Garlic: benefits, dosage, contraindications

Antibacterial

The antimicrobial effects of garlic have been attributed to its allicin content. Allicin and other sulfur-containing constituents in garlic can inhibit bacterial growth by interacting with the enzymes necessary for their growth. Ajoene, another constituent of garlic, also possesses in vitro antibacterial activity against Gram-positive and Gram-negative bacterial species, such as Escherichia coli, Lactobacillus casei, 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 also demonstrated antimicrobial activity against Streptococcus mutans, suggesting its potential to combat oral bacteria known to cause dental caries. Other in vitro research suggests that garlic extract might have antibacterial activity against several mycobacteria strains, including Mycobacterium tuberculosis, Mycobacterium avium-intracellulare, and Mycobacterium kansasii, three bacteria associated with lung infections. Several in vitro and in vivo tests have shown garlic's activity against Helicobacter pylori, although clinical study results are mixed. Two studies showed that a garlic and omeprazole combination produced synergistic effects against Helicobacter pylori. Rapid action of garlic oil was observed in an artificial gastric juice model, suggesting its significance. The antimicrobial activity of garlic oil appears more potent than 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 compounds in garlic activate key factors such as Nrf2, thus playing a protective role against oxidative stress. Additionally, aged garlic appears capable of directly scavenging superoxide radicals. Interestingly, laboratory studies have shown that garlic can enhance 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 by a meta-analysis of 12 clinical trials that showed garlic increases the serum's total antioxidant capacity and levels of superoxide dismutase SOD, while decreasing serum levels of malondialdehyde (MDA), a significant indicator of oxidative stress.

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Hypolipidemic

In patients with hyperlipidemia, garlic can lower cholesterol levels by acting as an HMG-CoA reductase inhibitor (an enzyme involved in the metabolic pathway producing cholesterol precursors). Garlic can also reduce the activity of enzymes involved in cholesterol synthesis such as hepatic glucose-6-phosphate dehydrogenase, malic enzyme, and squalene monooxygenase, as well as enzymes catalyzing fatty acid synthesis such as fatty acid synthase. On the other hand, garlic supplementation tends to increase HDL cholesterol (the good cholesterol) and decrease LDL cholesterol (the bad cholesterol). Moreover, the reduction in triglycerides 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 methylthiosulfinate, may have activity against certain viruses like cytomegalovirus, herpes simplex viruses type 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 allitridium, have shown antifungal activity against Scedosporium prolificans, a fungal species that can affect immunocompromised patients. Fresh garlic, aged garlic extract, and allicin, a constituent of garlic, have shown antifungal activity against Candida albicans in the lab. The antifungal activity of garlic against candida infections has been attributed to allicin's ability to downregulate SIR2, a gene involved in fungal formation. A randomized trial with 56 patients showed that a garlic-based 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 up to an hour after ingestion.

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Antiparasitic

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

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Anticancer

Preliminary data suggest that garlic and its sulfur compounds exert anticancer effects through several mechanisms. Garlic appears to have both humoral (via B lymphocytes against extracellular pathogens like bacteria) and cellular immunostimulant activities (via T lymphocytes against intracellular pathogens like viruses). For example, it may stimulate T cell proliferation, restore suppressed antibody responses, enhance macrophage cytotoxicity, and phagocytosis of tumor cells. It has also been demonstrated that S-allyl mercaptocysteine (a component of garlic) has activity against erythroleukemic cancer cells (a type of leukemia), breast cancer cells, and prostate cancer cells. The constituent ajoene has been observed to induce the death of human leukemic cells. Additionally, garlic could enhance selenium absorption and protect against tumor formation. 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 garlic powder extract reduces the production of cells responsible for inflammatory response. Garlic powder extract also seems to reduce the activity of factors involved in inflammation associated with autoimmune diseases such as arthritis and inflammatory bowel diseases, atherosclerosis. Garlic may also increase the number of natural killer (NK) cells and their activity against tumor cell lines. Additionally, garlic appears to stimulate the proliferation of immune cells (macrophages and lymphocytes). Animal research also suggests that long-term administration of garlic can improve the decline in immune function associated with aging.

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Antiplatelet/Anticoagulant

It has been shown that garlic powder and aged garlic preparations have antiplatelet properties in both cardiovascular patients and healthy volunteers. Garlic has antithrombotic properties and can increase fibrinolytic activity (prolongs bleeding time), decrease platelet aggregation, and increase prothrombin time (a coagulation factor). The antiplatelet activity may be attributed to garlic components, including adenosine, allicin, and paraffin polysulfides. Raw garlic seems to have more potent antiplatelet properties than cooked garlic. On the other hand, garlic oil does not seem to affect platelet aggregation.

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Vascular Effect

In hypertensive patients, garlic may reduce blood pressure by causing smooth muscle relaxation and vasodilation by activating the production of endothelium-derived relaxing factor (EDRF: endothelium derived relaxing factor). Allicin has been suggested as the garlic component responsible for vasodilatory effects mediated by nitric oxide. Garlic may also mitigate age-related increases in aortic stiffness, which can enhance vasoelasticity and protect against cardiovascular diseases.

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Metabolic

Animal research has shown that sulfur compounds in garlic are responsible for increased thermogenesis. These compounds promote thermogenesis by increasing norepinephrine secretion and reducing body fat accumulation.

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Hepatoprotective

There is promising evidence of the hepatoprotective effects of garlic. A clinical trial demonstrated a dose-dependent protective effect of a garlic oil supplement measured by levels of aspartate aminotransferase and alanine aminotransferase 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, thus strengthening the endogenous antioxidant protection mechanisms according to in vitro tests.

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