Does Garlic Reduce Risk of Colorectal Cancer? A Systematic Review1–3, abstract

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Does Garlic Reduce Risk of Colorectal Cancer? A Systematic Review1–3,

Suong N. T. Ngo4,7,*, Desmond B. 4, Lynne Cobiac5 and J.

Head6

4 Sansom Institute, School of Pharmacy and Medical Sciences, University of

South Australia, Adelaide, 5000 Australia; 5 Department of Nutrition and

Dietetics, Flinders University, Adelaide, 5001 Australia; 6 Preventative

Health National Research Flagship CSIRO, Adelaide, 5000 Australia; and 7

School of Science and Primary Industries, Darwin University,

Casuarina Campus, Darwin, 0909 Australia

Colorectal cancer (CRC) is the 3rd leading cause of cancer death in the

United States and the 2nd leading cause of cancer death in Australia.

Environmental factors play important roles in the multiple-stage process of

CRC and nutritional intervention has been identified as playing a major role

in its prevention. The aim of this study was to review systematically the

scientific evidence from all studies conducted over the last decade that

examined effects of garlic on CRC. Levels of evidence were ranked from level

I to level V according to study designs and the quality of each study was

assessed against a set of quality criteria based on those used by the

National Health and Medical Research Council in Australia. One randomized

controlled trial (RCT, level II) reported a statistically significant 29%

reduction in both size and number of colon adenomas in CRC patients taking

aged garlic extract. Five of 8 case control/cohort studies (level III)

suggested a protective effect of high intake of raw/cooked garlic and 2 of 8

of these studies suggested a protective effect for distal colon. A published

meta-analysis (level III) of 7 of these studies confirmed this inverse

association, with a 30% reduction in relative risk. Eleven animal studies

(level V) demonstrated a significant anticarcinogenic effect of garlic

and/or its active constituents. On balance, there is consistent scientific

evidence derived from RCT of animal studies reporting protective effects of

garlic on CRC despite great heterogeneity of measures of intakes among human

epidemiological studies.

Gallant, BSc, MNIMH, MCPP

Medical Herbalist

Canada