Higher blood levels of tyrosine, an amino acid sold worldwide in capsules that promise sharper focus, are linked to roughly 0.91 fewer years of life in men. Women showed no such shortfall.
The finding comes from an analysis of 272,475 UK Biobank participants published in the journal Aging in October 2025 by researchers at the University of Hong Kong and the University of Georgia. To move beyond a simple correlation, the team used Mendelian randomization, a genetic method that helps rule out confounding from diet, income or lifestyle.
One important caveat sits underneath the headline number. The study measured naturally occurring blood concentrations of tyrosine, not supplementation, and its authors have not called for people to stop taking anything. But tyrosine is one of the more widely marketed cognitive-performance ingredients in the US supplement aisle, sold to students, shift workers and athletes for focus under pressure.
What tyrosine actually does
Tyrosine is a non-essential amino acid the body can make from another amino acid, phenylalanine. It is a building block for the neurotransmitters dopamine, norepinephrine and epinephrine, the chemical messengers involved in alertness, motivation and the stress response. That biology is the pitch behind L-tyrosine capsules, which are typically promoted for concentration, energy and mental performance during stressful tasks.
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Dietary tyrosine is abundant in protein-rich foods such as chicken, cheese, fish, eggs, soy and nuts, so most people already get plenty without a supplement.
What the researchers found
In the Mendelian randomization analysis, genetically higher tyrosine was associated with 0.91 fewer years of life in men, with a 95 percent confidence interval of 1.60 to 0.21 years. In women, the estimate was 0.36 fewer years and did not reach statistical significance.
Phenylalanine, the amino acid the body converts into tyrosine, initially looked linked to earlier death, but the association vanished once tyrosine was taken into account. The cohort included 125,359 men and recorded 23,964 deaths.
Because Mendelian randomization relies on genetic variants people carry from birth, it is less prone to being distorted by daily habits than a plain observational study. That is why the authors frame the tyrosine link as evidence pointing toward causation, not just correlation.
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What it means for anyone taking a tyrosine capsule
The study did not test any supplement directly, and the authors do not claim that L-tyrosine pills shorten men's lives. What the paper does suggest is that men with elevated blood tyrosine may benefit from bringing those levels down, and that a plausible route is dietary rather than pharmaceutical. Reducing overall protein intake, especially very high-protein regimens, is one of the strategies the authors flag for future investigation.
Anyone already taking tyrosine for focus or mood, or considering it, should raise it with their own doctor, particularly men on thyroid medication, since tyrosine is a precursor to thyroid hormones and can interact with those drugs. The wider point from the paper is narrower than the headline: for men whose blood tyrosine is already high, adding more may not be doing them any favors.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.