Among the many quiet forces shaping how long a human life extends, a common amino acid called tyrosine has emerged as an unexpected variable — one that appears to shorten lifespan more sharply in men than in women. Researchers drawing on nearly 275,000 participants in the UK Biobank have found that higher circulating tyrosine correlates with reduced longevity, and that this effect may be amplified by the very hormones that define male biology. The finding does not yet carry a prescription, but it opens a door onto one of medicine's oldest questions: why, across nearly every culture and era, wo