Allium Vegetables and Cancer

Onions, garlic, leeks (allium vegetables) and cancer

Can Allium vegetables reduce the risk of cancer?

There is evidence that a regular consumption of vegetables reduces the risk of intestinal cancers. One review has investigated whether Allium vegetables (such as onions, leeks, garlic) are particularly important in the prevention of cancers.

Message

The evidence from this review is not strong enough to advise that we should eat Allium vegetables in preference to other types of vegetable to prevent cancer. However, one large study found eating half an onion a day had a protective effect against stomach cancers. It would therefore seem sensible to include onion as one of the other vegetables eaten regularly.

Search

Studies were searched for in MEDLINE between 1966-1996. The bibliographies of these papers were then reviewed.

Results

Twenty studies were found. Eighteen were case control studies and two were cohort studies.

Case Control Studies

Over the 18 studies, patient numbers ranged from 60 to 1,016. Five studies did not have comparison groups. Different allium vegetables and types of cancer were investigated. The daily consumption of allium vegetables either varied between studies or was not reported.

However, many of the studies examined the association between gastrointestinal cancers and onion. In a few of these, onion was reported to have a significant protective effect, but the results are either incomplete (confidence intervals are not given) or not reported at all.

Cohort Studies

The Iowa Women's Health Study examined the association between garlic and colon cancer in 41,837 women, aged 55-69 years. They completed a food questionnaire and were followed for 5 years: 212 cases of colon cancer were reported. There was no difference between women who ate garlic every day compared with women who ate garlic less than once a month.

The Netherlands Cohort Study examined the relation between onion, leek and garlic supplements with stomach cancers in 58,279 men and 62,573, women aged 55-69 years. After completing a food questionnaire they were followed for 3 years: 139 cases of stomach cancer were reported. A protective effect was not found for leek or garlic supplements. For those consuming half an onion a day a significant protective effect was reported (odds ratio = 0.50, confidence interval not given).

Comment

It was not possible to pool the results from the studies included in this review: different allium vegetables and different cancers were examined; the quantity of vegetables consumed was rarely reported; and the results were incomplete. It is also difficult to extrapolate high quality results from case control studies.

Although some studies found onion to have a protective effect against gastrointestinal cancers, it is not possible to say that allium vegetables in general, or even onions in particular, have a special role to play in the prevention of cancers. However, we are advised to eat at least five to six portions of fruit and vegetables a day, so it would seem sensible to regularly include onion as one of these.

Reference

E Ernst. Can Allium vegetables prevent cancer? Phytomedicine 1997 4: 79-83.