Human nutrition and metabolism — Which of the following is not an essential amino acid for humans (i.e., it can be synthesized endogenously)?

Introduction / Context:
Essential amino acids must be obtained from the diet because humans cannot synthesize their carbon skeletons. Nonessential amino acids can be synthesized de novo. Recognizing which amino acids are essential helps in clinical nutrition, formulation of culture media, and dietary planning.

Given Data / Assumptions:

• Aspartic acid, glutamic acid, and glycine are listed.
• We use the standard human classification (not specific to other organisms or life stages).
• “Essential” refers to dietary indispensability for adults.

Concept / Approach:
The canonical essential amino acids for adult humans are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (plus conditionally essential residues depending on state). Aspartate, glutamate, and glycine are nonessential because metabolic pathways (e.g., transamination, one-carbon metabolism) can supply them.

Step-by-Step Solution:

Verification / Alternative check:
Dietary references and biochemistry texts consistently classify these three as nonessential under normal conditions.

Why Other Options Are Wrong:

• Aspartic acid: not essential; humans synthesize it.
• Glutamic acid: not essential; central in nitrogen metabolism.
• Glycine: not essential; abundant, easily synthesized.

Common Pitfalls:
Confusing “conditionally essential” amino acids (e.g., arginine in rapid growth) with strictly essential; mixing up neurotransmitters (glutamate) with dietary essentiality.

Final Answer:
All of these