By: Brianna Choyce
There is an association between increased intake of garlic and reduced risk of several cancers including: gastric, colon, esophageal, pancreatic, and breast. Garlic possesses cancer-preventive and therapeutic potential at multiple stages of cancer development and has significant immune-enhancing effects. The anti-cancer effects of garlic, an Allium vegetable, are largely attributed to its metabolic byproducts and organosulfur components, which have been shown to inhibit the growth of certain tumor cells. Garlic is rich in many compounds that have been shown to have health benefits. Examples of such compounds include: arginine, flavonoids, selenium, and sulfur. Such compounds have anticarcinogenic and antitumorigenic properties that stem from a variety of attributes including: antibacterial, antioxidant, and detoxification properties, ability to prevent formation of carcinogens, propensity to promote cellular DNA repair, as well as antiproliferative and apoptotic effects.
Nitrosamines and heterocyclic amines are dietary carcinogens, which are substances that promote the formation of cancer by damaging the genetic makeup of cells or disrupting normal cell function. These carcinogens are not usually present in raw foods, but often arise during preservation and/or cooking. It is believed that these initiators play a large role in the development of cancer. Garlic intervenes by preventing initiation by inhibiting nitrosamines and heterocyclic amines. Garlic can also prevent the bioactivation of non-nitrosamines and non-heterocyclic amines. Many carcinogens only become carcinogenic after consumption through bioactivation. Allium vegetables and several of their allyl sulfur compounds block this bioactivation and subsequent carcinogenicity. Garlic is also shown to enhance detoxification by increasing the natural antioxidant, glutathione.
Garlic has been shown to reduce the proliferation, or growth and spread, of a wide variety of cancer cell types. By retarding the uninhibited progression of the cell cycle and inducing apoptosis, the promotion stage is halted. Garlic and other sulfur compounds are also hypothesized to suppress tumor proliferation by altering thiol status, which has antioxidant effects and allows the body to conduct anti-proliferative activities to its greatest potential.
Allium vegetables contain a wide variety of antioxidant compounds that protect cells from damage caused by unstable molecules called free radicals. Some of these compounds include flavonoids, glutathione, vitamin C, vitamin E, and various selenium compounds. Free radicals are unstable in the sense that they are highly reactive and have a great potential to harm cells, operating as initiators. Free radical scavenging by garlic’s compounds has also been shown to increase the body’s total phenolic content. This antioxidant strategy is vital to cancer prevention and treatment via inhibition of cancer cell proliferation. In addition to its antioxidant properties, there is evidence that garlic compounds can inhibit COX activities which cause inflammation in the body, as well as its release which is directed by chemical messengers called proinflammatory cytokines.
Substantial evidence indicates that garlic extracts can exhibit antifungal properties and inhibit a significant range of bacteria including gram-negative and gram-positive species. One such clinically significant bacterium related to cancer formation is Helicobacter pylori, or H. pylori, a virulent gram-negative organism that is associated with gastric cancers.
Garlic has also shown evidence in supporting both humoral and cellular immunity to help ward off disease. It can also stimulate T-cell proliferation, a means of restoring defenses that are often suppressed in cancer and chemotherapy. Garlic’s stimulation of macrophages can enhance the body’s ability to fight off tumor cells and subsequent increase in dietary selenium contained in garlic is hypothesized to prevent development of tumors. Finally, garlic may reduce or prevent cancer cell proliferation by halting uncontrolled cell cycle progression and replication and supporting induction of apoptosis of cancerous cells.
Much evidence of garlic’s health benefits has been conducted to date. A review of the cancer preventive effects of Allium vegetables, particularly garlic and onions, and their bioactive sulfur compounds was published in 2015. This article highlighted mechanisms by which garlic is hypothesized to aid in cancer prevention and therapy. The article also identified several types of cancer in which garlic has show promising efficacy including: stomach, colorectal, esophageal, and prostate cancers, among others.
A study conducted in 1998 suggested that H. pylori infection is a risk factor for formation of precancerous gastric lesions and that garlic may be protective. Factors such as variation in diet and genetics can significantly alter an individual’s risk for development of gastric cancer, complicating the association of H. pylori as it relates to gastric cancer. More recent studies focus on the mechanism by which H. pylori infection leads to increased acid production in the stomach, predisposing individuals to duodenal ulcers, gastritis, and theoretically gastric adenocarcinoma.
An analysis of studies in humans and animals evaluated published evidence of garlic’s anti-carcinogenic properties in over 300 previous studies. The conclusion of this review was that the published epidemiologic evidence supports garlic’s protective effects in stomach and colon cancer. The review explicitly recognized garlic for its anticarcinogenic properties and neoplastic inhibition.
Beginning in 1996, a much larger prospective study nicknamed the EPIC study, European Prospective Investigation into Cancer and Nutrition, has explored clinical and pathological data in cancer patients across seven countries. This study investigates the effects of nutrition, among other factors, on cancer and mortality. Results thus far have confirmed a negative association between garlic intake and incidence of intestinal/gastric cancer.
Dietary role on risk for breast cancer development was investigated in a study of 345 individuals with breast cancer as compared to a similar population of individuals without breast cancer. The most notable findings confirmed the protective effects of increased garlic, onion, and fiber intake with a statistically significant reduction in breast cancer risk.
Additional Health Benefits
Some additional health benefits associated with garlic use have been studied in the following conditions:
- Skin infections- Studies demonstrate garlic’s antibacterial, antifungal, antiviral, and antiparasitic activity when it comes to skin conditions.
- Heart disease- Clinical trials show that dietary and supplemental garlic reduces cholesterol and triglyceride levels. Reduction in these factors reduces the risk of heart disease.
- Atherosclerosis- A reduction in blood pressure, cholesterol, and triglyceride levels has been reported with garlic supplementation. These factors reduce the risk of developing atherosclerosis. Additionally, garlic’s antiplatelet activity can reduce the risk of blood clots often associated with this condition.
- High cholesterol- Clinical trials show that dietary and supplemental garlic reduces cholesterol and triglyceride levels.
- Circulatory disorders- Garlic has antiplatelet activity which can prevent clots in those with circulatory disorders.
- High blood pressure- A reduction in blood pressure has been associated with garlic’s ability to cause smooth muscle relaxation and vasodilation by activating the production of endothelium-derived relaxation factor.
An overwhelming amount of quality evidence supports the theory that garlic has anti-cancer effects. Its implications in various types of cancer have been studied with positive results. The compounds contained in garlic have many cancer-related benefits, most notably: antibacterial, antioxidant, anticarcinogenic, and antiproliferative properties. Numerous other non-cancer-related benefits are have been associated with garlic supplementation. Consideration of the mechanisms by which garlic exhibits these properties, among others, offers insight into the many ways that garlic can have chemopreventive and chemotherapeutic effects.
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