Introduction

Physical exercise shows promise as a modifiable risk factor to reduce the risk of dementia and related neurodegenerative diseases.1 As cognitive function declines with advancing age, a physically active lifestyle has an important role in reducing such declines,2 3 as well as the incidence of dementia.4 It is hypothesised that the neural and vascular adaptations to physical exercise improve cognitive function through promotion of neurogenesis, angiogenesis, synaptic plasticity, decreased proinflammatory processes and reduced cellular damage due to oxidative stress.5 While lifelong participation in physical exercise may be preferable, the adoption of exercise at any age to delay or reverse cognitive decline is worthwhile given the prevalence of physical inactivity and the increasing proportion of older adults in the population.

Although early meta-analyses, such as a study of aerobic exercise interventions,6 showed large benefits to cognitive function in older adults, more recent systematic reviews7 and meta-analytical studies8–10 are much less conclusive. For example, a recent meta-analysis of aerobic, resistance training and tai chi interventions in people older than 50 showed little benefit of exercise on cognitive function.9 The discrepancy in findings is partly because existing reviews are excessively restrictive in their inclusion criteria, often considering only one mode of exercise (eg, recent reviews of aerobic training only10 11) or a narrow range of publication years. Thus, the numerous meta-analyses published provide incomplete summaries of the available evidence in people aged 50 and over. Studies which prescribe a combination of both aerobic and resistance training components in one intervention (here on called multicomponent training) have not been reviewed in healthy older adults since the 2001 study of Colcombe and Kramer6 despite global guidelines recommending this type of training in older adults.12 13 Alternative modes of exercise such as yoga14 or tai chi15 may also be beneficial to cognitive function, yet randomised controlled trials (RCTs) of these modes in older adults have not been specifically reviewed. Importantly, prior reviews offer relatively little information about the optimal prescription of physical exercise for cognitive health. Physical exercise provides a complex stimulus for adaptation in the body and its dosage can be modulated by various parameters, including duration, frequency, intensity and the mode or type of exercise. Despite this, many reviews do not take into account the importance of exercise prescription variables in either the analysis or discussion of the literature. Consequently, there is an urgent need for guidelines on the type or amount of exercise a clinician should recommend to their patient.

To deal with these research gaps, we have completed a comprehensive meta-analysis which includes a larger number of studies by imposing no limit on publication date or exercise mode. This study examines four key issues including: (1) the effects of supervised exercise interventions of aerobic, resistance, multicomponent, tai chi and yoga training modes on cognitive function; (2) the influence of exercise training variables, including the duration, frequency, intensity and length of exercise; (3) the differentiation of exercise effects on global cognition and domains of cognition, including attention, executive function, memory working memory; and (4) the impact of study design, including the nature of the control group and the baseline cognitive status of participants.