Eating plenty of onions, leeks and garlic dramatically slashes the risk of bowel cancer - one of Britain's biggest killers, according to new research.

Each year 42,000 Britons are diagnosed with the cancer and it kills more than 16,000 but just one small onion a day is enough to prevent it.

A study of more than 1,600 men and women found those with the biggest intake were 79 per cent less likely to develop the disease than those with the least.

Allium vegetables - which also include chives and shallots - are rich in tumour-fighting compounds.

They are also known to protect against breast and prostate cancer.

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Senior author Dr Zhi Li, of the First Hospital of China Medical University in Shenyang, said: "It's worth noting that in our research there seems to be a trend - the greater the amount of allium vegetables, the better the protection.

"In general, the present findings shed light on the primary prevention of colorectal (bowel) cancer through lifestyle intervention, which deserves further in-depth exploration."

Dr Li and colleagues worked out that consuming at least 35lbs a year of allium vegetables such as onions and leeks may effectively reduce an individual's risk of the disease.

This would work out at around one-and-a-half ounces a day - equivalent to about one small onion.

The study published in the Asia Pacific Journal of Clinical Oncology compared 833 bowel cancer patients with the same number of healthy controls.

The Chinese participants were matched by age, sex and where they lived with demographic and dietary information collected via face-to-face interviews using a validated food frequency questionnaire.

Dr Li said bowel cancer is the third most common form of the disease worldwide - with about 1.23 million newly diagnosed cases annually. It's the second biggest cancer killer in the UK.

Rates in China are lower than in Europe and the US. But they are growing because of an increasingly westernised diet.

Dr Li said: "Allium vegetables, including garlic, garlic stalks, leek, onion, and

spring onion, belong to the most commonly consumed vegetables in the world.

"Bioactive compounds in allium vegetables, particularly, flavanols and organosulphur compounds, have been shown to provide beneficial effects against cancer.

But although lab experiments have demonstrated this results from epidemiological studies have been inconsistent.

Dr Li said: "The present findings shed light on the reduced risk of colorectal cancer associated with diet, specifically the intake of allium vegetables."

Alliums also contain polyphenols, including the flavonoid quercetin, which along with many of the sulphur compounds have important anti-inflammatory effects.

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To maximise the concentration of sulphur compounds experts advise allowing chopped onions, crushed or minced garlic, sliced leeks, or other alliums to sit for a few minutes.

This is before cooking them or adding to an acid - such as vinegar or lemon juice. This allows the enzymes released when the alliums' cells are broken to more completely react with sulphur-containing molecules and convert them to beneficial forms.

Alliums - Latin for 'garlic' - evolved their pungent odours and tastes to deter hungry animals.

But humans, ironically, have singled them out specifically for those characteristics. They have become vital to every cuisine in the world.

Bowel cancer kills about 16,400 people in the UK each year. Lung cancer is the only form that claims more lives.