Caffeine may interfere with the way the body deals with blood sugars, worsening Type 2 diabetes, US scientists suggest.
Researchers are divided over the effects of caffeine
Duke University Medical Center researchers found a strong link between caffeine intake at mealtimes and higher blood glucose and insulin levels.
Writing in Diabetes Care, they suggest people with diabetes cut down the amount of caffeine they have.
But UK experts said diabetics simply needed to check their blood sugar levels to see what triggers increases.
In Type 2 diabetes, the body either does not produce enough insulin, which the body needs to convert food into energy, or is unable to make proper use of it.
Without enough insulin, the body cannot move blood sugar into the cells. Sugar builds up in the bloodstream and causes health problems.
Other research into caffeine and coffee has shown varying results.
Some have suggested caffeine reduces the body's sensitivity to insulin.
However others have found components in coffee, such as magnesium and chlorogenic acid, could help prevent people developing Type 2 diabetes.
The US researchers studied 14 habitual coffee drinkers with type 2 diabetes.
Study participants were put on to a controlled diet in which they took their medications, had their blood tested and then were given caffeine capsules.
They then had another blood test before being given a liquid meal supplement and undergoing a third blood test.
It was found that caffeine had little effect on glucose and insulin levels when the volunteers fasted.
But after the liquid meal, those who were given caffeine
had a 21% increase in their glucose level and insulin
James Lane, professor of psychiatry at Duke, who led the study, said: "In a healthy person, glucose is metabolised within an hour or so after eating. Diabetics, however, do not metabolise glucose as efficiently."
"It appears that diabetics who consume caffeine are likely having a harder time regulating their insulin and glucose levels than those who don't take caffeine."
He added: "The goal of clinical treatment for diabetes is to keep the person's blood glucose down.
"It seems that caffeine, by further impairing the
metabolism of meals, is something diabetics ought to consider avoiding.
"Some people already watch their diet and exercise
regularly. Avoiding caffeine might be another way to better manage their disease.
"In fact, it's possible that staying away from caffeine could provide bigger benefits altogether."
However Debbie Hammond, a Care Advisor at Diabetes UK was cautious about the study's findings.
She told BBC News Online: "Although interesting, this is a very small study.
"Much more research is be needed to clarify the significance of these findings.
"To improve diabetes control we'd advise people to regularly test their blood glucose levels so they are aware of fluctuations and can act on them."