L-Tyrosine

Tyrosine is one of the clearest examples of a supplement whose effect depends entirely on context. Under ordinary conditions, tyrosine hydroxylase is saturated and adding more substrate changes nothing — which is why trials of tyrosine in rested, unstressed people generally find no cognitive benefit. Under acute stress, the picture reverses. Military and laboratory research going back to the 1980s shows that cold exposure, prolonged wakefulness, continuous noise and high cognitive load all deplete brain catecholamines faster than they are resynthesised, and that tyrosine at 100-150 mg/kg attenuates the resulting decline in working memory, vigilance and multitasking performance. The effect is protective rather than enhancing: it keeps stressed performance nearer to baseline, not above it. This context-dependence also predicts who benefits from it as a nootropic, namely people in genuinely demanding conditions rather than people at a desk after eight hours' sleep. A separate line of evidence suggests tyrosine benefits working memory more in people with lower dopamine function, and may be counterproductive in those already at optimal dopamine levels, following the inverted-U relationship. Tyrosine is also used as a levodopa-sparing idea in Parkinson's, but it does not cross into that role effectively, since the rate-limiting step is tyrosine hydroxylase, not substrate supply.