Garlic (Allium sativum), a common seasoning in many savory dishes, has been studied for its potential health benefits for decades. One of the biggest reasons it is studied is its ability to reduce the risk of certain cancers (1).

Researchers have examined garlic’s link to stomach, colon, prostate, and breast cancers. While many findings are promising, further large-scale studies are needed to clarify garlic’s role in cancer prevention.

Keep reading for summaries of the top hypotheses about how garlic may fight cancer. You’ll also learn about the key findings of relevant studies and clinical trials, plus cautions and considerations to note.

Tip
The smell of garlic can be removed from fingers by running hands under cold water while rubbing a stainless steel object.

Garlic’s Potential Anticarcinogenic Mechanisms

Scientists have identified several biochemical pathways through which garlic and its compounds may help fight cancer development. These mechanisms work at different stages of cancer progression, from preventing initial DNA damage to slowing the growth of existing tumors.

Organosulfur Compounds

Garlic contains sulfur-based compounds including allicin, S-allylcysteine, and diallyl trisulfide. Many researchers regard these as the “active ingredients” responsible for potential anticancer properties. When you crush or chop garlic, an enzyme (alliinase) converts alliin into allicin. This is thought to be especially advantageous in lab-based research.

Various forms of these organosulfur compounds have been found to slow tumor progression, prompt cancer cell self-destruction (apoptosis), and dampen harmful signaling pathways (2, 3).

Antioxidant and Anti-inflammatory Actions

Garlic provides antioxidant effects, which can help neutralize free radicals in your body. It also appears to reduce chronic inflammation, which researchers believe can create an environment favorable to cancer cell growth (4, 5).

Eradication of Helicobacter pylori

Chronic infection with H. pylori is highly associated with gastric (stomach) cancer. Some research suggests that polyphenolic compounds in garlic may lower or eradicate H. pylori, potentially reducing your stomach cancer risk (6, 7).

Immune System Enhancement

Garlic may also help your body recognize and destroy cancer cells. Some studies show it can enhance certain immune responses, including the activity of natural killer (NK) cells (8).

Tip
To maximize garlic’s potential cancer-fighting properties, crush or chop fresh garlic and let it sit for 10-15 minutes before cooking. This allows the beneficial compounds to form before heat exposure potentially reduces their effectiveness.

Research Connecting Garlic to Lowered Cancer Risk

The scientific literature contains numerous epidemiological studies, clinical trials, and meta-analyses examining garlic’s relationship with cancer risk. While the evidence varies by cancer type, certain patterns have emerged that suggest potential protective effects against cancer.

Gastric (Stomach) Cancer

When it comes to garlic and cancer prevention, stomach cancer is among the most thoroughly researched areas. Multiple meta-analyses find that higher garlic consumption is linked with a reduced risk of developing gastric cancer (9).

In a region of China known for high gastric cancer rates, garlic supplementation showed a protective influence against precancerous gastric lesions (10). Findings from this randomized trial (the Shandong Intervention Trial) suggests that garlic may have a lasting protective effect against gastric cancer for those with precancerous lesions.

Additional follow-up research noted that while vitamin and H. pylori treatments provided more consistent reductions in gastric cancer incidence, garlic still reduced mortality from stomach cancer in the long term (11, 12).

Summary
Garlic consumption has been linked to a reduced risk of gastric cancer. Studies show its protective effects against precancerous lesions and long-term reductions in mortality from stomach cancer.

Colorectal Cancer

Garlic consumption has also been extensively studied in relation to colorectal cancer. Early studies found that aged garlic extract might suppress the growth of precancerous adenomas in the colon (13). One double-blinded study showed that high doses of aged garlic extract significantly reduced both the size and number of colorectal adenomas over 12 months (14).

Several meta-analyses also suggest a modest reduction in colorectal cancer risk among frequent garlic consumers (15, 16, 17). However, not every study concurs. One prospective study indicated that garlic pills might increase colorectal cancer risk, showing the complexity of supplement-based approaches (18).

Other Cancers

Prostate Cancer: Various laboratory and animal studies indicate that diallyl trisulfide, a compound derived from garlic, can inhibit the growth of prostate cancer cells (3). In humans, however, a small study found that while sulforaphane (from broccoli) was measurable in prostate tissue, the alliin compound from garlic was not significantly detected in supplemented vs. non-supplemented groups (19).

More research is needed to pinpoint how garlic might prevent or slow prostate cancer in people.

Breast Cancer: In some populations, higher garlic consumption has been linked observationally to a lower risk of breast cancer (20). Larger, long-term clinical studies are needed to draw firm conclusions.

Esophageal Cancer: Diets high in garlic may help protect against esophageal cancer. This may be possible by reducing inflammation or neutralizing carcinogens in the esophagus (21).

Liver (Hepatocellular) Cancer: Preliminary findings suggest garlic’s sulfur compounds may protect your liver by aiding in detoxification and cutting oxidative stress, factors that may help slow tumor development in the liver (22).

Note
While research on garlic and cancer is promising, most evidence comes from observational studies. For the best potential benefits, consider including 1-2 cloves of fresh garlic in your daily diet rather than relying solely on supplements.

Proceed with Caution: Mixed Results and Risks

Although several studies suggest garlic may have a protective role against cancer, there are notable contradictions to consider.

One large study revealed a higher risk of colorectal cancer among those who used garlic pills (but not among those who consumed garlic in food) (18). Additionally, applying raw garlic to the skin can be hazardous and cause serious burns if not used properly (23).

Furthermore, garlic, especially in supplement form, can interact with certain prescription drugs (for example, blood thinners and some antihypertensives), potentially causing excessive bleeding or changes in blood pressure control (24, 25).

Limitations and Conflicting Evidence

Heterogeneity of Studies: Different forms of garlic (fresh vs. supplements vs. aged extracts), varying dosages, and diverse populations can make direct comparisons difficult (17).

Methodological Differences: Many studies use self-reported questionnaires for garlic intake, leading to possible recall bias or other misclassification (7).

Potential Publication Bias: Studies showing positive outcomes are likelier to get published than those with null findings, resulting in an inflated perception of garlic’s benefits (26).

Note
If you’re taking blood thinners, antihypertensives, or other medications, always consult with your healthcare provider before starting garlic supplements. While culinary use is generally safe, higher doses in supplement form can interact with medications and cause adverse effects.

Conclusion

Overall, most evidence from laboratory experiments, clinical trials, and population studies suggests a possible protective influence of garlic on certain cancers, particularly stomach and colon (27, 28). Researchers propose that garlic’s rich organosulfur compounds may help inhibit tumor cell growth, reduce inflammation, and possibly strengthen your immune system’s surveillance against cancerous changes. Nevertheless, study results are not always in agreement. Varied garlic preparations, differences in supplement quality, and interactions with other diet or medication factors can all influence findings.

For potential health benefits, consider moderate consumption of fresh or cooked garlic within your balanced diet. High-dose supplements can pose risks, especially if you’re on certain medications, so discuss this with your healthcare provider first.

Larger, carefully designed studies will continue clarifying whether and how garlic lowers cancer risk and at which dosages it may be most beneficial for you.

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