Standardized Horse Chestnut Seed Extract Supports Healthy Blood Vessels Dietary Supplement Horse chestnut extract supports a healthy fluid balance plus antioxidant, venotonic (helpful to veins), and vasoprotective (protective of vessels or ducts) actions. The key active ingredient in horse chestnut is a substance called escin. Venotone is an extract of horse chestnut, which has been standardized to ensure the highest quality, consistency and biological activity.