There are approximately 20 or so amino acids that can make up protein. Eight of them are considered essential and the body cannot make them on its own thus they are required from diet.
Technically, the non-essential aminos can be made from the essential aminos.
There are also amino acids considered "conditionally" essential under certain conditions and or populations.
If you link several aminos together you get a peptide. Keep linking peptides together and you get a protein. The shape of the individual amino acids and resulting proteins is quite unique and highly specific.
Proteins have many different roles in the body beside simple muscle. Protein, or more appropriately amino acids, is the only macro-nutrient that supplies nitrogen to drive lean tissue growth (anabolism).
Although athletes usually focus on the effect that protein has on skeletal muscle, it is equally important for people to understand that there are other disposal sites of amino acid nitrogen in the human body.
In simple terms, these include structural proteins, DNA, RNA, phospholipids, enzymes, immune function and bile acids to name a few. Bottom line? There are many uses for protein in the body unrelated to just building muscle.
We need protein to build or regenerate skeletal muscle. However, many people don't understand the other functions protein has within the body, as alluded to above.
Upon digestion, amino acids from ingested proteins enter what is called the "free amino acid pool." The amino acids can be diverted to different areas of the body for utilization depending on what the body needs. For example, some amino acids are used as an energy source through their conversation to glucose, using a process called gluconeogenesis.
Protein can also be converted to fat, though this is a very inefficient process in humans and is not a major source of body fat, contrary to what you may have been led to believe by some nutritional "authorities."
Protein is also a very thermo genic fuel substrate in the body, meaning that its digestion, metabolism and storage require a great deal of energy, which is released as heat. Have you ever wondered why you may feel hot after a large protein meal?
This could be the reason.
In fact, it has been shown that ingesting large amounts or protein can account for upward of 20 percent of daily energy expenditure.
This means that as much as 20 percent of the calories from protein you eat are lost as heat and can't be stored as fat on your glutes or hips! From a thermal, hormonal, and biochemical point of view, protein is the least likely macro-nutrient to be converted to body fat.