Optimum Nutrition Serious Mass 12 lb Chocolate is a premium weight gain formula for those who are in need of those extra calories but are having a hard time consuming enough of them. With 1263 calories per serving and 50 grams of protein for muscle recovery, this Serious Mass is highly recommended for weight gain. With 1,263 calories and 50 grams of protein per serving for muscle recovery support, this weight gain formula from Optimum Nutrition makes it the ideal post-workout and between meals shake to aid you achieve your goals. ON Serious Mass help you in developing the physique you have always wanted.