Scientists have uncovered fresh evidence that garlic can protect against some forms of cancer.

The research, by a team from the University of North Carolina at Chapel Hill, shows that people who eat raw or cooked garlic regularly cut their risk of stomach cancer by about a half compared with those who eat none.

They also cut their risk of colorectal cancer by as much as two-thirds.

There seems to be a strong, consistent protective effect

Professor Lenore Arab, University of North Carolina at Chapel Hill

Lead researcher Professor Lenore Arab said: "There seems to be a strong, consistent protective effect for people who are regular garlic consumers.

"It doesn't matter if they're consuming garlic in China or in the United States, the effect is still there."

However, the researchers found no such benefit from taking garlic supplements.

Professor Arab said it might be possible that this could be that the active ingredients are being destroyed in processing or by sitting on store shelves for a long time.

"Another possibility is that some of the people who turn to garlic supplements are sick already. That could skew the results."

The researchers based their findings on data from 22 previous studies from around the world on the impact of garlic on cancer.

Important compound

Professor Arab said previous research had shown that a compound in garlic called allium partially protects animals against cancer.

Some scientists believe it has the same effect in humans, she said.

Professor Arab said: "After controlling for various risk factors, we found that when we pooled the results, this preventive effect was largely confirmed.

"We didn't have enough information to be able to say the same about garlic's possible effects on other forms of cancer."

Co-researcher Professor Charles Poole warned that the possible benefits of consuming garlic might be somewhat overestimated in the study by what is called 'publication bias'.

This is the well-documented tendency of scientists and scientific publications to publish positive findings more often than results showing no effect.

"We also found the various studies were more inconsistent than we would have expected," he said.

Many scientists believe garlic helps prevent stomach cancer because it has anti-bacterial effects against a bacterium, Helicobacter pylori, found in the stomach and known to promote cancer there.

'Interesting'

A spokesman for the Cancer Research Campaign said: "It's interesting to see the results of many different studies pooled together, but it's still true that research on garlic has not led to straightforward answers.

"In order to come to any meaningful conclusions, there is a need for well-designed, large scale human studies.

"In the meantime, there's certainly no harm in including garlic as part of a healthy balanced diet."

A spokesman for the Imperial Cancer Research Fund agreed that larger studies were needed to prove the effects of eating garlic.

Rex and Christine Munday, a husband and wife research team based in New Zealand, published research earlier this year that to reduce the risk of cancer it would be necessary to eat half a clove or raw garlic a day, rising to four and half cloves if cooked.

A four-year, pan-European study was launched this summer to try to discover once and for all whether garlic does reduce the risk of cancer, and heart disease.

The research is published in the American Journal of Clinical Nutrition.