April 19, 2004

Allium family vegetables (garlic, leeks, onions, scallions) reduce the risk of cancer (and cardiovascular disease)

William B. Grant, Ph.D.

SUNARC

Introduction

When dietary factors explain 30-50% of the risk for cancer [Doll and Peto, 1981; Grant, 2002a, 2004], and when the approach of the health care system in the U.S. and other Western Developed countries has been to place more emphasis on secondary prevention (examinations and prescription drugs) and tertiary prevention (surgical operations) rather than on primary prevention through diet and lifestyle choices, it is worthwhile to identify simple dietary and lifestyle choices that offer significant risk reductions for many types of cancer.  Vitamin D, obtained though ultraviolet B (UVB) radiation, dietary sources, or supplements, is one such risk reduction factor [Grant, 2002b].  A dietary factor gaining increased prominence in this regard is allium family vegetables - onions, garlic, leeks and scallions.  This essay will review the epidemiologic evidence and the mechanisms and suggest a dietary amount effective for cancer risk reduction.

Epidemiologic evidence

A search was conducted at PubMed (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi) for articles reporting epidemiologic studies of allium family vegetables and cancer.  There are five cancers for which allium family vegetables have been found associated with reduced risk: breast, colon, gastric, prostate, and rectal (see Table 1).  Allium vegetables, collectively or individually, had the strongest (inverse) correlation with prostate cancer, 0.3 (95% confidence interval 0.18-0.51) to 0.7 (95% C.I. 0.5-0.9).  In two studies [Grant, 1994; Hodge et al., 2004], onions were found to have stronger correlations than tomatoes, even though tomatoes have received much more publicity in the past few years [Pohar et al., 2003]. 

Table 1.  Epidemiologic study findings regarding allium family vegetables and gastric and prostate cancer.

As for colorectal cancer, a meta-analysis of studies on garlic and colorectal cancer concluded that there was a protective effect [Fleischauer et al., 2000].

A meta-analysis of studies on the association of garlic with gastric cancer found a strong inverse association [Fleischauer et al., 2000], as did a subsequent paper on allium family vegetables and gastric cancer [De Stefani et al., 2001].  Many of the studies also investigated the correlations with fruits and other vegetables, indicating that the results for allium family vegetables are due to them and not other fruits or vegetables.  Thus, the evidence is strong regarding the inverse association between allium family vegetables and prostate cancer, but weaker regarding the protective effect for colon, gastric, and rectal cancer.  However, a review concluded that allium family vegetables were likely protective against colon and gastric cancer, although garlic supplements were not [Fleischauer et al., 2000; Fleischauer and Arab, 2001].

There is evidence from a French study that garlic and onions are protective against breast cancer [Challier et al., 1995].  On the other hand, a Dutch study failed to find a significant inverse correlation [Dorant et al., 1995].  The difference between the results in the two countries may be related to a larger consumption of allium family vegetables in France, which leans more towards the Mediterranean diet, than in the Netherlands, which leans more towards a Northern European diet.

Interestingly, allium family vegetables were not found to be significantly associated with risk reduction for other types of cancer.  However, given that other cancers are more difficult to study using case-control and cohort studies since the numbers of people who develop these cancers are lower, it is highly likely that allium family vegetables play a protective role for a large number of cancers.

As to the amount, the study by Fleischauer et al. [2000], reviewed studies for consumptions up to >28.8 g/wk of garlic, finding that compared to practically no consumption, cancer risk reductions were in the 0.5-0.7 range.  Onions, since they have lower densities of the active ingredients, would have to be consumed in higher amounts for optimal protection.

Active ingredients and mechanisms

A number of mechanisms have been tentatively identified by which allium family vegetables reduce the risk of cancer.  Some of the findings from the literature are listed in Table 2.  As can be seen, there are a number of ways that the organosulfur compounds likely reduce the risk of cancer.

Table 2.  Mechanisms whereby compounds in allium family vegetables reduce the risk of cancer.

Unlike the case of tomatoes, where cooked tomatoes seem to provide better access to the active ingredient, lycopene, there do not appear to be any studies indicating that cooked allium family vegetables are more or less effective than uncooked vegetables.  However, studies with garlic powder supplements have generally not found them to be effective, perhaps because they are often made to be tasteless to be more palatable, thus, perhaps destroying the active sulfur compounds. 

Summary and conclusion

There is strong epidemiologic evidence that frequent consumption of allium family vegetables is protective against cancer.  In addition, garlic compounds are also protective against arteriosclerotic plaque [Koscielny et al., 1999], reduce cholesterol levels [Yeh and Liu, 2001], and play a role in reducing the risk of cardiovascular disease in general [Kris-Etherton et al., 2002; Gardner et al., 2003].  Thus allium family vegetables should be considered to be an important component of a healthy, cancer and cardiovascular disease resistant diet. 

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