Garlic: benefits, dosage, contraindications

Antibacterial

The antimicrobial effects of garlic have been attributed to its allicin content. Allicin and other sulfur-containing garlic constituents can inhibit bacterial growth by interacting with enzymes required for their growth. Ajoene, another garlic constituent, also has in vitro antibacterial activity against both Gram-positive and Gram-negative bacterial species, such as Escherichia coli, Lactobacillus casei, and Helicobacter pylori. Interestingly, fresh garlic has shown activity against Escherichia coli, methicillin-resistant Staphylococcus aureus, and Salmonella enteritidis; it has even been suggested as a food additive to prevent food poisoning. Raw garlic extract has also demonstrated antimicrobial activity against Streptococcus mutans, suggesting potential for combating bacteria in the mouth known to cause dental caries. Other in vitro research suggests that garlic extract may have antibacterial activity against several strains of mycobacteria, including Mycobacterium tuberculosis, Mycobacterium avium-intracellulare, and Mycobacterium kansasii, three bacteria associated with pulmonary infections. Multiple in vitro and in vivo tests have shown that garlic has activity against Helicobacter pylori; however, clinical study results remain mixed. Two studies showed that a combination of garlic and omeprazole produced synergistic effects against Helicobacter pylori. A rapid action of garlic oil was observed in an artificial gastric juice model, which suggests its potential. The antimicrobial activity of garlic oil appears to be more potent than that of garlic powder.

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Antioxidant

In various laboratory studies, garlic and its constituents have demonstrated antioxidant activity. It has been observed that certain sulfur-containing components of garlic activate key factors such as Nrf2, thereby playing a protective role against oxidative stress. Moreover, aged garlic appears to be capable of directly scavenging superoxide radicals. Interestingly, laboratory studies have shown that garlic can strengthen the body's defenses against free radicals by stimulating the activity of antioxidant enzymes (glutathione peroxidase, catalase, superoxide dismutase (SOD)), while inhibiting lipid peroxidation. These results are confirmed in humans in a meta-analysis of 12 clinical trials that showed garlic increases total serum antioxidant capacity and levels of superoxide dismutase (SOD), while decreasing serum levels of malondialdehyde (MDA), which is an important marker of oxidative stress.

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Lipid-lowering

In patients with hyperlipidemia, garlic may lower cholesterol levels by acting as an HMG-CoA reductase inhibitor (an enzyme involved in the metabolic pathway that produces cholesterol precursors). Garlic may also reduce the activity of enzymes involved in cholesterol synthesis, including hepatic glucose-6-phosphate dehydrogenase, malic enzyme, and squalene monooxygenase, as well as enzymes that catalyze fatty acid synthesis such as fatty acid synthase. Furthermore, garlic supplementation tends to increase HDL cholesterol (the 'good' cholesterol) and decrease LDL cholesterol (the 'bad' cholesterol). Additionally, triglyceride reduction appears significant after garlic supplementation—according to meta-analyses—but remains minimal.

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Antiviral

In vitro evidence suggests that garlic compounds, such as ajoene, allicin and allyl methyl thiosulfinate, may have activity against certain viruses such as cytomegalovirus, herpes simplex virus types 1 and 2, parainfluenza virus type 3, and human rhinovirus type 2.

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Antifungal

The constituents of garlic, allicin and ajoene, are responsible for garlic's antifungal activity against tinea infections. An aged garlic extract and its constituents, ajoene and allitriduim, have shown antifungal activity against Scedosporium prolificans, a fungal species that can affect immunocompromised patients. Fresh garlic, aged garlic extract and allicin, a garlic constituent, have shown antifungal activity against Candida albicans in laboratory studies. Garlic's antifungal activity against candida infections has been attributed to allicin's ability to down-regulate SIR2, a gene involved in fungal development. A randomized trial of 56 patients showed that a garlic preparation was as effective as clotrimazole solution in suppressing signs of oral candidiasis. In another study, oral intake of a garlic extract was associated with anti-candidal activity in the serum for up to one hour after ingestion.

