With such a high percentage of water in meat, you can see why shrinkage can be a big problem in cooking meat. Protein is an important nutrient and the most abundant solid material in meat.
Protein coagulates when it is heated.
This means it becomes firmer and loses moisture. Too high heat toughens protein. Of course, there can be more fat surrounding the muscles. Juiciness Marbling is fat that is deposited within the muscle tissue. The juiciness we enjoy in well-marbled beef is due more to fat than to moisture.