Aging is accompanied by decreased grey matter volume (GMV), cerebral blood flow (CBF), and cerebrovascular reactivity (CVR), where the latter tends to decline the earliest in aging. Enhanced fitness in aging has been related to preservation of GMV and CBF, and in some cases CVR, although there are contradictory relationships reported between CVR and fitness. Therefore, to gain a better understanding of the complex interplay with fitness and GMV, CBF and CVR, it is necessary to study them

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... ently. Here, we aimed to disentangle the interactions between these outcomes in healthy older adults. MRI acquisitions collected anatomical, CBF and CVR information in all participants, as well as VO2max. Results revealed decreased CVR was associated with increased fitness throughout large areas of the cerebral cortex. Within these regions it was found that lower fitness was associated with higher CBF and a slower hemodynamic response to hypercapnia. Overall, results indicate that the relationship between age, cerebral health and cerebral hemodynamics are complex. Future studies should collect other physiological outcomes in parallel with quantitative imaging, such as measures of carbon dioxide sensitivity and autoregulation, to further understand the intricacy of the effects fitness has on the aging brain, and how this may bias quantitative measures of cerebral health.