Herbs & Supplements

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Note: Much of garlic's reputation is based on observation and tradition. While numerous animal and human studies have been carried out to test garlic's effectiveness for a wide range of health conditions, many of the studies have been small and short. Not all of the studies used the same or even comparable garlic products or similar dosing schedules. Therefore, comparing the study results is difficult and many of them are unreliable. However, taking garlic appears to have few or no serious side effects and it may be a useful addition to conventional treatments for certain conditions.

In the United States, oral garlic supplements are sold mainly to reduce the risk of heart disease. They have been studied most to treat high cholesterol. Results of numerous animal and human studies show that garlic does seem to be better than placebo (sugar pills with no medical effectiveness) for lowering high blood levels of total cholesterol and triglycerides. Garlic does not lower blood cholesterol as much as dietary changes plus prescription drugs known as HMG-CoA reductase inhibitors (HMGs). Also called statins, HMGs include Crestor, Lescol, Lipitor, lovastatin, pravastatin, and simvastatin. In general, these medications in combination with low-fat diets may produce a 28% to 60% reduction in total cholesterol. Garlic may be responsible for cholesterol reductions in the 6% to 12% range. Additionally, several studies appear to show that garlic's cholesterol-lowering effects may be only short-term. Lower cholesterol--generally seen after one month of treatment with garlic--may return to near pre-treatment levels after 6 months to 12 months of garlic therapy. One study of 30 children with an inherited type of high cholesterol known as familial hypercholesterolemia found no effect from garlic on their cholesterol levels.

While garlic also appears to have a small effect on lowering high blood pressure, overall results from a number of studies were generally not considered to be significant. On average, individuals who participated in several small clinical trials of a powdered garlic product had blood pressure reductions of 5% to 7% more than other participants who took an inactive placebo. It is believed that garlic may affect blood pressure by relaxing blood vessels, causing them to widen and allowing blood to flow more easily.

In addition, the thickness of the blood may be reduced by taking garlic, which may block the body's production of thromboxane, a chemical involved in blood clotting and blood vessel tightening. Garlic has also been proved to reduce the stickiness of platelets. Platelets are blood components that are partly responsible for forming atherosclerotic plaques--the cholesterol-associated deposits that can block arteries. In several studies conducted in laboratory animals, garlic appeared to stop or delay the formation of atherosclerotic plaques. Two separate human studies that each lasted for four years, also showed that existing plaques could be reduced in size by garlic supplementation.

Other protective effects may result from garlic's ability to lower the levels of homocysteine in the blood. Homocysteine is an amino acid (molecules that combine to form proteins) that is produced from the break down of another amino acid in the body. High levels of homocysteine raise the risk of having heart disease and other conditions. Because it irritates the inside of blood vessels, homocysteine causes artery damage, which may lead to blood clots. It may also be a factor in the complications (eye, kidney, and nerve damage) of diabetes. Additionally, high homocysteine levels may contribute to dementia. In general, dementia is an increasing deficiency in thought processes caused by brain damage or illnesses, such as Alzheimer?s disease. In animal studies and a few small studies in humans, supplemental garlic has shown a general reducing effect on homocysteine levels.

Additional animal studies have shown that either oral or injected garlic can lower blood sugar levels in some animals that have diabetes. Although not all types of supplemental garlic showed an effect, raw garlic has also reduced blood sugar levels even in laboratory rats that did not have diabetes. In some of the studies, animals receiving garlic increased the release of insulin and used insulin more efficiently. In one study, garlic supplementation prevented weight gain in laboratory rats with diabetes. However, one study of humans found that garlic was not any better than an inactive placebo for controlling type 2 diabetes. Although garlic has been used in Asia and other parts of the world to treat diabetes, much more research is needed to prove or disprove its effects on blood sugar levels.

Some garlic preparations have also been promoted for boosting immune function. Garlic has been shown in laboratory research to increase blood levels of at least two enzymes that have antioxidant properties. Antioxidants are thought to protect body cells from damage caused by a chemical process called oxidation. In one study, seven people living with AIDS showed improvements in natural immune function after 12 weeks of garlic supplementation. No further human studies have been published to support these results, although some animal research seems to confirm them.

Extensive research, conducted mostly in Asia, has associated high intakes of raw and/or cooked garlic and similar plant foods with a lower incidence of colorectal, esophageal, and stomach cancers. One study showed reductions in both the size and the number of pre-cancerous colon polyps (small growths along the inside of the intestines) among individuals who took aged garlic extract for one year. One five-year observational study that followed more than 40,000 American women between the ages of 55 and 69, also found that the women who used a lot of garlic in food preparation were less likely to have colon cancer than the women who did not cook with garlic. However, a study conducted over a three-year period in the Netherlands, did not show a reduction in breast, colon, lung, or rectal cancers among individuals who consumed garlic as compared with those whose diets did not include garlic. Much more research is needed to determine whether garlic can protect against prostate cancer.

Garlic has long been known to be anti-infective. Laboratory studies show that it kills or damages a wide range of microorganisms such as bacteria, fungi, protozoans, and viruses. The blood levels needed to control or kill most internal infective agents may be too high to achieve by taking garlic orally. It may be effective, though, for controlling infective agents on the skin's surface and in the mouth. Several studies carried out in humans show that a compound extracted from garlic is effective and safe for application on fungal infections such as athlete's foot and ringworm. Both laboratory and human studies have shown that chewing garlic or applying a garlic preparation kills or controls a number of different microorganisms that live in the mouth.