What Is Whey Protein Isolate? Whey protein isolate is protein isolated from the left overs from the process of making cheese. Whey contains everything that is soluble in the milk that is left over after the coagulation process and contains only about 5% lactose. Whey protein isolate is isolated protein where most of the remaining ingredients are filtered out, including the lactose. Whey Protein Isolate Benefits Whey protein isolate is a very rich source of protein that may support muscle growth and aid in the recovery of exhausted muscles after anaerobic exercise. This protein powder is a rich source of leucine and branched-chain amino acids (BCAAs), essential amino acids that are largely found in muscle tissues and used primarily in the production of proteins.