Gain

If you truly want to dramatically change your body, to learn how to build muscle mass fast, understand that it will be the power of all these factors working together that will propel you towards your goals - a precise combination of nutrition, exercise and rest will provide you maximum results.
 
So why is protein important in weight gain?
From a weight gain perspective protein is made up of amino acids. There are 20 in total and 8 essential amino acids must come from food. Weight training increases the demand for amino acids and will break existing muscle down if it does not get enough from a weight gain diet. Without adequate protein, and more specifically, amino acids muscle gain is unachievable.
 
Carbohydrate
Any weight gain diet worth its salt will contain plenty of unrefined carbohydrates. Just because you’re increasing your protein intake does not mean you should omit or even limit your carbohydrate intake.
Carbohydrate, which is converted into glucose and glycogen in the body, is the only macronutrient that can supply your body with an immediate source of energy – essential for any type of training.
Good sources of carbohydrate for a weight gain diet include whole meal bread, potatoes, brown rice, pasta, couscous, fresh and tinned fruit and dried fruit.
 
Fat
Certain dietary fats are crucial to both your well being and your ability to gain weight. One gram of fat contains more than twice the number of calories than 1g or protein or 1g of carbohydrate. A tablespoon of Flaxseed Oil contains as many calories as a banana for example so it makes sense to incorporate good fats into your weight gain diet.
the fats you want to consume are monounsaturated fats found in olive oil and avocados, and polyunsaturated fats found in oily fish, flax, sunflower, safflower and cod liver oil and some raw nuts.
Essential Fatty Acids also known as Omega-3 and Omega-6 fatty acids are found in polyunsaturated fats, particularly oily fish. As well as having a numerous health benefits they also play an important role in muscle building.
In short, a weight gain diet containing fish like mackerel, tuna and salmon (to name a few) or supplemented with a product like Flaxseed oil will not only help you build muscle but will keep you alive longer too.
 
Meal Frequency
Finally, forget about eating 3 large meals a day with a few snacks. The best approach to an effective weight gain diet is to eat 5 or 6 small meals a day. Separate them by 3 hours so your stomach has time to digest each meal fully.
Try eat a variety of fruits and vegetables. They are the richest source of vitamins and minerals (particularly antioxidants) and have both health and weight gain implications.