Garlic and colorectal cancer.

Background: In 2007, the WCRF published a systematic review with information about a large amount of life style variables in relation to a large variety of cancer types (1). The WCRF concluded that "garlic probably protects against colorectal cancer" (page 93 + 94 report).
This conclusion was based on data from 2 articles from the cohorts in the following table (Giovannucci E [6], Steinmetz KA [8]), and 6 case-control studies.

1) Garlic
Colon cancer risk:

The Health Professional's Follow-up Study: In an early article about this cohort (Giovannucci E [6]) after 6 years of follow-up, an inverse association was found, but neither the trend (P = 0.14), nor the effect (RR = 0.77; 95% CI = 0.51-1.16) was significant). No association was found in a later publication after 10 years of follow-up (Michels KB [6]).
The Iowa Women's Health Study: Data from this cohort is hard to interpret: A nonsignificant protective effect (trend) against colon cancer risk was found in an early article after 5 years of follow-up (Steinmetz KA [8]), but no association was found in a later publication after 10 years of follow-up (Sellers TA [8]). Given the fact that in both cohorts the amounts of cases were approximately the same, it is hard to find an explanation for these findings.

Rectal cancer risk: Data about the relation with rectal cancer risk was provided by 2 cohorts (6). No associations were found.

2) Garlic supplements Data about garlic supplements was provided by 1 cohort only (9). No association was found with colon or rectal cancer risk.

Conclusion: No significant effect with total colon or rectal cancer was found in any cohort. No (non)significant effects were found in the most recent publications. No evidence was found for an association between garlic and colon or rectal cancer.

Reference:
1) World Cancer Research Fund / American Institute for Cancer Research. Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. Washington DC: AICR, 2007. Full text

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Colon:

Rectum:

Garlic supplements use: RR = 1.26 (0.84-1.91).

Exclusive use of garlic as supplement: RR = 1.36 (0.79-2.35).

Garlic supplements use: RR = 0.77 (0.41-1.46).

Exclusive use of garlic as supplement: RR = 1.28 (0.63-2.60).

Stratified on family history of colon cancer:

No family history

Positive family history

RR = 1.2 (0.8-1.9; P = 0.4) for the highest vs lowest tertile of consumption.

Amount specific data (servings/wk):
0: RR = 1.
≤ 1: RR = 1.1 (0.7-1.5).
> 1: RR = 1.2 (0.8-1.9).

RR = 1.0 (0.4-2.5; P = 0.9) for the highest vs lowest tertile of consumption.

Amount specific data (servings/wk):
0: RR = 1.
≤ 1: RR = 0.6 (0.5-1.7).
> 1: RR = 1.0 (0.4-2.5).

Colon:

Proximal colon:

Distal colon:

RR = 0.68 (0.46-1.02; P = 0.10) for the highest vs lowest tertile of consumption.

Amount specific data (servings/week):
0: RR = 1.
0.5: RR = 1.07 (0.77-1.50).
≥ 1.0: RR = 0.68 (0.46-1.02).

RR = 1.00 (0.56-1.79; No P-value) for the highest vs lowest tertile of consumption.

Amount specific data (servings/week):
0: RR = 1.
0.5: RR = 1.32 (0.79-2.22).
≥ 1.0: RR = 1.00 (0.56-1.79).

RR = 0.52 (0.30-0.93; P = < 0.05) for the highest vs lowest tertile of consumption.

Amount specific data (servings/week):
0: RR = 1.
0.5: RR = 0.85 (0.53-1.36).
≥ 1.0: RR = 0.52 (0.30-0.93).

Cancer & Diet. A systematic review.

Literature review.

Relating various aspects of our diet
to colorectal cancer risk & mortality.

Garlic and colorectal cancer.

Cancer & Diet. A systematic review.

Literature review.

Relating various aspects of our dietto colorectal cancer risk & mortality.

Garlic and colorectal cancer.

Relating various aspects of our diet
to colorectal cancer risk & mortality.