Regular eating of raw or cooked garlic seems to halve the risk of getting gastric cancer, and it reduces the risk of getting colorectal cancer by about one-third, according to the research.
Researchers at the University of North Carolina, Chapel Hill, USA, identified 300 studies on the subject, picked out the 22 strongest pieces of research and performed a meta-analysis.
Regular garlic eating halves the risk of gastric cancer.
Research came from countries such as Argentina, China, Switzerland and Holland.
The researchers found strong links between consumption of garlic and preventing cancer - but could not find any evidence that garlic supplements had any benefit.
Researcher Professor Lenore Arab, professor of epidemiology and nutrition, 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."
Professor Arab said only a limited number of studies had examined supplements - so it was possible there were unrecognised benefits. She warned there could also be difficulties with publication bias - with "unsuccessful" research not appearing in publications.
The researchers said it was possible that garlic helped protect the stomach because it has antibiotic properties - which could affect H. pylori .
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