The Cancer Research
Laboratory research shows that allium compounds in garlic offer anti-cancer activity, but support for garlic lowering cancer risk is lacking in human studies.
More research is needed to understand how garlic’s role in cancer risk might vary based on the amount, absorption, preparation methods and individual differences.
AICR’s Third Expert Report and Continuous Update Project (CUP) found evidence too limited to draw any conclusions about garlic and cancer risk.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how foods and their nutrients affect the risk of developing cancer.
- Evidence categorized as “convincing” and “probable” means there is strong research showing a causal relationship to cancer—either decreasing or increasing the risk. The research must include quality human studies that meet specific criteria and biological explanations for the findings.
- A convincing or probable judgment is strong enough to justify recommendations.
• Evidence categorized as “limited suggestive” means results are generally consistent in overall conclusions, but it’s rarely strong enough to justify recommendations to reduce the risk of cancer.
Ongoing Areas of Investigation
- Laboratory Research
Most research on garlic and cancer focuses on garlic’s allium compounds. One of the most well-studied of these oil-soluble compounds is allicin, which is produced from an enzyme that is released when the garlic tissue is damaged or crushed. Allicin then quickly forms several other compounds.
Animal studies have shown garlic compounds decrease the development of several cancer types. In cell and animal studies, allium compounds from garlic show multiple effects that could play a role in cancer prevention.
These effects seem to be accomplished, at least in part, by influencing gene expression; for example, turning on tumor suppressor genes. Possible cancer-preventive mechanisms in these studies include:
- inhibiting enzymes that activate carcinogens (potentially cancer-causing compounds)
- boosting enzymes that remove the effects of carcinogens
- reducing inflammation that could support cancer development
- supporting DNA repair
- slowing growth and stimulating self-destruction of cancer cells without disturbing normal cells
- limiting cancer’s ability to spread by decreasing a tumor’s ability to grow new blood vessel
- Human Studies
Most human studies related to garlic look at cancer risk by comparing populations who consume high amounts of garlic to those who eat little garlic.
The majority of early human research on garlic and cancer came from a type of study that compares the diets of people who already have cancer with people who do not. This type of research, called case-control studies, suggested that garlic might be associated with a lower risk of colorectal cancer. However, these studies are prone to several potential inaccuracies, such as relying on people’s memory of what they ate years before.
Evidence now includes several higher-quality studies that followed a group of people over time. Called prospective cohorts, these studies followed people from 6 to over 20 years and collected data on their health and garlic consumption, along with many other relevant factors. Whether looking at garlic consumption of more than once a day or more than once a week, it was not associated with any significant reduction in colorectal cancer risk.
Some case-control studies have linked higher garlic consumption with less risk of cancers of the esophagus, pancreas, breast, endometrium and prostate. But the results of these studies are mixed, and this type of study is not considered a strong type of evidence.
One randomized controlled study of a single garlic-containing meal suggested that about 1½ teaspoons of garlic activated genes related to regulating the self-destruction of abnormal cells and immune function.
An analysis of randomized controlled trials found that extremely high amounts of garlic resulted in lower levels of some markers of inflammation. However, other inflammatory markers were not reduced, and the studies involved amounts of garlic far beyond what a person would regularly eat.
Questions remain on how well humans absorb the allium compounds formed from garlic, and the amount people need to experience the benefits that lab studies suggest may occur.
- Tips for Selection, Storage and Preparation
- Choose fresh garlic that is plump, dry and firm.
- Buy jars of pre-chopped garlic in oil to make cooking with garlic more convenient. Some of the allyl-sulfur compounds are oil-soluble, so you can use the oil too.
- Look for frozen, crushed or chopped garlic for other flavorful options.
- Fresh garlic will keep for several months at room temperature in a dark and dry space with some air circulation.
- Store jarred or frozen garlic as directed and look for expiration dates.
- To peel a garlic clove, place clove on a cutting board, cut off its bottom. Place the flat side of a large knife (sharp side away from you) on top of clove and hit it. It’s easy to pop the clove right out of its papery skin.
- Allow crushed or chopped garlic to stand for about 10 minutes before cooking. This allows time for the enzyme alliinase to create allicin, the source of bioactive compound.
- Use peeled whole garlic cloves for a hint of garlic flavor, slices for subtle flavor, chopped for medium flavor or quick-cooking dishes, or use crushed garlic – by hand or with a garlic press – for strongest flavor.
- Add garlic to sautéed vegetables, pasta sauces, dips, soups, salad dressings and more.
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