Quercetin is very effective in treating all types of inflammation, including arthritis. Like most flavonoids, it acts as an anticoagulant, preventing blood clotting. As an antioxidant, it protects LDL cholesterol from oxidation and prevents the development of atherosclerotic plaque. These two effects help reduce the risk of heart attack or stroke. Quercetin is thought to protect against the flu virus, and at least three studies have shown that it increases endurance during exercise.