If you are luckily a mesomorph than you will be delighted to know that you have the easiest road out of the rest to put on lean mass. You can follow the simple standards of a 40/40/20 macro nutrient ratio. 40% Carbohydrates, 40% Protein, and 20% Fat to make up your caloric intake for the day. Just like eating as an endomorph you want to stick to complex carbohydrates (whole grains mainly), lean protein sources (fish, chicken breast, eggs, and some beef), while fats you want to target things mainly high in omega 3 fatty acids such as almonds, natural peanut butter, fish oil, or flax seed.
While gaining mass as a mesomorph you want to find your daily caloric level to maintain and increase this number by 500, as you climb the scale every 3-4 pounds you want to readjust you caloric intake for your new body weight. Try not to exceed 500 calories a day or that body weight you are gaining wont look so lean. Train hard and you will get the results you want! My next post will be about ectomorph eating then I will teach you guys how to train for each body type :). Happy training all!
To find you daily maintenance calorie level: http://www.bodybuilding.com/fun/macronutcal.htm
A good website to calculate your caloric intake with what you are eating: http://www.myfitnesspal.com and it’s free to use!