MASS GAINERS provides protein, carbohydrates and possibly fats with the intention of helping to add muscle mass. Mass gainers also include L-glutamine, an amino acid that aids in muscle growth and muscle recovery. Glutamine increases leucine which helps lower muscle fatigue and increases growth hormone levels.
It is important to remember that to increase muscle growth when taking a Mass gaining supplement you must be training hard and regularly, or else you'll be consuming a high number of calories which can lead to an increase in body fat.