Cancer Epidemiol. 2013 Apr;37(2):152-5. doi: 10.1016/j.canep.2012.11.002. Epub 2012 Dec 21.
No association between garlic intake and risk of colorectal cancer.
Meng S, Zhang X, Giovannucci EL, Ma J, Fuchs CS, Cho E.
Source
Department of Epidemiology, Harvard School of Public Health, Boston, MA, USA.

Abstract
BACKGROUND:
Although experimental studies suggested beneficial role of garlic intake on colorectal carcinogenesis, limited prospective cohort studies have evaluated garlic intake in relation to colorectal cancer (CRC) incidence.
METHODS:
We followed 76,208 women in the Nurses' Health Study and 45,592 men in the Health Professionals Follow-up Study for up to 24 years and examined garlic intake and garlic supplement use in relation to CRC risk. Information on garlic intake and supplement use was assessed using a validated food frequency questionnaire and a Cox proportional hazard regression model was used to estimate the multivariable hazard ratio (MV-HR) and 95% confidence intervals (95% CIs).
RESULTS:
We documented 2368 (1339 women and 1029 men) incident CRC cases and fo und no association between garlic intake and CRC risk; the MV-HRs (95% CIs) associated with garlic (1 clove or 4 shakes per serving) intake ¡Ý 1/day compared with < 1/month were 1.21 (0.94-1.57; p-trend = 0.14) for women and 1.00 (0.71-1.42; p-trend = 0.89) for men. The MV-HRs (95% CIs) of CRC for garlic supplement use, which was used in 6% of the participants in each study, were 0.72 (0.48-1.07) for women and 1.22 (0.83-1.78) for men.
CONCLUSION:
Our prospective data do not support an important role of garlic intake or garlic supplement use in colorectal carcinogenesis.

Cancer Causes Control. 2012 Oct;23(10):1643-51. doi: 10.1007/s10552-012-0042-7. Epub 2012 Aug 23.
Garlic consumption and colorectal cancer risk in the CPS-II Nutrition Cohort.
McCullough ML, Jacobs EJ, Shah R, Campbell PT, Gapstur SM.
Source
Epidemiology Research Program, American Cancer Society, Atlanta, GA 30303-1002, USA. marji.mccullough@cancer.org

Abstract
PURPOSE:
The World Cancer Research Fund/American Institute for Cancer Research identified a probable role for garlic in colorectal cancer prevention based on preclinical evidence and epidemiologic studies, but prospective data are limited. The purpose of this paper was to contribute additional evidence on this topic for men and women in a large prospective cohort study.
METHODS:
In 1999, 42,824 men and 56,876 women in the Cancer Prevention Study II Nutrition Cohort completed a questionnaire with information on dietary garlic consumption. Garlic supplement use was assessed in 2001. Cox proportional hazards regression was used to estimate multivariable-adjusted hazard rate ratios (HRs) and 95 % confidence intervals (CIs).
RESULTS:
During 7 years of follow-up, 579 men and 551 women were diagnosed with colorectal cancer. Among men, daily garlic consumption was associated with a non-significant higher colorectal cancer risk (HR = 1.04, 95 % CI 0.99-1.08 for each additional clove or "4 shakes" of garlic per week), whereas the association was borderline inverse in women (HR = 0.95, 95 % CI 0.91-1.00, p heterogeneity by sex = 0.03). Garlic supplement use was not related to a lower risk of colorectal cancer, and in men, former use was associated with a higher risk of colorectal cancer (HR = 1.85, 95 % CI 1.13-3.03).
CONCLUSIONS:
These results provide weak support for a role of dietary garlic consumption in colorectal cancer prevention in women, but a possible increased risk in men. Further research is needed to confirm different associations by sex.