The hippocampus is one of the major brain sites of neuroplasticity, and has been identified as a brain region sensitive to the effects of physical activity, and in particular aerobic exercise interventions (Cotman et al., 2007). The impact of aerobic exercise on hippocampal size and function has been extensively studied in rodents (Van Praag et al., 2005). These animal studies have shown that voluntary exercise is associated with hippocampal neurogenesis (Van Praag, 2008) and prevents age-related decline in cell-proliferation in this brain structure.

Whilst there is clear evidence in animal studies that exercise training is associated with increased hippocampal neurogenesis, the evidence from human studies is equivocal. Along with a recent meta-analysis showing that physical activity is correlated with greater white matter volume (Sexton et al., 2016), there are also several studies reporting cross-sectional relationships between physical activity levels and hippocampal volumes (Erickson et al., 2010). However, determining causality from such correlational analyses is not possible.

A systematic review and meta-analysis to determine the impact of aerobic exercise interventions on hippocampal volumes in human subjects were conducted. These included controlled studies in human subjects, regardless of age or clinical status, as well as a range of subgroup analyses exploring effects of exercise in unilateral measures of left/right hippocampal volume, and establishing effects on the hippocampus in clinical and non-clinical populations.

Meta-regression techniques to examine how exercise effects differed with age found no effects of exercise on total hippocampal volume, but did find that exercise interventions retained left hippocampal volume significantly more than control conditions. As these positive effects were also observed among the subgroup of studies of healthy older adults, the findings hold promising implications for using exercise to attenuate age-related neurological decline.

Currently, the overall quality of the evidence is compromised by the fact that 10 of the 12 studies included some risk of bias, therefore more high-quality randomized controlled trials (RCTs) are now required. In additional to RCTs, a prospective meta-analysis examining how changes in physical activity and fitness predict hippocampal retention/deterioration across the lifespan would provide novel insights into longer-term neural effects of exercise, while also reducing the impact of methodological heterogeneity often found across exercise RCTs.

Further research is also required to determine effects in younger people (Riggs et al., 2016), and establish the neurobiological mechanisms through which exercise exerts these effects, in order to design optimal exercise programs for producing neurocognitive enhancements. However, the functional relevance of structural improvements has also yet to be ascertained. Nonetheless, the link between cardiorespiratory fitness with both structural and performance increases indicates this as a suitable target for aerobic training programs to improve brain health.

 

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