Mesomorphs tend to be in the middle when it comes to metabolic rate. While ectomorphs struggle to gain weight and endomorphs have trouble gaining too much weight (specifically fat: muscle ratio), mesomorphs seem to have the best of both worlds; they can gain muscle with limited fat gains.
Mesomorphs naturally have more muscle than ectomorphs and have better insulin sensitivity than endomorphs, which makes it easier for them to gain more muscle and less fat when eating a hypercaloric diet.
The primary factor that determines whether you gain or lose weight is your caloric intake (how many calories you eat)
. Mesomorphs trying to gain muscle need to eat a hypercaloric diet like anyone else who wants to gain weight, but they do not want to eat too many calories above maintenance.
 Limiting the bulk of your carbohydrates to times when your body needs them most will aid in muscle growth and limit fat gains. I recommend mesomorphs to limit the majority of their carbohydrates to breakfast and pre/post workout. For example, if someone workouts at 6 PM I would recommend they consume the majority carbohydrates at:
 Breakfast: Pre-workout: For all other meals mesomorphs should consume less carbs
Some mesomorphs may do very well using high glycemic carbs, but our general rule is for mesomorphs to stick to low glycemic carbs.
Skeletal muscle has the ability to uptake glucose post workout without "spiking" your insulin levels. Weight training increases GLUT-4 translocation in skeletal muscle, allowing the muscle to uptake glucose more efficiently.
