Garlic, Onions, and Cancer

Summarized by Robert W. Griffith, MD
December 4, 2006

Summary

A high consumption of onions, and, to a lesser extent, a high consumption of garlic is associated with a significantly reduced risk of cancer at 7 sites: oral cavity & pharynx, larynx, esophagus, kidney, colon & rectum, ovary, and prostate.

Introduction

Eastern countries, as well as many European countries, have long advertised the benefits of onions and garlic in providing good health. Twenty years ago garlic was being touted as the cure for atherosclerosis. Good studies are lacking, however; at best there have been epidemiological reports of the association between eating these vegetables and a reduced risk of some forms of cancer - stomach, colorectal, and pancreatic cancers.

Now Italian investigators have published the results of a series of case-control studies conducted in Italy and Switzerland. Here's a summary of the findings reported in the American Journal of Clinical Nutrition.

What was done

Case-control studies were done on several types of cancer in various areas of Northern Italy between 1991 and 2004. The 8 studies covered cancers of the oral cavity and pharynx, larynx, esophagus, breast, kidney, colon, rectum, ovary, and prostate. Each study had between one and 4 carefully matched controls; the controls had been admitted to the same hospital as the cases, but had non-cancerous conditions unrelated to known risk factors for the cancer concerned.

Cancer cases and their controls were interviewed about numerous lifestyle and sociodemographic factors, and dietary intake was obtained using a 78-item food frequency questionnaire. Two of the questions concerned the consumption of onion and garlic; for onion, the alternatives were low, medium, or large portions per week, and for garlic they were nonuse or low use, intermediate use, or high use.

The likelihood of developing a particular cancer, called an Odds Ratio (OR), was developed for each cancer, for the different frequencies of onion and garlic use. Adjustments were made, when appropriate, for factors that might affect the occurrence of the cancer, e.g. smoking, alcohol use, physical activity, number of children, and family history of the particular cancer.

What was found

The odds ratios - compared to onion non-users or garlic non- or low-users - are given in the table below:

* statistically significant difference between groups

These ORs show that high consumption of onions, and, to a lesser extent, high consumption of garlic is associated with a significantly reduced risk of cancer of seven organs. The calculations were after adjustments were made for total vegetable consumption, as this had been shown to be reduced in cancers at several sites.

What these findings mean

Onion (Allium cepa) and garlic (Allium sativa) are closely-related vegetables. There have been studies of experimental cancer in animals and cell cultures that show an inhibitory effect of several allium-derived compounds. The results of these case-control studies confirm that the experimental findings are relevant for human cancers. A previous study from China, reported in 2002, yielded similar findings for prostate cancer¹, and smaller studies from China have show such an effect with stomach cancer.

So, now that you know you should eat more onions (and garlic), why not make the effort? French onion soup, liver and onions, onion stuffing . . . Chinese and Mediterranean cuisine probably lend themselves more readily to incorporating allium compounds, but there's no reason Western-style cooking shouldn't adapt a bit.

Source

• Onion and garlic use and human cancer. C. Galeone , C. Pelucchi , F. Levi,  et al., Am J Clin Nutr, 2006, vol. 84, pp. 1027--1032

Footnotes
1. Allium vegetables and risk of prostate cancer: a population-based study. AW. Hsing, AP. Chokkalingham, Y-T. Gao,  et al., JNCI, 2002, vol. 94, pp. 1648--1651

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