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Antiparasitic

Laboratory evidence suggests that garlic oil has broad-spectrum activity against Trypanosoma, Plasmodium, Giardia, Leishmania and Cochlospermum planchonii. Animal research suggests that allicin, a garlic constituent, reduces infections and increases survival in mice exposed to Plasmodium, the parasite that causes malaria. In vitro, the garlic constituent, ajoene, inhibits enzymes present in Trypanosoma cells, a parasite that causes sleeping sickness.

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Anticancer

Preliminary data suggest that garlic and its sulfur-containing compounds exert an anticancer effect through multiple mechanisms. Garlic appears to have humoral immunostimulatory activity (via B lymphocytes against extracellular pathogens such as bacteria) and cellular activity (via T lymphocytes against intracellular pathogens such as viruses). For example, it may stimulate T cell proliferation, restore suppressed antibody responses, enhance macrophage cytotoxicity, and promote phagocytosis of tumor cells. It has also been shown that S-allylmercaptocysteine (a component of garlic) had activity against erythroleukemic cancer cells (a type of leukemia), breast cancer cells, and prostate cancer cells. The constituent ajoene has been observed to induce death of human leukemic cells. In addition, garlic may improve selenium absorption and protect against the development of tumors. Studies in humans suggest an association between garlic consumption and a reduced risk of stomach cancer and colorectal cancer.

Immunostimulant

Some in vitro evidence suggests that powdered garlic extract reduces the production of cells responsible for the inflammatory response. Powdered garlic extract also appears to reduce the activity of factors involved in inflammation associated with autoimmune diseases such as arthritis and inflammatory bowel disease, and atherosclerosis. Garlic may also increase the number of natural killer (NK) cells and activity against tumor cell lines. On the other hand, garlic appears to stimulate the proliferation of immune cells (macrophages and lymphocytes). Animal research also suggests that long-term administration of garlic may improve the age-associated decline in immune function.

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Antiplatelet/Anticoagulant

Preparations based on garlic powder and aged garlic have been shown to have antiplatelet properties in both patients with cardiovascular disease and in healthy volunteers. Garlic has antithrombotic properties and may increase fibrinolytic activity (prolong bleeding time), decrease platelet aggregation, and increase prothrombin time (one of the coagulation factors). The antiplatelet activity can be attributed to garlic components, notably adenosine, allicin, and paraffinic polysulfides. Raw garlic appears to have stronger antiplatelet properties than cooked garlic. However, garlic oil does not appear to affect platelet aggregation.

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Vascular effect

In hypertensive patients, garlic can reduce blood pressure by causing relaxation of smooth muscle and vasodilation by activating the production of the endothelium-derived relaxing factor (EDRF). It has been suggested that allicin is the garlic component responsible for the vasodilatory effects that are mediated by nitric oxide. Garlic may also attenuate age-related increases in aortic stiffness, which can improve vasoelasticity and protect against cardiovascular disease.

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Metabolic

In animal studies, sulfur-containing compounds in garlic were found to be responsible for increased thermogenesis. These compounds promote thermogenesis by increasing norepinephrine secretion and by reducing body fat accumulation.

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Hepatoprotective

There is encouraging evidence for the hepatoprotective effects of garlic. A clinical trial demonstrated a dose-dependent protective effect of a garlic oil supplement as measured by aspartate aminotransferase and alanine aminotransferase levels in patients with chronic hepatitis. Another study showed that garlic supplementation reduced aspartate aminotransferase (AST) levels, but not alanine aminotransferase (ALT). The benefit was observed for doses ranging from 100 to 150 mg of garlic oil. Aged garlic extract specifically increases reduced glutathione content, thereby enhancing endogenous antioxidant protective mechanisms according to in vitro tests.

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