Abstract
Aerobic fitness (AF) and self-reported physical activity (srPA) do not represent the same construct. However, many exercise and brain aging studies interchangeably use AF and srPA measures, which may be problematic with regards to how these metrics are associated with brain outcomes, such as morphology. If AF and PA measures captured the same phenomena, regional brain volumes associated with these measures should directly overlap. This study employed the general linear model to examine the differential association between objectively-measured AF (treadmill assessment) and srPA (questionnaire) with gray matter density (GMd) in 29 cognitively unimpaired community-dwelling older adults using voxel based morphometry. The results show significant regional variance in terms of GMd when comparing AF and srPA as predictors. Higher AF was associated with greater GMd in the cerebellum only, while srPA displayed positive associations with GMd in occipito-temporal, left perisylvian, and frontal regions after correcting for age. Importantly, only AF level, and not srPA, modified the relationship between age and GMd, such that higher levels of AF were associated with increased GMd in older age, while decreased GMd was seen in those with lower AF as a function of age. These results support existing literature suggesting that both AF and PA exert beneficial effects on GMd, but only AF served as a buffer against age-related GMd loss. Furthermore, these results highlight the need for use of objective PA measurement and comparability of tools across studies, since results vary dependent upon the measures used and whether these are objective or subjective in nature.
Highlights
A strong link has been established between higher physical activity (PA) levels and improved cognitive function in aging
OVERLAP BETWEEN AEROBIC FITNESS AND SELF-REPORTED PHYSICAL ACTIVITY ON GRAY MATTER DENSITY Based on the voxel-wise analyses described above, there was no overlap in regional gray matter density (GMd) between the areas significantly associated with Aerobic fitness (AF) and those significantly associated with self-reported physical activity (srPA) (Figure 1), suggesting that AF and srPA are associated with GMd in distinct and non-overlapping brain regions
RELATIONSHIP BETWEEN GRAY MATTER DENSITY AND SELF-REPORTED PHYSICAL ACTIVITY Voxel-wise analyses indicated that srPA was significantly associated with GMd in three discrete clusters after correcting for age: (1) bilateral occipital poles, lingual and left fusiform gyri, and right calcarine cortex; (2) left central opercular cortex and middle and superior temporal gyri; and (3) bilateral frontal poles, superior frontal gyrus, and paracingulate cortex
Summary
A strong link has been established between higher physical activity (PA) levels and improved cognitive function in aging. Mechanisms by which exercise affects cognitive function in humans include cell proliferation and increased synaptic density (Pereira et al, 2007), angiogenesis (Swain et al, 2003), changes in mitochondrial function (Steib et al, 2014), and alteration of trophic factor signaling, which in turn affects neuronal function and structure in areas that are critical for cognitive function (Phillips et al, 2014). One such example are exercise-induced increases in gene expression of brain derived neurotrophic factor (BDNF), which is critical for learning and memory formation by providing a propitious environment for neuroplasticity. Humans display decreased cognitive function and reductions in brain volume as they age, which are most pronounced in prefrontal, temporal, and parietal gray matter (Good et al, 2001; Tisserand et al, 2004; Raz and Rodrigue, 2006) and are ameliorated by exercise
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