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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.
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