BCAAs
Evidence: weak
A plausible fatigue mechanism that does not deliver in trials. Largely redundant for any runner eating enough complete protein.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
Keep it in proportion
The basics are where the real gains are: consistent volume, sleep and adequate fuelling. Weigh anything below honestly, and do not let a marginal or over-marketed aid pull attention from the things that actually move the needle.
Branched-chain amino acids are sold to endurance athletes on a plausible-sounding mechanism that does not survive contact with the trials, and they are not worth buying for any runner eating enough complete protein.
The central-fatigue hypothesis holds that BCAAs compete with tryptophan for entry to the brain, lowering the serotonin thought to contribute to fatigue. The biology is plausible, but the performance prediction is largely unsupported: BCAAs given during cycling failed to affect endurance performance, even though they can lower brain tryptophan (van Hall et al. 1995). Carbohydrate achieves the same anti-fatigue effect more effectively, making isolated BCAAs redundant for the purpose.
For recovery and adaptation, protein quality beats isolated BCAAs. BCAAs alone produce only a partial, plateauing muscle-protein-synthesis response, because the other essential amino acids become rate-limiting; a maximal response needs complete protein (Aragon & Schoenfeld 2013). A runner meeting their total daily protein from food gains nothing from a separate BCAA product. The AIS classes them Group C, no substantial evidence of benefit.