E ctomorph . The Ectomorph generally has a very high metabolic rate making it difficult for then to gain both muscle and fat. The Ectomorph is a naturally lean individual. Probably most professional body builders are mesomorphs. Endomorphs are people who tend to put on fat.
Endomorphs tend to have a lower metabolic rate.
Plus adding muscle through anaerobic exercise will increase the basal metabolic rate, muscle burns more calories at rest than fat. Simplistically these are the differences between the three basic genetic metabolic types. But ultimately metabolism is infinitely variable. Ultimately the Ectomorph has little Mesomorph in him or her. The Endomorph also has a little Mesomorph in him or her.