There is nothing like the feeling of filling out your T-shirt with newly added muscle. Mass gainers provide you with a little bit of everything that you need to effectively gain mass. These supplements are high in protein, carbohydrates, healthy fats and most importantly, calories. Many weight gainers contain close to 1,000 calories per serving. As with whey protein, mass gainers in powder form can be easily mixed with liquids to form an instant meal replacement shake.