Clinician Fact Sheet: Medicinal Foods - Garlic
Clinician Fact Sheet: Medicinal Foods
Garlic
(Allium sativum; Family: Liliaceae)Although the active constituents in garlic (allium sativum) have not been identified, studies suggest that garlic detoxifies chemical carcinogens, prevents carcinogenesis, stimulates immunity, and protects against the suppression of immunity by chemotherapy and ultraviolet radiation.1 Additionally, much of the garlic research under way today is focusing on garlic’s effects on serum lipid and lipoprotein concentrations.
Historical Medical Use
• Garlic is used to reduce high blood pressure; maintain normal cholesterol levels; enhance circulation; stimulate immune function; fight stress and fatigue; maintain healthy liver function; inhibit platelet aggregation; promote fibrinolysis; reduce lipid levels; and provide vasodilating and antioxidant effects.
• In Asian cultures, garlic is used for diarrhea, amoebic and bacterial dysentery, tuberculosis, bloody urine, diphtheria, whooping cough, scalp ringworm, hypersensitive teeth, and vaginal trichomoniasis.
• Traditionally, garlic has been used to treat colds, flu symptoms, fever, coughs, headache, stomachache, sinus congestion, athlete’s foot, gout, rheumatism, hemorrhoids, asthma, bronchitis, shortness of breath, arteriosclerosis, low blood pressure, hypoglycemia, hyperglycemia, cancer, old ulcers, snakebites, and as an aphrodisiac.
Dosage
• One clove of fresh garlic 1-2 times daily has been used. For cholesterol-lowering and antihypertensive effects, larger doses may be required.
• The equivalent of 6-10 mg/d alliin or 3-5 mg/d allicin has been used. Doses may vary depending on the preparation.
Adverse Reactions
• Taken orally garlic may cause breath odor, mouth and GI burning or irritation, heartburn, flatulence, nausea, vomiting, bloating, headache, dizziness, diarrhea, and outbreaks of sweating. These effects may be more pronounced with ingestion of raw garlic, in patients unaccustomed to eating garlic, and in high doses (8 cloves/d).
• Garlic may also cause changes to the intestinal flora.
• There is one report of spinal epidural hematoma and platelet dysfunction associated with the ingestion of fresh garlic and one report of post-operative bleeding and prolonged bleeding time associated with high dietary garlic consumption.
• Topically, garlic can result in contact dermatitis.
Contraindications
• Garlic is safe in pregnancy and lactation when ingested in typical food quantities; safety at therapeutic doses has not been demonstrated.
• There is a relative contraindication with drugs that affect coagulation.
Interactions
• Because of the antiplatelet aggregating effects of garlic, concomitant use with anticoagulant/antiplatelet drugs theoretically could increase the risk of bleeding in some people.
• Although never reported or observed, theoretically there also exists an increased risk of bleeding in some people using garlic concomitantly with herbs that have anticoagulant/antiplatelet potential. These herbs include: capsicum, danshen, feverfew, ginger, ginkgo, and ginseng (Panax sp.).
• Garlic ingestion may potentiate the effects of anti-hypertensive drugs.
• As reported in a single case report, garlic can enhance the effects of warfarin (Coumadin®) as measured by the INR.
• Theoretically, concomitant use of garlic might increase effects and adverse effects of hypoglycemic drugs; therefore, adjustments to insulin dosages may be necessary.
Clinical Research
Two studies from Japan examined the association between allium vegetable intake and gastric cancer. The first study compared histopathologically confirmed cancer cases (81 esophageal cancer patients; 153 stomach cancer patients) and 234 population-based controls.2 Using a questionnaire, researchers collected information on dietary habits, including consumption of allium vegetables, other foods, tea, smoking, and alcohol. The results demonstrated that frequent intake of allium vegetables (garlic, onion, Welsh onion, and Chinese chives), raw vegetables, tomatoes, snap beans, and tea was inversely associated with esophageal and stomach cancer risk.
In the second study, researchers again used a questionnaire to identify risk factors for gastric cancer.3 Subjects were randomly selected according to age and sex from two populations: one an area of high risk (414 subjects), the other an area of low risk (425 subjects). Smoking and drinking were more common in the low-risk area. However, allium vegetables were consumed in the low-risk area more frequently, with high consumption of raw vegetables, fruit, tomatoes, kidney beans, and soy products. In the low-risk area, 82% of men and 75% of women consumed garlic three or more times per week as compared to 1% of men and women in the high-risk area.
The collective body of research on garlic’s ability to reduce serum lipid and lipoprotein concentrations has produced conflicting results with recent studies demonstrating relatively negative results. A 1998 randomized double-blind, placebo controlled (RDBCT) by Isaacsohn et al found that 900 mg/d of garlic powder was ineffective in lowering cholesterol levels in patients with hypercholesterolemia after 12 weeks.4 Likewise, another 1998 RDBCT by Heiner et al showed that 5 mg of steam-distilled garlic oil taken twice daily had no influence on serum lipoproteins, cholesterol absorption, or cholesterol synthesis after 12 weeks.5
References
1. Lamm DL, Riggs DR. The potential application of Allium sativum (garlic) for the treatment of bladder cancer. Urol Clin North Am 2000;27:157-162.
2. Gao CM, et al. Protective effect of allium vegetables against both esophageal and stomach cancer: A simultaneous case-referent study of a high-epidemic area in Jiangsu Province, China. Jpn J Cancer Res 1999;90:614-621.
3. Takezaki T, et al. Comparative study of lifestyles of residents in high and low risk areas for gastric cancer in Jiangsu Province, China; with special reference to allium vegetables. J Epidemiol 1999;9:297-305.
4. Isaacsohn JL, et al. Garlic powder and plasma lipids and lipoproteins. Arch Intern Med 1998:158:
1189-1194.
5. Heiner K, et al. Effect of a garlic oil preparation on serum lipoproteins and cholesterol metabolism. JAMA 1998;279:1900-1902.
Additional Resources
Alternative Medicine Alert. Atlanta, GA: American Health Consultants; 1998;1:1-144; 1999;2:1-144.
McGuffin M, et al. American Herbal Products Association's Botanical Safety Handbook. Boca Raton, FL: CRC Press; 1997.
McDermott JH. Herbal Chart for Health Care Professionals. American Pharmaceutical Association; 1999.
Natural Medicines Comprehensive Database. Stockton, CA: Therapeutic Research Center, Inc.
PDR for Herbal Medicines. Montvale, NJ: Medical Economics Co.; 1998.
Schulz V, et al. Rational Phytotherapy. 3rd ed. Berlin: Springer-Verlag; 1998.
The Review of Natural Products. St. Louis, MO: Facts and Comparisons.
Foster S, Tyler VE. Tyler’s Honest Herbal. 4th ed. Binghamton, NY: The Haworth Herbal Press; 1999.
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