Garlic and Cancer Prevention: Questions and Answers
Key Points
- Preliminary studies suggest that garlic consumption may reduce the
risk of developing several types of cancer, especially cancers of the
gastrointestinal tract (see Question 3 and 4). Most of the studies
evaluated different types of garlic preparations and used them in varying
amounts (see Question 5).
- If garlic consumption does reduce the risk of developing cancer, the
amount needed to lower risk remains unknown (see Question
7).
- Although usual garlic consumption rarely causes problems, higher
intakes can cause side effects, including gastrointestinal distress
(see Question 8).
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The information included in this fact sheet is not to be used as the basis for a health claim.
- What is garlic?
Garlic is a vegetable (Allium sativum) that belongs to the Allium class
of bulb-shaped plants, which also includes onions, chives, leeks, and scallions.
Garlic is used for flavoring in cooking and is unique because of its high
sulfur content. In addition to sulfur, garlic also contains arginine, oligosaccharides,
flavonoids, and selenium, all of which may be beneficial to health (1).
The characteristic odor and flavor of garlic comes from sulfur compounds
formed from allicin, the major precursor of garlic’s bioactive compounds,
which are formed when garlic bulbs are chopped, crushed, or damaged (2).
Bioactive compounds are defined as substances in foods or dietary supplements,
other than those needed to meet basic nutritional needs, that are responsible
for changes in health status.
- What are the types of garlic preparations?
Garlic supplements can be classified into four groups: Garlic essential
oil, garlic oil macerate, garlic powder, and garlic extract.
• Garlic essential oil is obtained by passing steam through
garlic. Commercially available garlic oil capsules generally contain vegetable
oil, but only have a small amount of garlic essential oil because of its
strong odor.
• Garlic oil macerate products are made from encapsulated
mixtures of whole garlic cloves ground into vegetable oil.
• Garlic powder is produced by slicing or crushing garlic
cloves, then drying and grinding them into powder. Garlic powder is used
as a flavoring agent for condiments and food and is thought to retain the
same ingredients as raw garlic.
• Garlic extract is made from whole or sliced garlic cloves
that are soaked in an alcohol solution (an extracting solution) for varying
amounts of time. Powdered forms of the extract also are available (3,
4).
- Do findings from population
studies offer evidence that garlic may prevent cancer?
Several population studies show an association between increased intake
of garlic and reduced risk of certain cancers, including cancers of the
stomach, colon, esophagus, pancreas, and breast. Population studies are
multidisciplinary studies of population groups that investigate the cause,
incidence, or spread of a disease or examine the effect of health-related
interventions, dietary and nutritional intakes, or environmental exposures.
An analysis of data from seven population studies showed that the higher
the amount of raw and cooked garlic consumed, the lower the risk of stomach
and colorectal cancer (5).
The European Prospective Investigation into Cancer and Nutrition (EPIC)
is an ongoing multinational study involving men and women from 10 different
countries. This study is investigating the effects of nutrition on cancer.
In the study, higher intakes of onion and garlic were associated with a
reduced risk of intestinal cancer (6).
The Iowa Women’s Study is a large prospective study investigating
whether diet, distribution of body fat, and other risk factors are related
to cancer incidence in older women. Findings from the study showed a strong
association between garlic consumption and colon cancer risk. Women who
consumed the highest amounts of garlic had a 50 percent lower risk of cancer
of the distal colon compared with women who had the lowest level of garlic
consumption (7).
Several population studies conducted in China centered on garlic consumption
and cancer risk. In one study, investigators found that frequent consumption
of garlic and various types of onions and chives was associated with reduced
risk of esophageal and stomach cancers, with greater risk reductions seen
for higher levels of consumption (8). Similarly, in another
study, the consumption of allium vegetables, especially garlic and onions,
was linked to a reduced risk of stomach cancer (9). In
a third study, greater intake of allium vegetables (more than 10 g per day
vs. less than 2.2 g per day), particularly garlic and scallions, was associated
with an approximately 50 percent reduction in prostate cancer risk (10).
Evidence also suggests that increased garlic consumption may reduce pancreatic
cancer risk. A study conducted in the San Francisco Bay area found that
pancreatic cancer risk was 54 percent lower in people who ate larger amounts
of garlic compared with those who ate lower amounts (11).
