The body has more than one clock, says new study

A new study published in Nature reveals that the human body possesses multiple biological clocks rather than a single one, challenging the notion that one ageing clock can adequately describe everyone. The researchers built 38 ageing clocks, each covering different organ systems, using data from three sources: blood proteins, metabolites, and organ MRI measurements.

Crucially, they trained these models separately for males and females, discovering that some aspects of ageing are sex-specific, others are organ-specific, and many are shared. This finding transforms biological ageing clocks from simple measurements into complex modelling problems. The study suggests that biological ageing may be better understood as a collection of different trajectories rather than a single score, with each organ aging along its unique path.

Some ageing clocks performed better when females and males were modelled separately, particularly for measures involving body fat, the immune system, and metabolism. However, this approach was not universally advantageous, as separate models sometimes became less reliable due to a lack of data. Genetic analyses revealed 359 significant associations across the ageing measures, with patterns varying by sex and organ.

Some liver and metabolic ageing measures showed stronger genetic signals in females, while immune, skin, and spleen measures exhibited stronger signals in males. This indicates that different body parts may have distinct mixtures of genetic and environmental influences, potentially lacking a single genetic blueprint for ageing. The researchers also linked seven imaging-based ageing measures to proteins circulating in the blood, finding that the proteins associated with older-appearing organs differed between males and females.

This does not imply the existence of male or female ageing proteins, but rather that individuals with biologically older organs may follow distinct molecular pathways. These findings could eventually influence personalized medicine, as an ageing clock may detect disease-associated biological states without being the cause of the disease.

The study also found that older-appearing brains were associated with a higher risk of progressing to Alzheimer's disease among individuals already experiencing mild cognitive impairment, a pattern observed in both men and women. However, direct comparisons between the two groups did not yield statistically significant differences.

The researchers caution that ageing clocks remain research tools and are not predictive crystal balls. Further testing is needed to determine whether the same patterns apply across diverse populations, including those of South Asian, African, East Asian, indigenous, and other backgrounds.