Overview
The story of garlic’s role in human history could fill a book, as indeed it has, many times. Its species name, sativum, means “cultivated,” indicating that garlic does not grow in the wild. So fond have humans been of this herb that garlic can be found almost everywhere in the world, from Polynesia to Siberia.
From Roman antiquity through World War I, garlic poultices were used to prevent
wound infections. The famous microbiologist Louis Pasteur
performed some of the original work showing that garlic could kill bacteria. In
1916, the British government issued a general plea for the public to supply it
with garlic to meet wartime needs. Garlic was called Russian penicillin during
World War II because, after running out of antibiotics,
the Russian government turned to this ancient treatment for its soldiers. After
World War II, Sandoz Pharmaceuticals manufactured a garlic compound for intestinal
spasms, and the Van Patten Company produced another for lowering blood
pressure.
Uses and Applications
Garlic is widely used as an all-around treatment for preventing or slowing the
progression of atherosclerosis (the cause of most heart attacks and
strokes). However, there is actually relatively little in the way of meaningful
evidence that it works for this purpose. The balance of the evidence suggests that
garlic is not effective for treating high cholesterol; there is only minimal
evidence that it offers any benefits for people with high blood pressure.
According to some studies, garlic might have blood-thinning effects, but whether
this translates into any medical benefit remains unclear.
One study found preliminary evidence that the use of garlic could enhance blood
sugar control in diabetes. Also, garlic has a long folkloric history as a
treatment for
colds and is commonly stated to strengthen the immune system.
However, not until 2001 was there supporting evidence for this use. A
well-designed double-blind study suggested that the regular use of garlic extract
can help prevent colds.
In addition, folklore suggesting that garlic ingestion can ward off insect
bites may have some truth to it, at least when garlic is taken regularly for
several weeks. When applied topically, garlic can kill fungi, and there is
preliminary evidence suggesting that ajoene, a compound derived from garlic, might
help treat athlete’s foot. Topical garlic can also kill bacteria on contact;
however, if taken by mouth, garlic will not work like an antibiotic (that is,
throughout the body). Furthermore, oral garlic has failed to prove effective for
killing Helicobacter pylori, the stomach bacteria implicated as a
major cause of ulcers.
Traditionally, garlic was often combined with the herb mullein in
oil products designed to reduce the pain of middle ear infections (otitis media)
but not of external ear infections (known commonly as swimmer’s ear). Two
double-blind studies support this use. While these products may reduce pain, it is
very unlikely that they have any actual effect on the infection because the
eardrum prevents them from reaching the site of infection.
Preliminary evidence, including one small double-blind trial, suggests that regular intake of garlic as food or as aged garlic supplements may reduce the risk of various forms of cancer. Based on extremely weak evidence, garlic has been proposed as a treatment for problems related to the yeast Candida albicans, problems such as vaginal yeast infection, oral yeast infection (thrush), and the purported condition discussed in some alternative medicine circles as yeast hypersensitivity syndrome.
Scientific Evidence
Atherosclerosis. Scant evidence hints that garlic might help prevent atherosclerosis, the most common cause of heart attacks and strokes. Garlic preparations have been found to slow hardening of the arteries in animal studies.
In a double-blind, placebo-controlled study that followed 152 people for four years, standardized garlic powder at a dosage of 900 milligrams (mg) daily significantly slowed the development of atherosclerosis as measured by ultrasound. However, this study had some statistical problems that make its results less than fully reliable.
An observational study of two hundred people measured the flexibility of the aorta, the main artery exiting the heart. Participants who took garlic showed more flexibility, indicating less atherosclerosis. However, because this was not a double-blind trial, its results prove little.
Heart attack prevention. In one study, 432 people who had experienced a heart attack were given either garlic oil extract or no treatment for a period of three years. The results showed a significant reduction of second heart attacks and about a 50 percent reduction in death rate among those taking garlic.
High cholesterol. A number of studies published in the 1980s and early 1990s found evidence that garlic preparations can reduce high cholesterol. However, virtually all subsequent studies have failed to find any significant benefit. One carefully designed study failed to find benefits with raw garlic, garlic powder, or aged garlic. The accumulating impact of these repeated negative results indicates that garlic is not effective for improving cholesterol profile.
Hypertension. Numerous studies have found weak evidence that garlic lowers blood pressure slightly, perhaps in the neighborhood of 5 to 10 percent more than placebo. It remains unclear whether garlic supplements can help persons with high blood pressure safely eliminate or avoid antihypertensive medications.
