Alternative medicine has been touting the health benefits of garlic for centuries, from its anti-bacterial and antifungal properties, to its positive effects on the cardiovascular system.

Now US researchers say they have figured out precisely why the pungent clove makes such a valuable health tonic: it boosts the body’s own production of a compound that relaxes blood vessels, increases blood flow, and prevents blood clots and oxidative damage.

“This will help us standardize over the counter garlic supplements, and ensure they have the ingredients that produce the key compound,” said David Kraus, a physiologist in the department of environmental health sciences at the University of Alabama at Birmingham.

Much of the research into the pharmacological benefits of garlic has focused on the organic polysulphides that the clove is rich in - the best known of which is Allicin.

But the new research suggests that Allicin and similar biologically active compounds are only a piece of the puzzle, and that it’s the chemical messenger that is produced when these compounds are metabolized that is important.

In laboratory tests, the researchers at the University of Alabama found that it was this chemical messenger - hydrogen sulphide (H2S) - which is essential at low levels for cellular signaling, that appears to relax blood vessels, enhancing blood flow.

The team conducted a series of experiments, first extracting juice from supermarket garlic and adding minute amounts to red blood cells. The cells immediately began emitting hydrogen sulphide.

Further experiments showed that the key chemical reaction took place mainly at the membrane of the red blood cells, although a fraction of H2S was also produced inside the cells.

Further, when the team added a section of rat aorta, a heart blood vessel, to a solution containing organic polysulphides, it began to relax as it produced H2S.

The finding may explain why some studies showed there were no cardiovascular benefits to be gained from taking garlic supplements while many others showed that such supplements could halt the progress of cardiovascular disease, the authors said.

“In addition, our results suggest that the capacity to produce H2S can be used to standardize garlic dietary supplements,” the authors wrote.

The study appears in the Proceedings of the National Academy of Sciences.