Optimum Nutrition Serious Mass Vanilla 2.72kgSERIOUS MASS FOR SERIOUS MUSCLE. To gain weight, you need to consume more calories than you expend. For many aspiring to hit their calorie goals and put on size, a highly active metabolism, weak appetite or on the go lifestyle can make it a real challenge to get enough calories from food alone. SERIOUS MASS is the ultimate muscle building and weight gain formula. With 1,250 calories per 2 scoop serving and 50 grams of protein to support muscle