Nature's Pharmacopeia: A World of Medicinal Plants

GARLIC

Allium sativum

A close relative of onion, chives, and leek, garlic has been grown for thousands of years, originating in Central Asia, disseminated throughout Eurasia and North Africa in ancient times, and now grown worldwide.49 The culinary and medicinal parts are the fleshy leaf bases, which grow underground, surrounded by papery leaf sheaths (figure 14.7). Grasslike leaves can reach a height of 30 to 60 centimeters.50Although garlic, a perennial, occasionally produces flowers in starburst-like inflorescences at the end of long stems, it rarely sets seed. Therefore, most garlic propagation is vegetative, aided by the separation of individual vegetative buds and surrounding swollen leaf bases, the cloves.51

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FIGURE 14.7   Garlic. (Woodcut from Hamsen Schönsperger, Gart der Gesundheit [1487]; Peter H. Raven Library, Missouri Botanical Garden, St. Louis)

Widely employed in cooking and medicine, garlic found its way into the customs of East Asia, South Asia, Mesopotamia, Egypt, and the Greco-Roman world.52 At least as early as the Egyptian New Kingdom (from 1550 B.C.E.), garlic was a valued foodstuff, and, indeed, specimens of garlic bulbs accompanied numerous burials.53 The Greek herbalist Pedanius Dioscorides (ca. 40–90) recommended garlic as a warm and drying herb that had the capability to relieve flatulence, expel intestinal worms, increase urine output, treat bald spots, clear coughs, and cure snakebites, among other uses.54

By the seventeenth century, an English medical text declared garlic “a remedy for all diseases and hurts,” with warming properties particularly useful for “hydropick diseases, the Iaundise, Falling Sicknesse, Crampes, Convulsions, the Piles or hemorrhoides, and other cold diseases.”55 With such diverse applications across a long period of time, it is not surprising that modern researchers are engaged in testing garlic’s efficacy against a wide spectrum of health concerns.

Garlic’s medicinal and culinary properties derive in large part from an assortment of sulfur-containing molecules, some of them pungent, that are released when its tissue is crushed. Cells in the garlic bulb accumulate large amounts of the odorless chemical alliin.56 As cells experience damage (such as when crushed or chopped), alliin reacts with an enzyme released from specialized subcellular compartments and is converted to the characteristically acrid chemical allicin (figure 14.8).57 Allicin is unstable, and with heat or age it ultimately degrades into a diverse suite of potentially bioactive sulfur-containing compounds.58Because of the variety of allicin’s degradation products, garlic’s possible physiological effects depend on its fresh or aged state and method of preparation.59

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FIGURE 14.8   Allicin, a compound in freshly crushed garlic that gives rise to sulfur-containing chemicals with possible health-related effects.

The normal processes of cellular metabolism as well as the stresses of aging and illness can generate damaging (oxidizing) chemical byproducts in human cells, and some of garlic’s sulfur-containing compounds are suggested to have antioxidant properties that counter these effects.60 The antioxidant capacity of garlic appears to derive from the ability of the sulfur-containing compounds themselves both to neutralize oxidizing chemicals and to induce antioxidant gene expression in the body.61 Such properties could recommend garlic for use against neurodegenerative disease and cancer, although large studies targeting the prevention or treatment of these conditions have not been performed. Garlic’s antioxidant properties are credited in part for its potential role against cardiovascular disease, an area that has been investigated by biomedical researchers.

Mounting evidence indicates that garlic’s complex assortment of bioactive chemicals, including the sulfur-containing compounds, have effects in lowering blood-lipid levels and reducing cholesterol, interfering with blood clotting, and bringing down blood pressure, all potentially beneficial outcomes in certain populations.62 Laboratory assays, experiments in animals, and small-scale human trials have demonstrated garlic extract’s capacity to reduce the oxidation of low-density lipoproteins in blood, which would limit the formation of arterial plaques and improve the blood-lipid profile.63 While the results of trials investigating the effects of various preparations and dosing regimens of garlic on blood-lipid profile have been inconsistent, the collective data set indicates that garlic is probably efficacious in improving lipid parameters.64Furthermore, garlic in various forms has been demonstrated to reduce blood pressure in patients with hypertension.65 The mechanism by which garlic is thought to reduce blood pressure has been partially elucidated: the allicin-derived sulfur-containing compounds are transformed in the blood and arteries to the chemical hydrogen sulfide, which acts as a signal to relax the muscles lining the arteries.66 In addition to probable roles in improving blood-lipid profile and blood pressure, garlic is recognized to reduce blood clotting, a factor in cardiovascular disease and stroke.67

Garlic has an enduring reputation as a treatment for infectious disease, either by direct antimicrobial activity or by somehow boosting the immune system to deal better with pathogens.68 While many laboratory tests have demonstrated the antibacterial and antiviral properties of garlic and its extracts, this activity is generally attributable to the unstable, oxidizing compound allicin.69 Since allicin does not survive in the human system, it is not likely to be an effective antimicrobial in the body, except, perhaps, in topical applications. Although garlic has been suggested as an immune system modulator, studies of this possible function mostly remain preclinical. Several research centers have considered the use of garlic extracts as a preventive measure or treatment for infections such as the common cold, although to date the published experiments do not show an advantage of garlic over placebo treatment in this regard.70

In widespread use since ancient times and long associated with health and healing, garlic garnered esteem for treating diverse illnesses and adding its characteristic pungency to world cuisines. As garlic remains an important food item in diets around the globe, its role as a nutritive, flavorful ingredient will likely be further compounded with additional demonstrable medicinal properties in times to come.