For the purposes of their umbrella review, Ciria et al. included 24 meta-analyses of randomized controlled trials (RCT), constituting 109 separate primary studies, and over 11,000 participants. As noted by the authors themselves, observational and epidemiological studies may also have merit, but were not included in this study. Their results suggest that variations in study sampling, influential variables, baseline measures, control groups (such as physical or non-physical activity), and publication bias, have collectively resulted in an overestimation of the magnitude and significance of positive effects in the existing meta-analyses. The authors note that “the particular conclusions from the different meta-analyses cannot be taken as the empirical evidence accumulated over years, but as selective slices of it”. Their ultimate conclusion is therefore not that exercise does not improve cognition, but rather that, when subjected to further scrutiny, the existing meta-analyses do not conclusively support the existence of such a causal effect (7).
An interesting additional observation from the authors is that, despite the numerous studies evaluating the potential effects of exercise on cognition, “the absence of a firm theoretical model of the mechanisms involved in exercise-induced cognitive improvements in humans is surprising” (6). As they note, foundational preclinical research on numerous possible mechanisms of action is available. More detailed human studies are precluded for innumerable reasons, but previously proposed theories fail to completely account for existing observations and published evidence. It remains to be seen if an adequate and sufficiently complex model can be developed to account for the effect of exercise on cognition in human populations given the inherent complexity of the behavior and associated interactions.