Drinking copious amounts of coffee each day may increase the risk of developing rheumatoid arthritis, Finnish scientists said on Wednesday.

A study by doctors at the National Public Health Institute in Helsinki showed that the more coffee a person drank the greater the chances of getting the disabling disease which afflicts millions of people worldwide.

People who drank 11 or more cups a day were almost 15 times more likely to get the illness than non-coffee drinkers.

"Coffee drinking proved to be a risk factor for rheumatoid arthritis," Professor Kimmo Aho, one of the researchers, said in a telephone interview.

The scientists do not know why coffee seems to raise the risk of the disease, but they suspect an unknown ingredient could trigger the production of rheumatoid factor (RF), marker antibodies for the disease.

The antibodies appear years before a person develops symptoms of rheumatoid arthritis and predict the progression of the disease.

The scientists monitored the coffee intake and the development of the disease in 6,809 people over 15 years.

None of the people had signs of the disease when the study began but a decade later 126 developed the disease and 89 had RF antibodies at the time they were diagnosed.

In a report in the medical journal Annals of the Rheumatic Diseases, the researchers said "the number of cups of coffee drunk daily was directly proportional to the prevalence of RF positivity."

The increase risk was not due to age, sex, smoking, weight or cholesterol levels. Aho and his colleagues called for further studies to confirm their findings.

A spokesman for the Coffee Science Information Centre, which is financed by coffee manufacturers, said consumers should not be concerned by the research.

"You can't draw conclusions from one study," he said, adding that when the study was done coffee was usually boiled and not filtered.

"If they did the study now using coffee as consumed in Scandinavian countries, which is primarily now filtered, they would find very different results," he added.

The cause of rheumatoid arthritis is unknown but doctors suspect it is partly due to genetic and environmental factors. The illness, which causes inflammation of the joints, is more common in developed countries and occurs more often in women.

(Agencies)