Amino acid nutrition — Which of the following is NOT an essential amino acid for humans?

Introduction / Context:
Essential amino acids cannot be synthesized de novo in sufficient amounts by humans and must be obtained from the diet. Recognizing essential versus nonessential amino acids is fundamental in nutrition and metabolism.

Given Data / Assumptions:

• Histidine, leucine, and methionine are typically listed as essential.
• Glycine can be synthesized endogenously from serine and other precursors.
• Focus is on human requirements.

Concept / Approach:
Humans require nine dietary essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Nonessential amino acids (e.g., alanine, asparagine, aspartate, glutamate, glutamine, glycine, proline, serine, tyrosine in most contexts) can be made internally.

Step-by-Step Solution:

Verification / Alternative check:
Dietary guidelines and biochemistry references concur that glycine is nonessential for healthy adults.

Why Other Options Are Wrong:

• Histidine is essential, particularly important in hemoglobin synthesis.
• Leucine is essential and a key branched-chain amino acid.
• Methionine is essential and a methyl-group donor via S-adenosylmethionine.

Common Pitfalls:
Confusing “conditionally essential” states; for instance, in certain illness or rapid growth, requirements change, but glycine remains nonessential in general adult nutrition.

Final Answer:
Glycine