Filtering coffee does not remove a chemical linked to heart disease and stroke, researchers claim.

Scientists in the Netherlands said they were surprised that levels of homocysteine did not drop when the coffee was filtered.

It had been thought that by filtering coffee and removing chemicals called ditrepenes, responsible for raising cholesterol levels, homocysteine content might be reduced.

Diterpenes would be removed by sticking to the filter paper.

No reduction

But two sets of experiments testing people who had or had not drunk strong coffee over a two week period showed that there was no reduction.

Dr Martina Grubben of the University Hospital Nijmegen asked 30 adults to drink a litre of unfiltered coffee every day for two weeks. Another 34 people did not drink coffee.

The results, published in the American Journal of Clinical Nutrition, showed that unfiltered coffee raised homocysteine levels by around 10%.

A follow-up study by Dr Petra Verhoef, of the Wageningen Centre for Food Sciences, tested the thinking that the increase was caused by the presence of diterpenes.

She asked 26 volunteers to drink no coffee or strong filtered coffee every day for four weeks and found that homocysteine levels rose by 18 per cent in those drinking filtered coffee.

Twice as strong

The coffee they drank was around twice as strong as that in Dr Grubben's study, suggesting similar levels of homocysteine increase relative to the amount of coffee drunk, whether it was filtered or unfiltered.

Dr Verhoef said she was surprised to see the increase in homocysteine even without the presence of diterpenes.

The aim now is to find other compounds which could be responsible for raising homocysteine levels.

Dr Verhoef told BBC News Online: "Now we know it must be another compound other than disterpenes. We still don't know what the responsible compound is in coffee which raises homocysteine. It could be caffeine."

Previous studies have shown that coffee drinkers have higher homocysteine levels, but it was thought this could have been due to them smoking more or having lower folic acid levels due to eating less fruit and vegetables.

The Coffee News Information Service said that raised levels of homocysteine had yet to be conclusively confirmed as a factor in the development of cardiovascular disease.

In a statement, the association said: "To date, evidence for an association between coffee consumption and plasma homocysteine levels is thin and contradictory and associations do not prove cause and effect relationships."

The studies are reported in New Scientist magazine.