Switching to decaffeinated coffee could increase the risk of heart disease, according to published research.

The drink may lead to a rise in harmful cholesterol by increasing a specific type of blood fat linked to the metabolic syndrome, researchers have found.

Metabolic syndrome is the umbrella term for a cluster of several harmful heart disease risk factors.

Three groups of people were used for the study which was presented at the American Heart Association's Scientific Sessions 2005.

One group drank three to six cups of caffeinated coffee a day, the second drank the same amount of decaf, and a control group that drank no coffee.

Researchers measured the level of caffeine in the blood of the 187 participants, as well as levels of the key heart-health indicators before and after the three-month study.

They found no significant changes among the participants' levels of blood insulin and glucose, or other major risk factors.

However, they reported for the first time that, after three months of coffee drinking, the decaffeinated group experienced an 18% rise in fatty acids -- the fuel in the blood that can drive the production of bad cholesterol.

The levels of fatty acids did not change in the other groups.

A protein linked to bad cholesterol (apolipoprotein B) went up 8% in the decaffeinated group but did not significantly change in the other two groups.

Studies suggest this protein may be a better predictor of cardiovascular disease risk than cholesterol level.

Dr. Robert Superko, lead author of the study and chairman of molecular, genetic, and preventive cardiology at the Fuqua Heart Centre in Atlanta, said, "These results are very surprising and have never been reported before for coffee consumption.

"There is a real difference between caffeinated and decaffeinated coffee, and contrary to what people have thought for many years, I believe it is not caffeinated but decaffeinated coffee that might promote heart disease risk factors that are associated with the metabolic syndrome, an expanding heart-health hazard."

Previous studies have linked coffee drinking to heart disease, but others have suggested that it is not harmful.

"If you only drink one cup each day, the results of our study probably have little relevance, because at that level your daily coffee dose is relatively low," Superko added.