How might garlic act to prevent cancer?

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Protective effects from garlic may arise from its antibacterial properties (17) or from its ability to block the formation of cancer-causing substances (18), halt the activation of cancer-causing substances (19), enhanceDNA repair (20), reduce cell proliferation, or induce cell death (10).

How much garlic may be useful for cancer prevention?

The National Cancer Institute, part of the National Institutes of Health, does not recommend any dietary supplement for the prevention of cancer, but recognizes garlic as one of several vegetables with potential anticancer properties. Because all garlic preparations are not the same, it is difficult to determine the exact amount of garlic that may be needed to reduce cancer risk. Furthermore, the active compounds present in garlic may lose their effectiveness with time, handling, and processing. The World Health Organization’s (WHO) guidelines for general health promotion for adults is a daily dose of 2 to 5 g of fresh garlic (approximately one clove), 0.4 to 1.2 g of dried garlic powder, 2 to 5 mg of garlic oil, 300 to 1,000 mg of garlic extract, or other formulations that are equal to 2 to 5 mg of allicin.

What are the safety considerations?

Although garlic has been used safely in cooking, excessive consumption can cause some side effects, in addition to strong breath and body odors (4, 21). Garlic occasionally causes allergies that can range from mild irritation to potentially life-threatening problems. Ingestion of fresh garlic bulbs, extracts, or oil on an empty stomach may occasionally cause heartburn, nausea, vomiting, and diarrhea. Some animal and human studies suggest that garlic can lower blood sugar levels and increase insulin.

Garlic has been shown to interfere with several prescription drugs, especially the HIV medication saquinavir (brand names Invirase® and Fortovase®). Garlic can lower the serum levels of saquinavir by as much as 50 percent (22). Garlic also acts as a natural blood thinner and, thus, should be avoided by pregnant women, people about to undergo surgery, and people taking blood thinners, such as warfarin (brand name Coumadin®).

Garlic bulbs are sometimes contaminated with the bacterium Clostridium botulinum. C. botulinum can grow and produce botulinum toxin in garlic-in-oil products that are not refrigerated and do not contain antibacterialagents.

In addition, chemical burns, contact dermatitis, and bronchial asthma can occur when garlic is applied to the skin. Garlic should also be avoided by people who are prone to stomach conditions, such as ulcers, as it can exacerbate the condition or cause new ones (4).

Selected References

1. Milner JA. Garlic: Its anticarcinogenic and antitumorigenic properties. Nutrition Reviews 1996; 54:S82–S86.

2. Ross SA, Finley JW, Milner JA. Allyl sulfur compounds from garlic modulate aberrant crypt formation.Journal of Nutrition 2006; 136(3 Suppl):852S–854S.

3. Amagase H, Petesch BL, Matsuura H, Kasuga S, Itakura Y. Intake of garlic and its bioactive components.Journal of Nutrition 2001; 131(3s):955S–962S.

4. Amagase H. Clarifying the real bioactive constituents of garlic. Journal of Nutrition 2006; 136(3 Suppl):716S–725S.

5. Fleischauer AT, Arab L. Garlic and cancer: A critical review of the epidemiologic literature. Journal of Nutrition 2001; 131(3s):1032S–1040S.

6. Gonzalez CA, Pera G, Agudo A, et al. Fruit and vegetable intake and the risk of stomach and oesophagus adenocarcinoma in the European Prospective Investigation into Cancer and Nutrition (EPIC-EURGAST).International Journal of Cancer 2006; 118(10): 2559–2566.

7. Steinmetz KA, Kushi LH, Bostick RM, Folsom AR, Potter JD. Vegetables, fruit, and colon cancer in the Iowa Women’s Health Study. American Journal of Epidemiology 1994; 139(1):1–15.

8. Gao CM, Takezaki T, Ding JH, Li MS, Tajima K. Protective effect of allium vegetables against both esophageal and stomach cancer: A simultaneous case-referent study of a high-epidemic area in Jiangsu Province, China. Japanese Journal of Cancer Research 1999; 90(6):614–621.

9. Setiawan VW, Yu GP, Lu QY, et al. Allium vegetables and stomach cancer risk in China. Asian Pacific Journal of Cancer Prevention 2005; 6(3):387–395.

10. Hsing AW, Chokkalingam AP, Gao YT, et al. Allium vegetables and risk of prostate cancer: A population-based study. Journal of the National Cancer Institute 2002; 94(21):1648–1651.

11. Chan JM, Wang F, Holly EA. Vegetable and fruit intake and pancreatic cancer in a population-based case-control study in the San Francisco bay area. Cancer Epidemiology Biomarkers & Prevention 2005; 14(9):2093–2097.

12. Challier B, Perarnau JM, Viel JF. Garlic, onion and cereal fibre as protective factors for breast cancer: A French case-control study. European Journal of Epidemiology 1998; 14(8):737–747.

13. Li H, Li HQ, Wang Y, et al. An intervention study to prevent gastric cancer by micro-selenium and large dose of allitridum. Chinese Medical Journal (English) 2004; 117(8):1155–1160.

14. You WC, Brown LM, Zhang L, et al. Randomized double-blind factorial trial of three treatments to reduce the prevalence of precancerous gastric lesions. Journal of the National Cancer Institute 2006; 98(14):974–983.

15. Tanaka S, Haruma K, Kunihiro M, et al. Effects of aged garlic extract (AGE) on colorectal adenomas: A double-blinded study. Hiroshima Journal of Medical Sciences 2004; 53(3–4):39–45.

16. Tilli CM, Stavast-Kooy AJ, Vuerstaek JD, et al. The garlic-derived organosulfur component ajoene decreases basal cell carcinoma tumor size by inducing apoptosis. Archives of Dermatological Research 2003; 295(3):117–123.

17. Ruddock PS, Liao M, Foster BC, et al. Garlic natural health products exhibit variable constituent levels and antimicrobial activity against Neisseria gonorrhoeae, Staphylococcus aureus and Enterococcus faecalis.Phytotherapy Research 2005; 19(4):327–334.

18. Shenoy NR, Choughuley AS. Inhibitory effect of diet related sulphydryl compounds on the formation of carcinogenic nitrosamines. Cancer Letters 1992; 65(3):227–232.

19. Milner JA. Mechanisms by which garlic and allyl sulfur compounds suppress carcinogen bioactivation. Garlic and carcinogenesis. Advances in Experimental Medicine and Biology 2001; 492:69–81.

20. L’vova GN, Zasukhina GD. Modification of repair DNA synthesis in mutagen-treated human fibroblasts during adaptive response and the antimutagenic effect of garlic extract. Genetika 2002; 38(3):306–309.

21. Boon H, Wong J. Botanical medicine and cancer: A review of the safety and efficacy. Expert Opinion on Pharmacotherapy 2004; 5(12):2485–2501.

22. Piscitelli SC, Burstein AH, Welden N, Gallicano KD, Falloon J. The effect of garlic supplements on the pharmacokinetics of saquinavir. Clinical Infectious Diseases 2002; 34(2):234–238.