Garlic consumption and cancer prevention: meta-analyses of colorectal and stomach cancers

Background: Animal and in vitro studies have provided evidence of an antica rcinogenic effect of active ingredients in garlic. Objective: The objective was to conduct meta-analyses of the epidemiologic literature on the association between garlic consumption and risk of stomac h, colon, head and neck, lung, breast, and prostate cancers. Design: Meta-analyses were conducted for all cancers mutually and separatel y for colorectal and stomach cancers in relation to consumption of exclusiv ely raw garlic, cooked garlic, or both (RC garlic). Eighteen studies report ed a relative risk estimate for RC garlic consumption and cancer risk. Results: In the meta-analyses of colorectal and stomach cancer, the referen ce categories ranged from no consumption to consumption of 3.5 g/wk, wherea s the highest categories ranged from any consumption to >28.8 g/wk. The ave rage difference between the highest and lowest categories was 16 g/wk. The random-effects relative risk (RR) estimate of colorectal cancer and RC garl ic consumption, excluding garlic supplements, was 0.69 (95% CI: 0.55, 0.89) . For stomach cancer, the random-effects RR estimate was 0.53 (95% CI: 0.31 , 0.92). The heterogeneity among studies for the latter outcome (P = 0.0002 ) indicates the questionableness of the generalizability of this summary es timate. An indication of publication bias for all cancers combined is evide nt from a funnel plot of RC garlic consumption and cancer risk and from the results of the Begg and Mazumdar test (P = 0.039). Conclusions: High intake of RC garlic may be associated with a protective e ffect against stomach and colorectal cancers. Heterogeneity of effect estim ates, differences in dose estimation, publication bias, and possible altern ative hypotheses (eg, confounding by total vegetable consumption) preclude sole reliance on summary effect estimates.