How many amino acids compose body proteins, and how are they categorized?

• A. 18 amino acids—none essential
• B. 22 amino acids—all essential
• C. 20 amino acids—some essential and some non-essential
• D. 12 amino acids—half essential

Answer: (C)

Proteins in the body are built from 20 standard amino acids, and they’re categorized by whether the body must obtain them from the diet or can synthesize them. Some amino acids are essential, meaning the body cannot make them in sufficient amounts and they must come from food. Others are non-essential because the body can synthesize them from other compounds. In adults, nine amino acids are essential while the remaining eleven are non-essential, making up the total of 20 that typically compose body proteins. (Some contexts also recognize a couple of additional amino acids used in special proteins, but the usual count for most body proteins is twenty.)