Garlic is one of the most exhaustively researched herbs-the National Library of Medicine's website lists more than 3,700 studies addressing garlic's effect on everything from elevated cholesterol and various types of cancer to fungal infections.
So why is there still so much confusion about the health benefits of garlic?
Even though garlic has been used medicinally by some cultures for thousands of years, much of the contemporary research on garlic is mixed-some studies show that it has positive effects, while others indicate no significant benefits.
Here's what the research shows...
Over the years, scientists have investigated garlic's ability to reduce cholesterol levels and blood pressure and act as an anticlotting agent to prevent blood platelets from being too sticky-a main cause of heart attack.
Key scientific finding: A recent meta-analysis in China looked at 26 randomized, double-blind, placebo-controlled trials-the "gold standard" in scientific research. In that meta-analysis, researchers concluded that garlic reduces total cholesterol by 5.4% and triglyceride levels by 6.5% compared with a placebo. Garlic powder and aged garlic extract were found to be the most effective at lowering total cholesterol, while garlic oil had a greater effect on lowering triglyceride levels.
When it comes to high blood pressure, some credible research shows that garlic can help lower it.
Important scientific findings: Two meta-analyses showed that garlic reduced systolic (top number) blood pressure by 8 mmHg to 16 mmHg and diastolic (bottom number) blood pressure by 7 mmHg to 9 mmHg in people with high blood pressure.
As for garlic's antiplatelet effect-that is, its ability to make blood less sticky and therefore less prone to clotting-a meta-analysis of 10 trials showed a modest, but significant, decrease in platelet clumping with garlic treatment when compared with placebos in most of the studies.
Bottom line: Garlic does help reduce risk for cardiovascular disease, with positive effects on both total cholesterol and blood pressure. It also has enough of an effect on clotting that I recommend patients discontinue garlic supplements seven to 10 days before surgery because it may prolong bleeding.
My advice: If you have a personal or family history of heart disease, ask your doctor about using garlic (in food or supplements) as part of a heart-healthy lifestyle. Be sure to consult your doctor first if you take a blood pressure or statin drug.
Large population studies have shown that people who live in countries where a lot of garlic is eaten-as well as onions and chives are at lower risk for certain cancers.
Key scientific findings: In China, high intake of garlic and other alliums, including onions, was associated with a reduced risk for esophageal and stomach cancers. Specifically, the study found that people who ate alliums at least once a week had lower incidence of both forms of cancer than people who ate these foods less than once a month.
Meanwhile, the European Prospective Investigation into Cancer and Nutrition, which involves 10 different countries, found that higher intakes of garlic and onions lowered the risk for intestinal cancer.
My advice: If you are concerned about cancer-especially if you have a family history or other risk factors for stomach or esophageal cancer-include one to two cloves of garlic in your diet each day.
Historically, garlic has received attention as a potent antibacterial agent. In 1858, Louis Pasteur touted garlic as an antibiotic. Garlic was later used in World War I and World War II as an antiseptic to prevent gangrene.
Bottom line: There have been few contemporary studies looking at the use of garlic to treat infections. However, preliminary research suggests that it may reduce the frequency and duration of colds when taken for prevention and may speed the healing of a fungal infection or wart.
My advice: For most people, garlic is worth trying as a preventive/treatment for these infections (see options described below).
Should you use garlic?
It's wise to make garlic part of a healthful diet that includes plenty of fruits, vegetables, whole grains and fiber.
Caution: Consuming large quantities of garlic-either in the diet or as a supplement-may cause body odor and/or bad breath. Chewing a sprig of fresh green parsley, mint or cardamom can work as a breath freshener. Hot tea also can help by rinsing away garlic oil still in your mouth. Drinking a glass of milk-full-fat or fat-free-may be effective as well. Garlic, especially on an empty stomach, can cause gastrointestinal upset and flatulence
Because garlic may also interact with certain prescription drugs, such as warfarin (Coumadin), consult your doctor before significantly increasing your intake of the herb if you take any medication or have a chronic medical condition.
Options to consider...
• Raw garlic. If you prefer raw garlic, try eating one or two cloves a day. You can chew and swallow it or use it in pesto, guacamole or a salad dressing. Cooked garlic is less powerful medicinally-heat inactivates the enzyme that breaks down alliin, the chemical precursor to allicin.
• Aged garlic extract (AGE). If you prefer liquid, AGE is available in this form, which is popular in Europe. Follow label instructions.
• Powdered garlic supplements. These are typically sold as capsules or tablets and standardized to contain 1.3% alliin. They usually contain 300 mg.
Typical dose: Two or three capsules or tablets a day.
Facts about garlic
Garlic has been used since ancient times as a medicinal remedy. It is a member of the allium family of plants, which also includes onions, chives and leeks. Garlic's medicinal powers are attributed to its sulfur compounds, including a substance called allicin, which is formed when garlic is crushed or chopped. Besides fresh cloves, garlic is available in supplement form made from fresh, dried or freeze-dried garlic, garlic oil and aged garlic extracts.