Garlic consumption and colorectal cancer risk in man: a systematic review and meta-analysis

Abstract

Objective: Colorectal cancer shows large incidence variations worldwide that have been attributed to different dietary factors. We conducted a meta-analysis on the relationship between garlic consumption and colorectal cancer risk.

Design: We systematically reviewed publications obtained by searching ISI Web of Knowledge, MEDLINE and EMBASE literature databases. We extracted the risk estimate of the highest and the lowest reported categories of intake from each study and conducted meta-analysis using a random-effects model.

Results: The pooled analysis of all fourteen studies, seven cohort and seven case-control, indicated that garlic consumption was not associated with colorectal cancer risk (OR=0·93; 95 % CI 0·82, 1·06, P=0·281; I 2=83·6 %, P≤0·001). Separate analyses on the basis of cancer sites and sex also revealed no statistically significant effects on cancer risk. However, when separately analysed on the basis of study type, we found that garlic was associated with an approximately 37 % reduction in colorectal cancer risk in the case-control studies (combined risk estimate=0·63, 95 % CI 0·48, 0·82, P=0·001; I 2=75·6 %, P≤0·001).

Conclusions: Our results suggest that consumption of garlic is not associated with a reduced colorectal cancer risk. Further investigations are necessary to clarify the discrepancy between results obtained from different types of epidemiological studies.

Keywords: Colorectal cancer; Garlic; Meta-analysis.

Fig. 1

Flowchart of the study selection process

Fig.2

Funnel plot for the association between garlic consumption and colorectal cancer

Fig. 3

Forest plot for the association between garlic consumption and colorectal cancer by study design: (a) case–control, (b) cohort and (c) supplement studies. The study-specific effect size (ES) and 95 % CI are represented by the black square and horizontal line, respectively; the area of the black square is proportional to the specific-study weight to the overall meta-analysis. The centre of the diamond presents the pooled RR risk and its width represents the pooled 95 % CI (F, females; M, males)