GNC Pro performance L-Carnitine 500mg 60tablets. Fuels Nitric Oxide & Muscle Recovery. Helps Use Fatty Acids as Energy. L-Carnitine is a popular amino acid among bodybuilders, weight trainers and serious athletes because of its role in facilitating the metabolism of long chain fatty acids. Pro Performance L-Carnitine 500 provides 500mg of this important nutrient per pill.