Kim JT, Kwon O. Garlic intake and cancer risk: an analysis using the Food and Drug Administration's evidence-based review system for the scientific evaluation of health claims. Am J Clin Nutr. 2009;89: 257-264.

The health benefits of many foods are conveyed on their product labeling, and the information provided on such labels has become an important factor in the purchasing decisions of consumers. Thus, it is imperative that such health claims are not misleading. To this end, regulations concerning the accuracy of health claims have been implemented worldwide. In the United States, the Food and Drug Administration (FDA) authorized health claims on food labeling by enacting the Nutrition Labeling and Education Act in 1990. The FDA uses an evidence-based review system to assess the relation between disease and a food component or food with the aim of protecting consumers against false or misleading claims. More recently, the European Union established a new regulation that requires all health claims on food labels to be approved by the European Commission. In Korea, the Health/Functional Food Act was enacted in 2002 to aid in the regulation of health claims.

Garlic (Allium sativum) is widely consumed in Korea. This vegetable is characterized by a high content of organosulfur compounds and flavonoids, which are claimed to confer health benefits by acting synergistically. Despite the availability of several scientific articles suggesting the health benefits of garlic, few systematic reviews or meta-analyses of human studies have been conducted to evaluate the association between cancer risk and garlic intake. The objective of this study was to evaluate the scientific evidence concerning the relation between garlic intake and cancer risk.

The MEDLINE and EMBASE databases were searched to identify studies in humans published in Korean or English between January 1955 and November 2007 that evaluated the effects of garlic or garlic-containing products on cancer. The following search terms were used: garlic, Allium sativum, vegetables, diet, and nutrition, in combination with cancer, neoplasm, and individual cancer names. Review articles, meta-analyses, and studies that did not include a control group or a statistical analysis were excluded. Nineteen studies meeting the inclusion criteria were reviewed; some of the studies evaluated the risk of more than one type of cancer. Four of the articles addressed the relation between garlic consumption and risk of gastric cancer, 9 the risk of colon or rectal cancer, 4 the risk of prostate cancer, 3 the risk of breast cancer, 2 the risk of lung cancer, and 1 the risk of larynx, esophageal, oral, ovary, and renal cell cancers. The FDA's evidence-based review system was used to evaluate the strength of the evidence supporting a relation between garlic intake and a reduced risk of these cancers with respect to food labeling.

No "credible evidence" was observed in support of an association between garlic intake and a reduction in the risk of gastric, breast, or lung cancer. Six studies showed no reduction in risk, and 3 "weaker and more limited studies" (2 case-control studies and 1 prospective study) suggest that garlic intake may reduce the risk of colon cancer. Three studies showed no reduction in risk and one "weaker, more limited study" (case-control study) suggests that garlic intake may reduce the risk of prostate cancer. One small study suggests that garlic intake may reduce the risk of larynx, esophageal, oral, ovary, and renal cell cancers. The authors conclude that the following wording be used on food product labels "to convey the level of scientific evidence for a health claim": "it is highly unlikely that garlic intake reduces the risk of colon cancer," "it is highly uncertain whether garlic intake reduces the risk of prostate cancer," and "the existence of such a relation [reduction in risk] between garlic intake and these cancers [larynx, esophageal, oral, ovary, and renal cell] is highly uncertain."

The authors suggest that additional studies are needed to assess the relation between cancer risk and garlic consumption, because many variables can affect the chemical composition of garlic, including cultivation conditions and preparation methods. Furthermore, because garlic is generally used in small amounts as a food seasoning, it is difficult to determine accurate intakes on food-frequency questionnaires.