Study Links Cardiovascular Fitness to Brain Health

The white matter of the brain is made up of areas of the central nervous system that are made up of myelinated axons. Its name comes from the pale appearance of the lipids that make up myelin. Myelin is a segmented sheath that insulates axons and conducts neuronal signals. Myelin loss has been documented in a number of neurodegenerative diseases, including Alzheimer’s disease and Parkinson’s disease, and perhaps most notably, multiple sclerosis. With age, demyelination becomes more likely.

Researchers have long suspected a relationship between cardiorespiratory fitness and the integrity of the brain’s white matter as people age. However, the lack of specific evidence led researchers at the National Institutes of Health to conduct a study examining the strength of this correlation, now published in the journal Proceedings of the National Academy of Sciences.

To establish a correlation between cardiovascular fitness and brain myelination, the researchers recruited a cohort of 125 participants aged 22 to 94 years. The participants’ cardiovascular fitness was quantified as the maximal rate of oxygen consumption, commonly and succinctly referred to as VO₂max. Myelin content was defined as the myelin water fraction, which the researchers estimated using an advanced multicomponent relaxometry MRI method.

Previous studies using conventional techniques have failed to isolate myelin from the rest of the brain matter; the new MRI technique used here is more sensitive and specific for measuring myelin content in vivo. In fact, recent studies using multicomponent relaxometry MRI have established correlations between local myelin water fraction, cerebral blood flow, and motor function, both of which are influenced by cardiorespiratory fitness, motivating the NIH researchers to continue the current study using the same technology.

Results

The researchers reported that better cardiorespiratory fitness was strongly correlated with better brain myelination. Additionally, better cardiorespiratory fitness was associated with better myelin integrity, which was particularly notable in middle-aged and older participants.

Notably, they found significant positive correlations between the two measures in the frontal lobes and white matter tracts, regions that are likely to experience early degeneration associated with neurological disorders in early old age. They suggest that cardiorespiratory fitness is likely to have a significant protective effect on these sensitive brain regions, particularly in subjects with good physical condition throughout life.

The researchers note that they were unable to establish a cause-and-effect relationship between improved cardiorespiratory fitness and improved myelin integrity and that their results represent only a correlation.

“Nevertheless, our results suggest that cardiorespiratory fitness is likely to be a valuable indicator of overall health and a potential target for interventions aimed at promoting brain health,” they write.

The study also notes the association of aerobic exercise with neuroprotective adaptations in the brain, as well as upregulation of neurotrophins and brain-derived neurotrophic factor, which increases brain mitochondrial function. Decline in mitochondrial function has previously been associated with diseases resulting from demyelination.

The researchers suggest that future studies could use their work to investigate the relationship between fitness, brain health and myelin integrity to support brain aging and prevent neurological disorders.

They write: “Furthermore, this work lays the foundation for further research into the potential therapeutic applications of improving cardiorespiratory fitness or myelination to promote healthy brain aging and combat age-related neurodegeneration, including in Alzheimer’s disease.”

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