Mitochondrial function can change with age, but there is no single decline curve that describes every organ or every person. Mitochondria form dynamic networks, and their performance depends on tissue demands, physical activity, disease, nutrition and cellular maintenance.

What can change

Researchers examine energy production, mitochondrial DNA, protein composition, network structure and the removal of damaged mitochondria. These measures describe different aspects of the system. Having more mitochondrial DNA, for example, does not necessarily mean each mitochondrion works better.

A human skeletal-muscle study reported age-associated differences in mitochondrial function and several mitochondrial proteins. It was informative about the tissue and participants studied, but it did not establish that all age-related functional change has one mitochondrial cause. Short and colleagues, PNAS, 2005

Aging and activity are difficult to separate

Older study groups may differ from younger groups in fitness, medications and chronic conditions. Cross-sectional comparisons cannot fully distinguish the effects of age from these other influences. Longitudinal studies and controlled interventions help address that problem.

In a small study of older adults, exercise was associated with changes in muscle mitochondrial content and electron-transport-chain activity. The small sample limits generalization, but it illustrates that an age-associated measure need not be completely fixed. Menshikova and colleagues, 2006

Why “mitochondrial rejuvenation” is a strong claim

A laboratory improvement should be connected to a meaningful outcome: better physical function, reduced disease burden, or another prespecified clinical measure. It should also persist for a useful period and have an acceptable safety profile. A single molecular result cannot establish that an entire person’s aging process has reversed.

The accurate picture is more nuanced than “old mitochondria stop working.” Mitochondrial maintenance and adaptation change with age and context, and scientists are still determining which changes are causes, consequences or compensation.