"I remember when I used to read Flex magazines and they would offer the most unhealthiest diets to getting big. Telling people to eat tons of peanut butter, red meat, loads of milk, and a lot of starchy carbs, etc."
Pretty much the eating regiment of how we add muscle mass to under weight newer high school and/or college level football players. 12-15lbs (in 4 weeks on average) of confirmed increases of muscle mass, some with even inches lost around the waist. Confirmed by the medical staff. Strength advancement can improve greatly......just saying.
Also shakes based on whole milk can include raw eggs, banana's, peanut oil/olive oil, whey powder, etc. The property's and values of whole milk have of late induced extended study.
BB'ers may need to follow different avenues of eating as far as muscle gain may go....their choice. The above examples are for active and competing athletes and other sports minded individuals in others sports.
Good Luck.