From The March 2000 Issue of Nutrition Science News
by Carmia Borek, Ph.D.
Can a clove a day keep the doctor away? Can garlic supplements do the same job? The November 1998 conference in Newport Beach, Calif., on garlic and garlic supplements, sponsored by the Bethesda, Md.-based National Cancer Institute and Pennsylvania State University in University Park, showed that garlic supplements may prevent heart disease and cancer, help memory and extend life.  Garlic does not have to be eaten raw or fresh to be effective. The characteristic odor of garlic is also not needed for health benefits. For example, aged, deodorized garlic, taken as a supplement, may work even better than fresh garlic in protecting against cell damage that leads to disease and aging. 
Garlic contains many substances that studies show act together to prevent disease and age-related conditions. Sulfur-containing compounds are responsible for most of the health benefits of garlic and its preparations. [2, 3] Nonsulfur compounds in garlic include proteins, carbohydrates, saponins, flavonoids (notably allixin) and selenium. 
Whole garlic cloves are relatively odorless and contain alliin, a sulfur-containing amino acid derivative also known as S-allyl-cysteine sulfoxide. Alliin is converted to allicin with the help of the enzyme alliinase, which is activated when garlic is cut or crushed. Allicin, a volatile compound also known as allyl-2-propenethiosulfinate, gives garlic its pungent smell. Allicin is a reactive oxidant with antibiotic activity. In fact, chopped garlic, which releases allicin, was used to treat wounds before the discovery of antibiotics. In vivo, however, allicin is not bioavailable and is virtually undetected in the blood after eating garlic or garlic preparations.  Ultimately, allicin decomposes into oil-soluble organosulfur compounds to bring other health benefits. 
The 4 Forms
Research supports the recommendation to include a daily garlic supplement. There are four basic choices in garlic products.
Fresh garlic is an excellent source of garlic compounds, but can cause indigestionnot to mention pungent breathwhen eaten in amounts large enough to have therapeutic effects. Fresh garlic's benefits have been confirmed for various cardiovascular risk factors, specifically its ability to reduce blood cholesterol levels, inhibit aggregation of blood platelets and break down clots. However, all these effects were noted in a review only at fairly high dosage levels. 
Garlic oil is produced by steam distilling crushed raw garlic. It is diluted about 200-fold with vegetable oils to reduce the powerful odor of the volatile oil-soluble sulfides. Garlic oil contains a fraction of the original oil-soluble sulfides and no water-soluble sulfur compounds. Because of high-heat distillation, garlic oil loses nonvolatile nutrients.
Garlic powder is made from chopped cloves that are oven-dried then pulverized. The allicin yield, also called allicin potential, of a garlic powder tablet depends on the alliinase activity. If the preparation was dried at a high temperatureabove 158 F (70 C)the enzyme is inactive and the remaining molecular alliin in the unchopped part of the clove is unconverted. To avoid alliinase inactivation in the stomach, capsules should be enterically coated with cellulose esters so they are de-coated only by intestinal enzymes.
Aged garlic extract is produced by extracting and aging garlic in diluted alcohol without heat for up to 20 months. The aging process, which results in an odor-free preparation, retains garlic nutrients, converts the unstable allicin to beneficial stable compounds, and produces water-soluble S-allylcysteine and S-allylmercaptocysteine. Garlic chemistry is complex, and a number of other compounds are also produced by the aging process. Aged garlic extract supplements are standardized by levels of the stable S-allylcysteine. Commercially available aged garlic extract has a higher content of the S-allyl compounds and antioxidant activity compared to fresh garlic and many other commercial preparations. 
