Why do pimples apear on ones back from eating lots of protein? why is 2g per 1kg of body weight recommended for optimal muscle gains in bodybuilding?

Possibly dairy: Muscles are mainly protein, so high-protein diets are often recommended when trying to gain muscle mass. It has been shown that milk intake is strongly correlated with acne. Whey protein is a milk derivative, so bodybuilders are getting lots of milk protein. This, plus the sweating from exercise and possibly hormones related to muscle-building may all contribute to acne flares.