Grape seed extract is rich in plant flavonoids, substances that are considered to be powerful antioxidants. Antioxidants prevent the breakdown of substances in the body that can release "free radicals". These highly energetic compounds have been implicated in deleterious conditions such as coronary artery disease, certain forms of arthritis, cancer, and aging.
Grape seed extract may also increase levels of vitamin C (another antioxidant) within cells. They are also thought to decrease capillary fragility and may also inhibit or slow the destruction of collagen molecules that are essential for maintaining the integrity of bone and soft tissue. In some animal studies, grape seed extract appeared to help reduce the amount of cholesterol in the blood and actually shrink cholesterol deposits in arteries. (Biochem Pharmacol 1984; 33: 491).
However, a recent study reported that high doses of antioxidant vitamins did not lower blood cholesterol or prevents the growth of plaque in people with coronary artery disease (N Engl J Med, 2001; 345:1583). Antioxidant vitamins actually interfered with the beneficial effects of certain cholesterol lowering drugs on HDL ("good") cholesterol. No large-scale controlled studies have been performed to assess the effects of grape seed extract in people with heart disease.