Muscles are made of protein. This statement is rather obvious to all of us. So to build muscle, equally obviously, you need to take proteins. This can be found in eggs, fish meat, cottage cheese or protein concentrates like protein powder.
To appreciate the importance of proteins in your body, you have to understand that your body cells are constantly being replaced by new ones. The tissue, made of protein, is in a state of flux, constantly being changed as old cells die and new cells replace them. Rheo Blair states "It is as if you lived in a building whose bricks were systematically taken out and replaced every year. If you keep the same blueprint then it will still look like the same building. But it won't be the same in actuality. The human body is in a constant state of flux, it loks the same from day to day, but through multiple biological processes, it is always rebuilding itself."
Science has proven that 99% of the atoms in your body are replaced within one year. Every cell in your body is always being recycled. Protein is what is used in the creation of new cells. It is the building blocks of your body. Skin, hair, bones, hormones, and antibodies are formed of protein. Save for water, protein is the most abundant substance in your body. Like other nutrients, proteins are made of carbon, hydrogen and oxygen. But unlike other nutrients, proteins are the only nutrients that bring nitrogen into the body. Thus by measuring the amount of nitrogen in ones excreta, compared to the amount taken in, one can estimate the amount of protein used for muscle growth. If the difference is positive, then muscle is being made. But if the difference is negative, there is a negative nitrogen balance, and the body literally begins feed on its own muscle to produce energy. Proteins are broken down by digestive acids like protease to smaller units called peptides. This occurs in the stomach where there are acidic conditions necessary for the digestion of proteins. The peptides are in turn are digested by peptidase, found in the duodenum, into amino acids. These are what the body actually absorbs and utilizes to form body tissue, including muscle.
There are 20 amino acids that are required for the normal growth of the body. Eleven of these are naturally made in the body and thus are called non-essential amino acids. The other nine have to be ingested into the body as it cannot synthesize them and are thus called essential amino acids. Foods containing both types of amino acids in the exact amounts as they are needed by the body are called complete proteins. Only when all the essential amino acids are available can the body grow muscles. Otherwise it starts breaking down body tissue to suffice the amount of essential amino acids for growth and repair purposes. Thus lack of these amino acids actually leads to muscle loss.