Muscle gain during caloric restriction is affected by a number of variables. Anabolic hormones include IGF-1 and others that help with muscle growth. Anabolic hormones may not be as effective when the body is in a caloric deficit. Because of this, muscle protein synthesis does not increase as much as would be optimal for muscle development. Calorie restriction triggers a reaction in these hormones. The loss of hormone function is proportional to the magnitude of the calorie deficit. Which makes it more challenging to gain muscle.