A2
L-tyrosine
- Product declaration
- L-Tyrosine: 6750 mg
- Selected studied range
- 2000 to 2000 mg
- Protocol and duration
- per day for five days, in the cadet study
- Studied context
- Studied as the dietary precursor to norepinephrine for mitigating cognitive performance decrements under acute stress. A 2015 Military Medicine evidence review (rapid evidence assessment, 14 trials: 10 RCTs + 4 controlled clinical trials) issued a "weak recommendation" in favor of tyrosine for cognitive stress specifically, stating "all studies showed a positive effect" on that outcome. A widely cited primary study (Deijen et al. 1999, Brain Research Bulletin) gave cadets 2 g tyrosine daily for five days during a combat training course and found better memory and tracking-task performance, plus lower systolic blood pressure, versus a carbohydrate control.
- Limitations
- The 2015 Military Medicine review states explicitly that "no recommendation could be made" for tyrosine's effect on physical performance under stressful physical conditions, and concludes "available evidence is insufficient to make confident recommendations on the effectiveness of tyrosine for mitigating stress effects on physical/cognitive performance" overall, despite the positive signal on cognition. A separate heat-stress study found tyrosine ingestion did not influence cognitive function or physical performance during exercise heat stress despite raised serum tyrosine. NIH ODS's exercise/athletic-performance fact sheet does not cover tyrosine at all.
Selected evidence: Deijen et al., Brain Research Bulletin, 1999
A1
L-citrulline
- Product declaration
- L-Citrulline: 4000 mg
- Selected studied range
- 3000 to 9000 mg
- Protocol and duration
- per day, in the studies NIH ODS summarizes
- Studied context
- Studied as a nitric-oxide pathway precursor (converted to arginine via the urea cycle) for aerobic exercise performance measures such as oxygen-uptake (VO2) kinetics, blood lactate, and rate of perceived exertion, and for acute resistance-exercise performance and muscle oxygenation. NIH ODS describes evidence for enhancing performance as limited, with few studies and conflicting results.
- Limitations
- A 2022 systematic review/meta-analysis found no significant benefit for endurance-performance outcomes specifically. A single 8 g acute dose did not enhance isometric force production, muscle endurance, or muscle-oxygenation parameters in one resistance-exercise study. NIH ODS characterizes overall performance-enhancement evidence as limited.
Selected evidence: NIH ODS, Dietary Supplements for Exercise and Athletic Performance · Open source
A1
Betaine anhydrous
- Product declaration
- Betaine Anhydrous: 2000 mg
- Selected studied range
- 2000 to 5000 mg
- Protocol and duration
- per day, studied for up to 15 days
- Studied context
- NIH ODS describes betaine as studied for strength- and power-based performance in bodybuilders and cyclists, with "potential but modest" improvements from a limited set of small, sometimes conflicting studies. A 2024/2025 systematic review focused specifically on endurance performance (VO2max, time-to-exhaustion, time-to-completion, power output).
- Limitations
- The endurance systematic review explicitly states "limited and inconclusive evidence regarding the definitive ergogenic effect of betaine supplementation on endurance exercise performance". Only 2 of 5 reviewed studies showed a significant improvement, and the authors call for "additional, more rigorous research."
Selected evidence: NIH ODS, Dietary Supplements for Exercise and Athletic Performance · Open source