Scientists from the University of Bonn in Germany say the traditional belief that garlic thins the blood and combats heart disease has no basis in fact.

Dr Heiner Berthold and colleagues tested commercial garlic oil preparations and found no evidence of lowered cholesterol levels.

Groups who promote garlic have already attacked the research. They say the work undertaken by the German team is fundamentally flawed.

Accurate tests

Dr Berthold designed "the most accurate test possible" of the effect of garlic after conflicting results from previous studies.

At the start of the study, 25 volunteers with moderately high cholesterol levels were given a placebo for four weeks, not realising it was a dummy preparation.

Some then received the garlic preparation and some a further placebo for 12 weeks, after which they all took a placebo again for another month.

Finally, the 12-week treatment was repeated, but reversed. Those who took the garlic preparation before now took the placebo, and vice-versa.

Cholesterol concentrations from blood samples at the beginning and end of each phase were measured. In addition, participants were given cholesterol in a pill form to see if garlic affected the way it was broken down by the body.

No significant effects

The research could undermine sales

The research showed no significant effects of garlic, either on cholesterol blood levels or metabolism.

The Journal of the American Medical Association, which published the findings, said: "The authors state that this study provides no evidence to recommend garlic therapy for lowering serum lipid levels, in spite of the fact the dosage of garlic administered (the equivalent of four to five grams of fresh garlic cloves) in the study was relatively high."

The trial volunteers were given two doses of garlic a day, equivalent to a total of three to four garlic cloves.

Surprise result

Dr Berthold said: "We were expecting to see a cholesterol lowering of 5-10%, but there was nothing. It was a surprise.

"Everyone has been led to believe garlic can lower cholesterol, but it doesn't appear to have any effect at all. It's a deception on the public. Garlic supplements have a huge market here in Germany, worth about £50 million a year.

"Everyone has been led to believe garlic can lower cholesterol, but it doesn't appear to have any effect at all. It's a deception on the public. Garlic supplements have a huge market here in Germany, worth about £50 million a year.

"Our concern is that people who might need effective treatment are not getting it because they think all they have to do is take garlic."

He said garlic was supposed to have other benefits, such as reducing the effect of platelets in making the blood sticky and anti-oxidant properties, that had not been tested. But there did not seem to be good evidence to support those claims either.

'It's just the oil'

However, Peter Josling, from the Garlic Information Centre, an independent organisation set up to promote the benefits of the plant, said the research only told half the story.

Mr Josling said he was 'not surprised' that the research found no benefit from garlic oil, but said fresh garlic, or even powdered garlic was effective in cutting both cholesterol and blood pressure levels.

"I would not expect garlic oil to reduce cholesterol because it does not contain the compound allicin, which is found in fresh and powdered garlic, and which reduces cholesterol." he said.

"Garlic oil products take raw garlic and boil it up, that starts a chemical reaction which destroys allicin.

"I am concerned that research like this could end up with the statement that garlic does not reduce cholesterol, because the evidence that it does is overwhelming."