Muscle mass can be gained through moderate to intense strength training several times each week, coupled with taking in extra calories.
For each pound gained as muscle in a week, you will need to consume about 500 extra calories each day. The extra calories should come from a variety of foods: milk, meat, fruits, vegetables, and grains.
The key is to be consistent. Eating three meals a day with snacks in between is an essential part of gaining lean body mass. If you sleep in and skip breakfast, you miss a chance to add extra calories to your diet.
Eat enough to satisfy your appetite and then try to eat a little more. This can be done by:
-eating larger than normal portions
-eating an extra snack or meal
-drinking commercial liquid meals or milkshakes with regular meals or as snacks.
Some good snacks if you are trying to gain weight are:
-peanut butter sandwich
-low-fat milkshake (with skim milk and low-fat ice cream)
-dried fruit
-cottage cheese
-pasta with sauce.
Commercial protein supplements will not help you gain weight and will probably add too much protein to your diet. If you need a liquid supplement, make sure it provides the extra calories you need as carbohydrates, not protein.
-http://www.med.umich.edu/1libr/sma/sma_weigain_sma.htm
