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Garlic vs Cancer

It's important to start this article with the bad news: as far as I'm aware there is no evidence to suggest that eating garlic can cure an established cancer. It's possible that the organic compounds in garlic might one day be used in specialised anti-cancer drugs, but is eating garlic a miracle cure? Unfortunately not.

That said, there is some evidence to suggest that eating garlic might help to reduce (not eliminate) the risk of developing certain types of stomach cancer in the first place.

Research

One study suggesting a cancer prevention role for garlic was published in 2000(1). This was a meta-study by a team from the University of North Carolina at Chapel Hill. Combining data from 22 other studies the paper concludes:
"High intake of [raw or cooked] garlic may be associated with a protective effect against stomach and colorectal cancers"
However it also cautions:
"Heterogeneity of effect estimates, differences in dose estimation, publication bias, and possible alternative hypotheses (eg, confounding by total vegetable consumption) preclude sole reliance on summary effect estimates."
In other words, meta-studies are not always reliable and more research is needed.

How Much?

If eating garlic can reduce the risk of stomach cancer, how much is needed? Some studies have suggested that it would be a massive amount - up to your own weight in garlic! However a 1999 study(2) in rats showed that diallyl disulphide (DADS) - found in garlic - could increase activity of detoxification enzymes at levels equivalent to less than one clove of raw garlic a day for the average human. Of course results in rats don't always apply to us.

So there is some suggestion that eating garlic regularly might help to protect against stomach cancers. However most of the studies published to date are inconclusive. An interesting survey of the research situation is available in a factsheet from the US National Cancer Institute(3).

References:
(1) Garlic consumption and cancer prevention: meta-analyses of colorectal and stomach cancers
(2) Low Doses of Diallyl Disulfide, a Compound Derived From Garlic, Increase Tissue Activities of Quinone Reductase and Glutathione Transferase in the Gastrointestinal Tract of the Rat
(3) Garlic and Cancer Prevention