Mass Gainers - The main reason any person is looking for weight or mass gainer is that "they're having trouble eating enough to actually gain pounds." Adding a weight gainer can help you hit caloric requirements that might otherwise seem out of reach.
A mass gainer 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.
Mass gainer could be taken in the morning to fuel you for the day and instantly boost your calorie intake, so you don’t have to constantly worry about getting enough calories throughout the day. You can also take it after you've expended a high number of calories from your work out.
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.