A large new study suggests that higher levels of a common amino acid called tyrosine may be linked to a shorter lifespan in men.

The research, published recently in the journal Aging-US, examined whether blood levels of two amino acids, phenylalanine and tyrosine, were connected to how long people live.

Amino acids are building blocks of protein. They are found in protein-rich foods such as milk, eggs and meat, and are also sold as dietary supplements.

Researchers from the University of Hong Kong and the University of Georgia analyzed health and genetic data from more than 270,000 people in the U.K. Biobank, a large long-term health study in the U.K.

At first, both amino acids appeared to be tied to a higher risk of death. But after closer analysis, only tyrosine showed a strong and consistent link, specifically in men.

Genetic modeling suggested that men with higher tyrosine levels could live nearly one year less on average. The researchers did not find the same link in women.

"Phenylalanine showed no association with lifespan in either men or women after controlling for tyrosine," the authors explained.

Tyrosine plays an important role in the body. It helps produce brain chemicals such as dopamine, which affect mood, focus and motivation. Because of this, tyrosine supplements are sometimes promoted to improve mental performance.

Researchers are still trying to understand why higher tyrosine levels might affect lifespan in men. 

One possible reason: Insulin resistance, a condition linked to diabetes and other age-related diseases. 

Tyrosine also plays a role in stress-related brain chemicals, which may affect men and women differently.

Researchers also noted that men tend to have higher tyrosine levels than women. This difference could help explain why men often have shorter lifespans in general.

The study did not test tyrosine supplements directly. But the findings raise questions about whether very high levels of the amino acid over time could affect lifespan.

The researchers suggested that people with high tyrosine levels might even benefit from adjusting their diet. For example, lowering overall protein intake could possibly lower tyrosine levels.

But experts warn that more research is needed before making recommendations like these.

Kaixiong Ye, an associate professor of genetics at the University of Georgia, is a co-author of the study.