L-Tyrosine

McAllister et al. (Stress, 2024; PMID 38975711): 80 young adults randomized to 2,000 mg L-tyrosine, 200 mg L-theanine or placebo before a virtual-reality active-shooter drill and cognitive challenge; neither amino acid changed salivary alpha-amylase, secretory IgA, heart rate or state anxiety, and the only tyrosine result was fewer missed responses on the Stroop task. Mahoney et al. (Physiol Behav, 2007; PMID 17585971): 19 volunteers, double-blind within-subject, 300 mg/kg tyrosine in food bars during two 90-minute cold-water immersions at about 10 degrees Celsius; cold impaired a match-to-sample memory measure and tyrosine restored correct responses; US Army laboratory, indexed as a controlled trial rather than a randomized one. Deijen et al. (Brain Res Bull, 1999; PMID 10230711): 21 cadets during a combat training course, five daily 2 g doses in a protein drink against a carbohydrate drink; memory and tracking performance better on tyrosine, no mood effect; the control was a calorie-matched carbohydrate drink, not a matched placebo. Null: Coull et al. (Med Sci Sports Exerc, 2016; PMID 26285023), 150 mg/kg before a load-carriage protocol in 40 degree heat, 8 participants, no cognitive benefit; Donnan et al. (PLoS One, 2025; PMID 39820592), 150 mg/kg in 8 soccer players during intermittent exercise in 32 degree heat, no effect on decision making, power or affect. Harm signal in older adults: van de Rest et al. (Nutrients, 2017; PMID 29168741), 17 adults aged 60 to 75 given 100, 150 or 200 mg/kg; working memory declined with higher doses, most in those with the largest plasma rise. Grade C: small, short trials on objective tests under acute stress, positive in cold and combat settings, null in heat, with a dose-dependent decline in older adults. A 2017 tyrosine and brain-stimulation paper by another group was retracted and is not cited.