Cardiorespiratory fitness and hippocampal subfield volume in healthy older adults
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The increasing incidence of Alzheimer’s disease (AD) combined with recent evidence suggesting that its neuropathologies begin years prior to symptomatic onset has produced an immense focus on ways to attenuate the related structural and cognitive decline of certain brain regions. One low cost intervention is aerobic exercise. Rodent models have demonstrated aerobic exercise induces adult hippocampal neurogenesis, the birth of new neurons, in the dentate gyrus (DG) subregion of the hippocampus (HC) as well as increased performance on a spatial memory task. Further, human studies have demonstrated the association between increased cardiorespiratory fitness (CRF) and increased HC volume, and its translation to increased episodic memory performance. The goal of this study was to assess the relationship between CRF, as measured by VO2max, and brain region of interest (ROI) volumes notably including the left HC, left DG/Cornu Ammonis 3 (CA), and right entorhinal cortex (ErC). A secondary goal was to assess the relationship between CRF and cognitive performance on the Rey Auditory Verbal Learning Test. Baseline data was collected from 31 healthy older adults as a part of two larger clinical trials on aerobic exercise and HC function. Data included a CRF assessment as measured by VO2max, and structural MRI data including a high-resolution whole-brain T1-weighted image, and a T2-weighted image with higher-in-plane resolution. Automatic Segmentation of Hippocampal Subfields (ASHS) was the neuroimaging software utilized to segment the HC and medial temporal lobe cortices into its appropriate subfields. Multiple linear regressions ran in IBM SPSS 25 to determine if CRF predicted ROI volumes yielded no significant results when controlling for age, sex, intracranial volume, education, and scanner location. Multiple one-way between-subject ANOVAs conducted to compare ROI volumes in high-fit versus low-fit individuals revealed marginal significance for the left HC, but no other ROI. Multiple one-way between-subject ANOVAs conducted to compare cognitive performance in high-fit versus low-fit individuals also revealed no significant results. Considering the marginal significance achieved by the one-way between-subjects ANOVA for CRF and left HC, a larger sample size is needed to potentially achieve significant statistical significance. Given these remaining null results, further investigation is suggested using additional neuroimaging analyses that split the DG/CA3 into its anterior and posterior sections, as well as examining different aspects of the RAVLT or utilizing more sensitive episodic memory tests.