Protein, while used to build lean tissue (muscle) and synthesize enzymes, can also be used to provide energy. Amino acids concentrated upon their structures, they can either be converted to carbohydrates or fat. Our main dietary sources of protein are meat & fish, dairy products, nuts, eggs soy and soy products such as tofu. Body builders often use depending milk (whey) and soy protein to gain muscle. Our body can also synthesize some amino acids. The ones that we must get from our diet are called essential amino acids.