A large study involving more than 270,000 people has found that higher blood levels of the amino acid tyrosine are linked to a shorter lifespan in men. Researchers from the University of Hong Kong and the University of Georgia investigated whether two amino acids—phenylalanine and tyrosine—play a role in determining lifespan. These amino acids are essential for metabolism and brain function. The study found a significant difference between men and women, with higher tyrosine levels in men associated with a shorter life expectancy, while no such link was found in women. The researchers used observational data and genetic analysis, including a technique called Mendelian randomization. This method uses natural genetic variations to assess whether a biological factor might influence a health outcome. Their findings suggest that high tyrosine levels could have a causal effect on reduced life expectancy in men. Based on their estimates, elevated tyrosine levels might reduce a man's lifespan by nearly one year. Tyrosine is naturally present in protein-rich foods such as meat, dairy, and legumes, and it is also sold as a dietary supplement to enhance focus and mental performance. While the study did not directly test the effects of tyrosine supplements, the results raise questions about the long-term consequences of consistently high tyrosine levels. The researchers suggest that individuals with unusually high tyrosine levels might benefit from dietary adjustments, such as reducing protein intake, to lower those levels. The exact reason behind the link between high tyrosine levels and shorter lifespans in men is not yet clear. One possible explanation involves insulin resistance, which is connected to several age-related diseases. Tyrosine is also used by the body to produce neurotransmitters involved in the stress response. Additionally, hormones and signaling pathways can function differently in men and women, which might explain why the effect was observed only in men. The study, published in the journal Aging-US under the title "The role of phenylalanine and tyrosine in longevity: a cohort and Mendelian randomization study," was conducted by researchers including Jie V. Zhao, Yitang Sun, Junmeng Zhang, and Kaixiong Ye. While the findings could eventually help scientists understand the differences in aging and longevity between men and women, more research is needed before these results can be translated into practical health recommendations.