The answer is simple, but read this carefully. The 80% refers to the concentration level of the protein that is in the ingredient – whey protein concentrate.
If a product is called 100% whey protein, this usually refers to the protein source used, meaning only whey protein is used, not egg, soy or milk protein. Therefore, a product that is named 100% whey protein (if it’s from a concentrate) would contain about 80% protein.
