Skeletal muscle is like a wall made up of building blocks, or bricks, called amino acids. The wall is constantly being broken down and rebuilt throughout our lives. When we’re young and growing, the walls get bigger, as we add more amino acids than we lose. During adulthood and into our 40s, the bricks going into the wall and coming out are about equal. As we age, more bricks leave the wall than we can produce. We lose, albeit very gradually, muscle.
When we stop using our muscles, it decreases the rate at which we replenish our amino acids (our muscular bricks), and it increases the rate at which bricks come out of the wall. But when we load a muscle through resistance exercise, we can reverse that process, and the muscle grows in size. We call the process of putting bricks into the wall muscle protein synthesis, and the process of taking bricks out of the wall muscle protein breakdown.
For optimal muscle health and performance, you should trigger synthesis 3 to 4 times per day with 25 to 30 grams of high-quality protein at least once after a workout and once before sleep, as the biggest period of daily muscle loss is overnight sleep.
 Phillips, S. and Devries, M. “Supplemental Protein in Support of Muscle Mass and Health: Advantage Whey,” 2015.