Garlic (Allium sativum)
Garlic (Allium sativum) is a species of bulbous flowering plant in the genus Allium. Its close relatives include the onion, shallot, leek, chive, Welsh onion, and Chinese onion. It is native to Central Asia and northeastern Iran and has long been a common seasoning worldwide, with a history of several thousand years of human consumption and use. Numerous cuneiform records show that garlic has been cultivated in Mesopotamia for at least 4,000 years. The use of garlic in China and Egypt also dates back thousands of years. Well-preserved garlic was found in the tomb of Tutankhamun (c. 1325 BC).
It was consumed by ancient Greek and Roman soldiers, sailors, and rural classes (Virgil, Eclogues ii. 11), and, according to Pliny the Elder (Natural History xix. 32), by the African peasantry. Garlic was placed by the ancient Greeks on the piles of stones at crossroads, as a supper for Hecate (Theophrastus, Characters, The Superstitious Man). Currently, China produces over 70% of the world's supply of garlic.
Garlic is primarily used as a spice and to treat hyperlipidemia, hypertension, atherosclerosis, cancer, and infections.
Garlic has a variety of bioactive compounds, including organosulfur compounds, saponins, phenolic compounds, and polysaccharides. The major active components of garlic are its organosulfur compounds, such as diallyl thiosulfonate (allicin), diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS), E/Z-ajoene, S-allyl-cysteine (SAC), and S-allyl-cysteine sulfoxide (alliin). In general, organosulfur compounds in raw garlic have higher digestibility than those in cooked garlic. In addition, saponins were found to be more stable in the cooking process. The total amount of saponin in purple garlic was almost 40 times higher than that in white garlic, and several saponin compounds were only found to exist in purple garlic, such as desgalactotigonin-rhamnose, proto-desgalactotigonin, proto-desgalactotigonin- rhamnose, voghieroside D1, sativoside B1-rhamnose, and sativoside R1. Moreover, garlic contained more than 20 phenolic compounds, with higher contents than many common vegetables. The main phenolic compound was β-resorcylic acid, followed by pyrogallol, gallic acid, rutin, protocatechuic acid, as well as quercetin. Furthermore, garlic polysaccharides were reported to contain 85% fructose, 14% glucose, and 1% galactose.
Processing can have a substantial effect on the chemical content of garlic because the volatile oil components are heat-sensitive and certain enzymes are acid-labile. The best measure of total activity of garlic is its ability to produce allicin, which in turn, results in the formation of other active constituents. Several oral garlic formulations are available, and clinical studies have evaluated a variety of the proposed claims.
Epidemiologic studies show an inverse correlation between garlic consumption and progression of cardiovascular disease. Garlic has been shown to inhibit enzymes involved in lipid synthesis, decrease platelet aggregation, prevent lipid peroxidation of oxidized erythrocytes and LDL, increase antioxidant status, and inhibit angiotension-converting enzyme. Probably one of the most profound effects observed is garlic's ability to reduce platelet aggregation.
Data on whether garlic preparations may lower blood pressure are also mixed, but other meta-analyses suggest reductions in cardiovascular risk factors due to antihyperlipidemic effects, reduced inflammatory biomarkers, and improved glucose levels. Garlic extract also improved endothelial biomarkers associated with cardiovascular risk in obese individuals. Data also suggest immunostimulatory effects and benefits in patients with hepatopulmonary syndrome.
Several studies have evaluated whether garlic products can have protective effects against various cancers. In a large, randomized intervention trial, long-term garlic supplementation was associated with reduced risk of gastric cancer mortality but not incidence. Preliminary studies suggest aged garlic extract may reduce number and size of subsequent colorectal adenomas in patients with a history of adenoma and improve natural killer cell number and activity. Garlic supplementation may also be associated with reduced risk of hematologic malignancies.
The intact cells of garlic contain an odorless, sulfur-containing amino acid derivative known as alliin. When the cells are crushed, alliin comes into contact with the enzyme alliinase located in neighboring cells and is converted to allicin. Allicin has antibacterial and antimicrobial activity but is highly odoriferous and unstable. It also has antiplatelet and antihyperlipidemic activities. Most authorities agree that the best measure of the total activity of garlic is its ability to produce allicin, which, in turn, results in the formation of other active constituents.
In patients with hyperlipidemia, garlic might lower cholesterol levels by acting as an HMG-CoA reductase inhibitor. For atherosclerosis, garlic is believed to reduce oxidative stress and low-density lipoprotein oxidation and have antithrombotic effects. It is also thought to reduce blood pressure by causing smooth muscle relaxation and vasodilation by activating the production of endothelium-derived relaxation factor.
Garlic may stimulate both humoral and cellular immunity, causing T-cell proliferation, restoring suppressed antibody responses, and stimulating macrophage cytotoxicity on tumor cells. It may increase selenium absorption with possible protection against tumorigenesis. In addition, garlic may protect against certain cancers by halting cell cycle progression and inducing apoptosis of cancer cells as well as by decreasing angiogenesis and influencing carcinogen metabolism.
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