Which of the following is NOT considered a conditionally essential amino acid?

Cystine is classified as a non-essential amino acid because the body can synthesize it from methionine, which is considered an essential amino acid. Consequently, the body does not require cystine from dietary sources under normal circumstances, as it can produce it as needed.

In contrast, the other amino acids listed—tyrosine, glutamine, and glycine—are considered conditionally essential. This means that under certain stressful situations, such as illness, injury, or intense exercise, the body's demand for these amino acids may exceed its ability to synthesize them. For instance, during periods of metabolic stress, the body may require more glutamine than it can produce, making it conditionally essential during those times. Similarly, tyrosine can become essential when phenylalanine, its precursor, is inadequate, while glycine's synthesis might not suffice during times of high demand like growth or healing.

Understanding the classification of amino acids helps in nutritional planning, especially for athletes or individuals undergoing physiological stress, where maintaining an adequate supply of these conditionally essential amino acids is crucial for recovery and overall performance.