The antioxidant quercetin - naturally found in apples, capers and citrus fruits - had previously been shown to reduce oxidative stress in muscles after heavy exertion, and now a recent study published in the Journal of the International Society of Sports Nutrition has shown it has the ability to improve muscle growth. The study found that a combination of quercetin and whey protein, compared to whey protein alone, led to greater increases in muscle mass. The researchers suggest quercetin’s antioxidant properties help boost endurance and reduce fatigue.