One study followed forty-seven persons with an average starting blood pressure of 171/101. In a period of twelve weeks, one-half were treated with 600 mg of garlic powder daily standardized to 1.3 percent alliin, while the other one-half were given placebo. The results showed a statistically significant drop of 11 percent in the systolic blood pressure and 13 percent in the diastolic pressure. In comparison, blood pressure fell in the placebo group by 5 and 4 percent, respectively. However, this study had a significant problem: The average starting blood pressures of the placebo and the treated groups were quite different, making comparisons unreliable.
Prevention of colds. The herb garlic has a long history of use
for treating or preventing colds. An American study reported in 2001 provides
meaningful preliminary evidence that garlic might possess cold-fighting powers. In
this twelve-week, double-blind, placebo-controlled trial, 146 people received
either placebo or a garlic extract between the months of November and
February.
The results showed that participants receiving garlic were almost two-thirds less likely to catch cold than those receiving placebo. Furthermore, participants who did catch cold recovered about one day faster in the garlic group compared with the placebo group. Thus, the regular use of garlic might help prevent colds. However, there is no evidence that taking garlic at the onset of a cold will help a person recover more quickly.
Insect repellent. A twenty-week, double-blind, placebo-controlled crossover trial followed eighty Swedish soldiers and measured the number of tick bites they received while undergoing the garlic and the placebo treatments. The results showed a modest but statistically significant reduction in tick bites when soldiers consumed 1,200 mg of garlic daily for eight to ten weeks. However, the type of garlic used in this study was not stated. Another study failed to find one-time use of garlic helpful for repelling mosquitoes.
Cancer prevention. Evidence from observational studies suggests that garlic may help prevent cancer, particularly cancer of the stomach and colon. In one of the best of these trials, the Iowa Women’s Study, 41,837 women were questioned as to their lifestyle habits (beginning in 1986) and then followed in subsequent years. At the four-year follow-up, questionnaires showed that women whose diets included significant quantities of garlic were approximately 30 percent less likely to develop colon cancer.
The interpretations of studies like this one are always a bit controversial. For example, it is possible that the women who ate a lot of garlic also made other healthful lifestyle choices. While researchers looked at this possibility carefully and concluded that garlic was a common factor, it is not clear that the researchers are right. What is really needed to settle the question is an intervention trial, in which some people are given garlic and others are given placebo. However, no studies have been performed to evaluate garlic for cancer prevention.
Antimicrobial. There is no question that raw garlic can kill a wide variety of microorganisms, including fungi, bacteria, viruses, and protozoa, by direct contact. A double-blind study reported in 1999 found that a cream made from the garlic constituent ajoene was just as effective for fungal skin infections as the standard drug terbinafine. These findings may explain why garlic was traditionally applied directly to wounds to prevent infection (but it also can burn the skin). Nevertheless, there is no real evidence that taking garlic orally can kill organisms throughout the body. Thus, it is not an antibiotic in the usual sense; it is more of an antiseptic.
Oral garlic could theoretically offer benefits against organisms in the stomach or intestines because it can come into direct contact with them. However, there is only the slightest evidence that it works for any specific infection of this type. For example, despite test-tube evidence that garlic can kill H. pylori, studies in people have not been promising.
Dosage
A typical dosage of garlic is 900 mg daily of a garlic powder extract standardized to contain 1.3 percent alliin, providing about 12,000 micrograms of alliin daily, or 4 to 5 mg of “allicin potential.” Alliin-free aged garlic is taken at a dose of 1 to 7.2 grams daily.
Alliin is a relatively odorless substance found in garlic. When garlic is crushed or cut, an enzyme called allinase is brought in contact with alliin, turning it into allicin. Allicin is responsible for much of the typical odor of garlic. It is very active chemically and probably helps the garlic bulb defend itself from attack by insects and other threats. However, allicin is unstable, and it soon breaks down into a variety of other substances. When garlic is ground up and encapsulated, the effect is similar to cutting the bulb: Alliin contacts allinase, yielding allicin, which then breaks down. Unless something is done to prevent this process, garlic powder will not have any alliin or allicin left by the time it is purchased.
Some garlic producers believe that alliin and allicin are not essential for garlic’s effectiveness and do not worry about this breakdown. Aged garlic, for example, has very little of either compound, but other manufacturers believe that allicin is the primary active ingredient in garlic. Because allicin is an unstable chemical, these manufacturers are faced with a challenge.
