Men lose nearly a year of life to a molecule that's supposed to boost their brains. Tyrosine, a dopamine precursor taken by millions as a cognitive supplement. @uk_biobank Mendelian randomization (MR), n=272,475: it causally shortens male lifespan by 0.91 years (p=0.01) No significant effect in women. Men naturally carry higher circulating tyrosine than women. The authors suggest this single amino acid difference may partly explain the male-female lifespan gap. Nuance: MR captures lifelong endogenous exposure, not supplement effects. But the finding held across multiple MR methods and every sensitivity test. If you're supplementing tyrosine daily, this paper deserves your attention.