Garlic is rich in antioxidant phytochemicals that include organosulfur compounds as well as flavonoids such as allixin, which are capable of scavenging free radicals.  Garlic also contains selenium, required for optimal function of the antioxidant enzyme glutathione peroxidase.  Although the mechanisms of all garlic's components are not known, many of its cardiovascular-protective effects as well as its anti-inflammatory, anti-cancer and anti-aging effects are due to its antioxidant actions. [2, 4, 9, 10]
Research shows garlic can lower cholesterol and other blood lipids, prevent lipid oxidation, protect blood vessels, and prevent blood platelet "stickiness." Reducing blood cholesterol and triglyceridesthe form in which fat is transported in the bloodhas been the focus of much research on garlic supplementation during the past 15 years. Some studies have produced conflicting results due to differences in the kind of garlic preparation, quality of standardization, doses and treatment periods.
Findings using aged garlic extract show it lowers blood cholesterol, low-density lipoproteins (LDL, or "bad" cholesterol) and triglycerides. In a double-blind study conducted at Memorial Hospital of Rhode Island in Pawtucket, aged garlic extract given at 2.44.8 g a day for six months on average lowered total serum cholesterol levels by 5 to 7 percent in 41 men with moderately elevated cholesterol levels. It also reduced triglycerides, reduced LDL cholesterol by 4 percent, and reduced systolic blood pressure by 5.5 percent. 
In a meta-analysis of five studies in which 75 percent of the patients had cholesterol levels of 200 mg/dL, one-half to one fresh garlic clove a day decreased total serum cholesterol by 9 percent in the groups of patients tested. 
A German study reported that patients with high cholesterol who were given garlic powder tablets for four months showed mean values of 12 percent cholesterol reduction and 17 percent triglyceride reduction. However, a garlic smell was noted by 21 percent of patients. 
Protecting LDL cholesterol from free radical-induced oxidation is one way to stave off atherosclerosis. Oxidized LDL injures cells that line blood vessels, which increases plaque-forming cholesterol deposits in the vessel wall. Aged garlic extract, its components S-allylcysteine and the flavonoid allixin have been shown to protect LDL cholesterol from oxidation and prevent cell injury in the blood vessels.  Garlic's oil-soluble organosulfur components also show an ability to protect LDL from oxidation. 
The anti-clotting effects of garlic reduce plaque formation in blood vessels. In this way it helps protect against heart disease and stroke. Garlic prevents blood platelets from clumping (aggregation) and sticking to blood vessels (adhesion). Allicin produces this effect in vitro during its brief and transient presence.  However, in vivo, other bioavailable compounds produce the anti-platelet effect because allicin does not adequately get into blood circulation. 
Beyond the Blood
Cancer prevention is another indication for garlic. A study of a Chinese population with a high incidence of stomach cancer revealed that those in the highest quarter of garlic consumption had 40 percent less cancer risk than those in the lowest quarter. 
Animal and cell studies show that aged garlic extract and its organosulfur components act as anti-cancer agents by virtue of their antioxidant effects that prevent cancer-causing DNA damage.  These components also block the action of carcinogens, preventing colon, esophagus, lung, breast, prostate and stomach cancers. [2, 16-18]
Brain function improvement has been shown in preliminary animal studies. Experiments in mice that are genetically susceptible to early aging showed that aged garlic extract supplements increased the mice's life span and enhanced learning ability and memory.  The garlic compounds also protected the brain against free radicals thought to contribute to human Alzheimer's disease.  These experiments suggest that aged garlic extract supplements may help prevent physiological aging and may protect against age-related cognitive disorders in humans. Nerve cells exposed to these compounds showed an unusual ability to grow and branch, which may be associated with the enhanced memory function. No similar studies have been conducted with other garlic forms.
Ancient Egyptians swore to their gods in the presence of garlic. Science confirms reasons for their trust. But we now know that well-standardized garlic supplementsespecially those containing stable sulfur compounds such as the water-soluble compoundsmay serve better than fresh cloves to prevent disease, delay aging and help maintain health.
Commission E Monograph: Garlic
Carmia Borek, Ph.D., is a research professor at Tufts University School of Medicine in Boston. She is author of "Maximize Your Healthspan with Antioxidants: The Baby Boomer's Guide" (Keats Publishing, 1995).
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