Packing on muscle can be done in two steps: lifting weights and eating more. When you workout, you cause tiny tears in your muscles that are later repaired which is what makes you bigger, faster and stronger. When your body is repairing, it's in an anabolic state. To gain the most weight, you need to keep your body from going in the opposite direction, what's known as a catabolic state - when your body is breaking tissue down. The longer you can keep your body in an anabolic state, the more muscle you'll build and the quicker you'll gain weight.
Why would you go into a catabolic state?
The two main reasons that your body would fall into a catabolic state are: extended periods of activity (exercise) and fasting. When you workout for a long time, your body starts to use up its preferred sources of energy: glycogen and glucose. When this happens, it can start to breakdown tissue in order to burn the protein found in the muscle as energy. A similar process happens when you go without food for a long time.
Eating breakfast is very important if you're trying to gain weight. You need protein to keep those muscle repairs going and carbohydrates for energy. Breakfast is particularly important because you haven't eaten since the night before which could have been more than 8 hours ago. Think about how long that is when your body is used to getting food every 2-4 hours. You might think your body doesn't use a lot of energy during sleep, however this is when most of the repairing and rebuilding of damaged muscle tissue takes place. By the time you wake up, your body needs a fresh supply of nutrients.
Avoiding a Catabolic State During Exercise
Working out, as counterintuitive as it may seem, actually puts your body into a catabolic state. You can lessen the impact of this by using proper pre, during and post workout nutrition. Eating before a workout will give your body the energy it needs to exercise. Just as you wouldn't go on a road trip without filling up your gas tank, you shouldn't workout on an empty stomach. Doing so will increase the chances that your body will breakdown muscle tissue for energy.
Long workouts can also put you into a catabolic state. The longer you workout, the higher your energy needs are which can force your body to start breaking down muscle tissue.
If your workout lasts longer than an hour consider drinking a sports drink such as Powerade or Gatorade to keep your body out of a catabolic state. The quick digesting sugars will give you an extra supply of energy so your body won't need to breakdown muscle tissue to convert into sugar.
Carbs Can Help Keep Away Catabolic States
Your body stores carbohydrate energy in two main ways: as blood glucose and as glycogen which is kept in the muscle and liver. When you're done working out, these stores are low and your body's immediate goal is to return those levels back to normal. Post workout nutrition should consist of a food that is digested quickly so that your body can move onto its next goal of repairing damaged muscle tissue. Protein shakes are one of the best options because they will get digested a lot quicker than any whole foods you eat.
By consuming simple carbs (sugar), you'll give your body a quick shot of energy that it can use to refill its energy supplies. If you don't take in any carbohydrates, your body will begin taking apart muscle to convert into sugar which will then be used to return glucose levels back to normal. If you're trying to gain weight, this would be very counterproductive. You also need to consume protein after a workout. This will help your body with its second goal of beginning to repair muscle tissue that was damaged during the workout.
Post Workout Nutrition
Muscle repair doesn't stop after you drink a protein shake. It continues all day and night. It's very important that you get on a normal eating schedule throughout the day to keep your body from going into a catabolic state. Rebuilding muscle tissue is a very energy intensive process. If you aren't providing your body with the nutrients it needs (fats, carbs and protein) throughout the day, you won't build a lot of muscle. Eating smaller more frequent meals will allow your body to better utilize the calories rather than storing them as fat.
The Bottom Line
To build any sort of structure, you need a form of energy and building materials. Your body works in the same way. If you want to gain weight in the form of muscle, you need to stimulate your muscles through strength training and give it enough nutrients (both energy and protein) to grow.