Nitrogen balance is a measure of nitrogen consumption minus nitrogen excretion. Lifting weights puts your body into a catabolic state–that is, it breaks down muscle tissue for later rebuild, thus excreting nitrogen. Protein is the only macronutrient that has nitrogen as a constituent inside its chemical make-up. As such, the fundamental way to achieve a state of positive nitrogen balance is to have nitrogen consumption exceed nitrogen excretion, i.e. increase protein intake.