One solution might be to chemically stabilize allicin so that it does not break down. However, allicin has a strong garlic smell, and a relatively odorless product is preferable. Many manufacturers of garlic powder products seek to stabilize the alliin in the product, and to do so in such a way that the alliin converts to allicin after it is consumed. How well their methods work remains a matter of controversy.
One should not confuse essential oil of garlic with garlic oils. The term
“garlic oil” refers to garlic extracted by means of oil. Garlic essential oil is
the pure oily component of the herb, and, like other essential
oils, it is potentially toxic.
Safety Issues
As a commonly used food, garlic is on the GRAS (Generally Recognized As Safe)
list of the U.S.
Food and Drug Administration. Test rats have been fed
gigantic doses of aged garlic (2,000 mg per kilogram of body weight) for six
months without any signs of negative effects. Long-term treatment with
standardized garlic powder at a dose equivalent to three times the usual dose,
along with fish oil, produced no toxic effects in rats.
The only common side effect of garlic is unpleasant breath odor. Even “odorless garlic” produces an offensive smell in up to 50 percent of those who use it. Other side effects occur only rarely. For example, a study that followed 1,997 people who were given a normal dose of deodorized garlic daily for sixteen weeks showed a 6 percent incidence of nausea, a 1.3 percent incidence of dizziness on standing (perhaps a sign of low blood pressure), and a 1.1 percent incidence of allergic reactions. There were also a few reports of bloating, headaches, sweating, and dizziness.
When raw garlic is taken in excessive doses, it can cause numerous symptoms, such as stomach upset, heartburn, nausea, vomiting, diarrhea, flatulence, facial flushing, rapid pulse, and insomnia. Topical garlic can cause skin irritation, blistering, and even third-degree burns.
Because garlic might “thin” the blood, it is probably imprudent to take garlic
pills immediately before or after surgery or labor and delivery because of the
risk of excessive bleeding. Similarly, garlic should not be combined with
blood-thinning drugs such as warfarin (Coumadin), heparin,
aspirin, clopidogrel (Plavix), ticlopidine (Ticlid), or pentoxifylline (Trental). In addition, garlic could
conceivably interact with natural products with blood-thinning properties, such as
ginkgo, policosanol, or high-dose vitamin E. However, a placebo-controlled study
found that actual raw garlic consumed in food at the fairly high dose of 4.2 mg
once daily did not impair platelet function. In addition, volunteers who continued
to consume the dietary garlic for one week did not show any changes in their
normal platelet function.
Garlic may also combine poorly with certain medications for human immunodeficiency virus (HIV) infection. Two HIV-positive persons experienced severe gastrointestinal toxicity from the HIV drug ritonavir after taking garlic supplements. Garlic might also reduce the effectiveness of some drugs used for HIV infection. Garlic is presumed to be safe for pregnant women (except just before and immediately after delivery) and nursing mothers, although this has not been proven.
Important Interactions
Persons should not use garlic except on medical advice if also taking blood-thinning drugs. Taking garlic at the same time as ginkgo, policosanol, or high-dose vitamin E might conceivably cause a risk of bleeding problems. Finally, one should not use garlic if also taking medications for HIV infection.
Bibliography
Gardner, C. D., et al. “Effect of Raw Garlic vs Commercial Garlic Supplements on Plasma Lipid Concentrations in Adults with Moderate Hypercholesterolemia.” Archives of Internal Medicine 167 (2007): 346-353.
Khoo, Y. S., and Z. Aziz. “Garlic Supplementation and Serum Cholesterol.” Journal of Clinical Pharmacy and Therapeutics 34 (2009): 133-145.
Ngo, S. N., et al. “Does Garlic Reduce Risk of Colorectal Cancer?” Journal of Nutrition 137 (2007): 2264-2269.
Rajan, T. V., et al. “A Double-Blinded, Placebo-Controlled Trial of Garlic as a Mosquito Repellant.” Medical and Veterinary Entomology 19 (2005): 84-89.
Ried, K., O. R. Frank, and N. P. Stocks. “Aged Garlic Extract Lowers Blood Pressure in Patients with Treated but Uncontrolled Hypertension.” Maturitas 67 (2010): 144-150.
Sabitha, P., et al. “Efficacy of Garlic Paste in Oral Candidiasis.” Tropical Doctor 35 (2005): 99-100.
Sobenin, I. A., et al. “Metabolic Effects of Time-Released Garlic Powder Tablets in Type 2 Diabetes Mellitus.” Acta Diabetologica 45 (2008): 1-6.
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