Garlic, onion, linked to decreased cancer risk

Eating lots of onions, garlic and leeks has been linked to reduced risk of bowel cancer in a Chinese study of more than 1600 volunteers.

The vegetables, collectively in the genus Allium, contain several bioactive compounds known as phytonutrients, including sulfur-rich flavonoids, which have been linked to anti-bacterial, anti-fungal and antioxidant outcomes in humans.

To explore links between allium vegetables and bowel cancer, the researchers led by Xin Wu from the First Hospital of China Medical University in Shenyang compared the previous 12 months’ food intake of 833 patients with colorectal cancer to that of 833 people free from the disease. The cohorts were matched for age, sex and residence.  

Results showed eating allium vegetables was associated with 50% to 80% lower colorectal cancer risk after controlling for various factors such as smoking, family history of colorectal cancer, alcohol and diet. The research is published in the Asia-Pacific Journal of Clinical Oncology.

Mary Flynn, research dietitian and associate professor of medicine at Brown University, US, says the study is interesting. But she adds that it’s important to note the patients had higher family history of colorectal cancer than controls – 5.9% versus 1.0%.

The cancer patients also smoked more, ate more calories and reported consuming less fruit, other vegetables and milk, more alcohol, and nearly double the amount of red meat than controls.

Although the link with allium vegetables remained after factoring these differences into analyses, higher red meat and lower plant food intake has previously been linked to higher bowel cancer risk.

Flynn notes a strength of the study is the number of foods assessed. But she says the difference in allium consumption between patients and controls was not huge: an average daily intake of 43.6 grams versus 58.5 grams. 

The difference equates to around three to five cloves of garlic per day.

People have enjoyed the bulbs and leaves of allium vegetables – which also include shallots, spring onions and chives – for their flavour and medicinal benefits for more than 4000 years.  

More recently, test tube and non-human animal research has shown that sulfer-containing allium compounds can fight tumours at each stage of their formation by inhibiting the growth of precancerous cells and stifling tumour-promoting micro-environments through antioxidant and antimicrobial properties.

The compounds are released when the fresh vegetable is cut or crushed. The effects can be reduced by cooking.

Several population-based studies with humans have linked allium vegetables to lower risk of different cancers including pancreatic, prostate, breast and colorectal cancer.

But the findings have been mixed. Some reviews have concluded that allium vegetables lower risk of bowel cancer, while others found little to support the contention.

Wu and colleagues suggest inconsistencies may be attributed to differences between studies, including how allium intake is measured, whether the vegetables are cooked or eaten raw, and which confounding factors are included in analyses.