Measuring the strength of the evidence

The review, published in Gastroenterology, analysed the findings from 21 studies with a total of 534,220 participants. Most of the studies compared the amount of allium vegetables eaten by people with stomach cancer to people without stomach cancer. The others looked at stomach cancer rates over time in people who ate lots of allium vegetables and compared them to people who did not eat allium vegetables.

Onions and garlic prevent stomach cancer

The combined analysis yielded the following results:

• People who ate the most allium vegetables were 46% less likely to develop stomach cancer than people who ate the least.
• For each 20 grams of allium vegetable eaten each day, the risk was reduced by 9%. This would be about one twentieth of a large onion, or 3 to 4 cloves of garlic.
• Welsh onions had the strongest protective effect; garlic bulbs and stalks, onions in general, and Chinese chives all had similar protective strength; while leeks and scallions were slightly weaker.
• Eating onion leaves did not prevent stomach cancer.

“This meta-analysis provides strong evidence that increased consumption of [allium] vegetables is associated with a reduced [stomach] cancer risk,” the study’s authors said. Since stomach cancer is the second most frequent cause of cancer death in the world, affecting one million people every year, identifying effective prevention measures could have an important impact.

Reducing your stomach cancer risk

One of the key risk factors for stomach cancer is the presence of the bacterium, Helicobacter pylori, in the stomach. H. pylori infection is best known as a cause of gastritis and peptic ulcer disease. If you have either of these conditions, or if you have a family history of stomach cancer, getting tested for H. pylori infection and treating it (usually with a combination of antibiotics) is a top priority for cancer prevention. To improve your chance of avoiding stomach cancer, eat lots of onions, garlic, and chives; treat H. pylori if it is present; and consider these other steps:

• Quit smoking. Smoking is an established risk factor for stomach cancer.
• Eat lots of fruits and vegetables. Emphasise the allium vegetables but remember that fruits and vegetables in general, and the fibre they contain, reduce stomach cancer risk.
• Limit overly salty foods, smoked and pickled foods, and cured meats. These foods contain high amounts of known cancer-causing compounds.

(Gastroenterology 2011;141:80–9)