Several studies from around the world have found that people who eat more garlic seem to have a lower risk of certain types of cancer. In particular, large human studies that looked at diet and cancer have suggested that people who eat more garlic have a lower risk of stomach, prostate, mouth and throat, kidney, and colorectal cancer. The effect on risk of breast, bladder, ovarian, and lung cancers is less clear. As always in population-based studies, it is possible that other factors may account for the differences in cancer risk. The few human studies that have looked at garlic supplements have not found them to be helpful against cancer.
Many laboratory studies done in cell cultures and animals suggest garlic may help reduce tumor growth. Cell culture studies have shown garlic can help cancer cells die off normally, a process called apoptosis. Other studies in cell cultures have found that substances in garlic seem to be able to act as antioxidants. Some studies have also suggested that garlic can act against Helicobacter pylori, a bacterium thought to be a major cause of stomach cancer. Studies in laboratory animals have found garlic may help protect against cancer of the colon, skin, liver, and breast, among others.
Although results of some observational studies are encouraging, randomized clinical trials in which people assigned by researchers to receive either garlic or an inactive control substance provide more reliable information. Very few studies of this type have studied garlic and cancer risk. In one recent study conducted in China, where stomach cancer is quite common, aged garlic extract and steam-distilled garlic oil did not prevent this disease.
While some research on garlic is promising, it is very hard to determine the exact role a particular food may have against cancer. It is even more difficult when the food in question is often used in small amounts, as is garlic. A balanced diet that includes 5 or more servings a day of fruits and vegetables along with foods from a variety of other plant sources such as nuts, seeds, whole grain cereals, and beans is likely to be more effective than eating one particular food in large amounts.
Some studies suggest that garlic can lower blood cholesterol levels, although a recent clinical study funded by the National Center of Complementary and Alternative Medicine did not confirm any effect. This California study compared raw garlic with aged garlic extract, powdered garlic, and a placebo in nearly 200 randomly assigned volunteers. The garlic was given in doses of 4 grams per day over 6 months. At the end of the study, there was no significant difference in LDL ("bad") cholesterol among the 4 groups. Other studies suggest that garlic makes blood less likely to form clots, which might help prevent heart disease and stroke. However, there is no reliable direct clinical evidence that garlic can actually prevent heart attacks or strokes. Evidence on garlic and blood pressure is mixed.
While some research on garlic is promising, it is very hard to determine the exact role a particular food may have against cancer. It is even more difficult when the food in question is often used in small amounts, as is garlic. A balanced diet that includes five or more servings a day of fruits and vegetables along with foods from a variety of other plant sources such as nuts, seeds, whole grain cereals, and beans is likely to be more healthful than eating one particular food in large amounts.