Proteins are essential for the proper growth, repair and function of the body. They are made up of amino acids linked together by peptide bonds. The human body uses approximately 20 amino acids, 8 are essential, 10 are nonessential, and 2 are semi-essential. Some sources categorize the two semi-essential amino acids within the essential category. Neither are incorrect.
Essential amino acids cannot be made in the body, and must be obtained directly from food sources. Nonessential amino acids are able to be manufactured in the body and thus are not required to be consumed through food. Semi-essential amino acids can be manufactured in the body, but not at a rate that can support/sustain growth, so a percentage must be consumed through food sources. Proteins from food sources must be first denatured in the stomach (broken down) into amino acids before the body can use them.
Chronic over consumption of protein (over 35%) is associate with high saturated fat intake, low fiber intake, increased urea, possible dehydration, decreased glycogen stores and decreased performance.