Which amino acid is considered conditionally essential?

Glycine is considered a conditionally essential amino acid because, under normal conditions, the body can synthesize it from other compounds. However, in certain situations such as illness, trauma, or high levels of stress, the body's demand for glycine exceeds its capacity to produce it. During these times, dietary intake becomes more important, and additional sources of glycine may be necessary to maintain proper physiological functions, such as supporting collagen synthesis and neurotransmitter function.

Leucine and threonine, on the other hand, are essential amino acids, which means that they cannot be synthesized by the body and must be obtained through diet at all times. Proline, while sometimes classified as conditionally essential, is typically synthesized in sufficient quantities to meet the body's needs under most conditions, making glycine's classification more prominent in discussions of conditional essentiality.