The active ingredient in garlic, allicin, produces hydrogen sulfide when metabolized. This helps cardiovascular health by relaxing blood vessels and increasing blood flow, according to research appearing in the Proceedings of the National Academy of Sciences (published online on October 19, 2007, 10.1073/pnas.0705710104).
The researchers added a small amount of red blood cells to garlic juice. Allicin is unstable in liquid and breaks down to form various sulfides (diallyl sulfide, diallyl disulfide, diallyl trisulfide, and ajoene). When exposed to theses chemicals, the red blood cells release hydrogen sulfide. Muscular rings from the aortas of rats were exposed to the hydrogen sulfide-emitting red blood cells—it caused them to relax. The garlic caused the vessel relaxation in a dose-dependant manner. The greater the concentration of garlic, the more relaxation in the blood vessel.