What distinguishes essential and nonessential amino acids?

• A. Essential amino acids must be obtained from the diet; nonessential can be synthesized by the body.
• B. Essential amino acids can be synthesized by the body; nonessential must be obtained from the diet.
• C. Both essential and nonessential amino acids must be obtained from the diet.
• D. Some essential amino acids can be synthesized by the body, others must be obtained from the diet.

Answer: (A)

Essential amino acids must come from the diet because the body cannot synthesize them in adequate amounts, while nonessential amino acids can be produced by the body. So the key distinction is whether synthesis by the body is enough: if not, they’re dietary essentials; if yes, they’re nonessential. In adults, nine amino acids are considered essential for this reason, though some nonessential ones can become essential under certain conditions.