High-intensity-interval-training (HIIT) has been suggested to have beneficial effects in multiple populations across individual systematic reviews, although there is a lack of clarity in the totality of the evidence whether HIIT is effective and safe across different populations and outcomes. The aim of this meta-review was to establish the benefits, safety and adherence of HIIT interventions across all populations from systematic reviews and meta-analyses. Major databases were searched for systematic reviews (with/without meta-analyses) of randomised and non-randomised trials that compared HIIT to a control. Thirty-three systematic reviews (including 25 meta-analyses) were retrieved encompassing healthy subjects and people with physical health complications. Evidence suggested HIIT improved cardiorespiratory fitness, anthropometric measures, blood glucose and glycaemic control, arterial compliance and vascular function, cardiac function, heart rate, some inflammatory markers, exercise capacity and muscle mass, versus non-active controls. Compared to active controls, HIIT improved cardiorespiratory fitness, some inflammatory markers and muscle structure. Improvements in anxiety and depression were seen compared to pre-training. Additionally, no acute injuries were reported, and mean adherence rates surpassed 80% in most systematic reviews. Thus, HIIT is associated with multiple benefits. Further large-scale high-quality studies are needed to reaffirm and expand these findings.

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