In addition, a study in France found that increased garlic consumption
was associated with a statistically significant reduction in breast cancer
risk. After considering total calorie intake and other established risk
factors, breast cancer risk was reduced in those consuming greater amounts
of fiber, garlic, and onions (12).
- Do findings from clinical trials offer
evidence that garlic may prevent cancer?
Few clinical trials (research studies with people) have been done to examine
the potential anticancer effects of garlic.
Three randomized clinical trials have evaluated the effect of garlic intake
on gastric cancer risk. In one study, which involved over 5,000 Chinese
men and women at high risk for stomach cancer, researchers compared the
effects of taking a combination of 200 mg synthetic allitridum (an extract
of garlic used as a medicine in China for over 3,000 years) daily and 100
micrograms selenium every other day with taking a placebo (an inactive substance
or treatment that looks the same as, and is given the same way as, an active
drug or treatment being tested) for 5 years. In the group that received
allitridum and selenium, the risk for all tumors combined was reduced by
33 percent and the risk for stomach cancer was reduced by 52 percent in
comparison with the group that received only the placebo (13).
In contrast, findings from another randomized trial involving individuals
with precancerous stomach lesions found that garlic supplementation (800
mg garlic extract plus 4 mg steam-distilled garlic oil daily) did not improve
the prevalence (number of existing cases) of precancerous gastric lesions
or reduce the incidence (number of new cases) of gastric cancer (14).
A third randomized study in Japan compared the effects of daily high-dose
(2.4 mL) and low-dose (0.16 mL) intake of aged-garlic extract after 6 and
12 months of use on individuals with colorectal adenomas (noncancerous tumors).
At the end of 12 months, 67 percent of the low-intake group developed new
adenomas compared with 47 percent in the high-intake group (15).
The results of a small, nonrandomized study indicate that the application
of garlic extracts to some skin tumors may be beneficial. In the study,
which involved 21 persons with basal cell carcinoma, the application of
ajoene (a sulfurous chemical found in garlic) to the skin for 1 month markedly
decreased the size of 17 tumors, increased tumor size in 3 patients, and
resulted in no change in 1 other patient (16). Changes
in tumor size ranged from an 88 percent reduction to a 69 percent increase,
with an overall median reduction of 47 percent.
- What are the current issues and controversies surrounding
the use of garlic in cancer prevention?
Study limitations, including the accuracy of reporting the amounts and
frequency of garlic consumed, and the inability to compare data from studies
that used different garlic products and amounts make an overall conclusion
about garlic and cancer prevention extremely difficult. Since many of the
studies looking at garlic use and cancer prevention have used multi-ingredient
products, it is unclear whether garlic alone or in combination with other
nutritional components may have the greatest effect.
Well-designed dietary studies in humans using predetermined amounts of
garlic (intervention studies) are needed to determine potentially effective
intakes. Studies directly comparing various garlic preparations are also
needed.
- How might garlic act to prevent cancer?
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), enhance DNA 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 antibacterial
agents.
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
- Milner JA. Garlic: Its anticarcinogenic and antitumorigenic
properties. Nutrition Reviews 1996; 54:S82–S86.
- Ross SA, Finley JW, Milner JA. Allyl sulfur compounds
from garlic modulate aberrant crypt formation. Journal of Nutrition
2006; 136(3 Suppl):852S–854S.
- 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.
- Amagase H. Clarifying the real bioactive constituents
of garlic. Journal of Nutrition 2006; 136(3 Suppl):716S–725S.
- Fleischauer AT, Arab L. Garlic and cancer: A critical
review of the epidemiologic literature. Journal of Nutrition 2001;
131(3s):1032S–1040S.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Shenoy NR, Choughuley AS. Inhibitory effect of diet
related sulphydryl compounds on the formation of carcinogenic nitrosamines.
Cancer Letters 1992; 65(3):227–232.
- 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.
- 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.
- Boon H, Wong J. Botanical medicine and cancer: A
review of the safety and efficacy. Expert Opinion on Pharmacotherapy
2004; 5(12):2485–2501.
- 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.
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