What Aging Clocks Measure, Decoded for the First Time; Five Clocks Reveal Distinct Biological Pathways

Researchers have, for the first time, revealed what aging clocks actually measure. A U.S. research team decoded the biological meaning of five widely used biomarker clocks and used the findings to develop a new tool that more accurately predicts aging.

According to a report published on the 28th in the international journal Nature Chemical Biology, a team at the University of Southern California reached these findings by analyzing blood samples from more than 3,200 participants in the Health and Retirement Study project.

The researchers compared DNA methylation patterns with transcriptome data. The transcriptome refers to the full set of gene activity in a cell that is transcribed into RNA and directs protein synthesis. The analysis focused on the five most widely used biomarker clocks.

The results were unexpected. The five clocks, which appeared similar on the surface, were in fact linked to substantially different biological pathways. Some were closely associated with energy balance, cell growth and metabolism, while others focused more on immune cell activation and inflammatory signaling.

All five clocks pointed to common features of aging, including changes in the immune system, metabolic abnormalities and impaired communication between cells, but they differed markedly in the aspects they emphasized and how they interpreted them. It is like five judges scoring the same gymnastics competition: Their total scores may be similar, but each focuses on different deductions and technical points.

Building on this discovery, the researchers developed a new measurement tool called the “transcriptomic aging gene score,” using rich transcriptome data. The tool is intended to complement and extend existing biomarker clocks rather than replace them.

In tests, the transcriptomic aging gene score performed better than existing biomarker clocks in predicting a range of health measures, including physical frailty, slower walking speed, heart disease, diabetes, lung disease and even overall mortality risk.

The study offers two benefits to aging research. For basic researchers, it clearly decodes the biological meaning of five mainstream clocks for the first time, explaining the strengths and scope of each.

For example, to assess the effects of an anti-aging intervention targeting the immune system, researchers should prioritize a clock closely linked to immune pathways; another clock may be more suitable when the focus is metabolic function. The ability to select tools suited to specific goals is expected to substantially improve the precision and efficiency of future study designs.

For clinical applications, the findings open a path for biomarker clocks to move beyond the laboratory and into clinical practice. When physicians and scientists can clearly explain the molecular logic behind test results, using these tools for personalized health care, early disease alerts and medical decision support can gain a reliable scientific basis.

“By revealing the molecular programs behind these biomarkers, we have greatly improved their interpretability,” said Professor Eileen Crimmins. “This will help guide their rational application in geroscience, epidemiology and future clinical practice.”

This study did not create a more precise “clock”; rather, it opened the backs of several existing “luxury watches” to examine how their internal gears and springs actually work. This gives users a way to know which tool to choose, depending on their destination, when assessing the long and complex journey of aging.