blood sugar levels.
He said there are two possible ways that caffeine produces the effect.
It could be that caffeine interferes with the process that moves glucose from the blood and into muscle and other cells in the body.
Caffeine could also trigger the release of adrenaline which can also boost sugar levels.
"My advice would be, if patients are having trouble controlling their blood glucose and they are coffee drinkers, particularly heavy coffee drinkers, they might want to give it a try to see if it makes a difference to them."
He said he suspects in some people it would make a very big difference whereas others may not be so sensitive to it.
Cathy Moulton, care advisor at Diabetes UK, said: "Although this is interesting research, the study only examines a sample of 10 people for a 72-hour period, which proves very little.
"More research is needed before we ask people with diabetes to stop drinking coffee.
"The best way to control glucose levels is through healthy eating and exercise."