Garlic has no detectable effects on levels of cholesterol in the bloodstream, a trial suggests.

While this does not prove that garlic is ineffective in protecting the heart, it shows that any effects it may have are not caused by it reducing cholesterol, a claim often made by health-food companies.

“Garlic supplements, many of which seek to package the benefits of raw garlic in more palatable forms, are promoted as cholesterol-lowering agents and are among the top-selling herbal supplements,” the authors of the study say in Archives of Internal Medicine.

“Based on our results and those of other recent trials, physicians can advise patients with moderately elevated LDL [or “bad”] cholesterol concentrations that garlic supplements or dietary garlic in reasonable doses are unlikely to produce lipid benefits.”

When garlic is crushed, it forms allicin, a compound shown in the laboratory to prevent cholesterol. But clinical trials in humans have been inconsistent. Some previous studies had suggested that garlic could reduce cholesterol by about 12 per cent.

Christopher Gardner, of Stanford University Medical School, in California, divided 192 people aged 30 to 65 into four groups. One group was given raw garlic, one a powdered garlic supplement called Garlicin, one an aged garlic supplement called Kyolic 100 and one a placebo. All had moderately elevated levels of LDL cholesterol and took the garlic or pills for six months.

Levels of good and bad cholesterol and triglycerides in the bloodstream remained unchanged in each groups. The main sideeffect was predictable: bad body and breath odour in 57 per cent of those on raw garlic.

“The results should not be generalised to other populations or health effects,” the team writes. “Garlic might lower LDL in specific subpopulations, such as those with higher LDL concentrations, or may have other beneficial effects.”

In an editorial, Mary Charlson and Marcus McFerren, of Weill Cornell Medical College, New York, urge people not to assume that garlic has no health benefits at all.

“The results do not demonstrate that garlic has no usefulness in the prevention of cardiovascular disease,” they say.

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