Protein – Whey Protein is the most important supplement for all athletes, but it is especially indispensable for those involved in long-term, high intensity endurance training. It is the fuel for the build-up, mass and toning of the muscle. In fact, protein is essential for growth and development. The proteins that make up the human body are not obtained directly from the diet; rather, dietary protein is broken down into its constituent parts known as amino acids, which are the basic building blocks of life. That means that it is the amino acids, and not protein per se, which are essential nutrients.