Abstract
Background
High intensity interval training (HIIT) has been shown to improve physical and mental health in healthy individuals and those with physical illnesses, such as cardiovascular and cardiometabolic diseases. Initial work has shown that HIIT may have similar benefits in people experiencing mental illnesses including schizophrenia. Thus, it has been proposed that HIIT may be a promising exercise intervention that has the potential to target the poor health of those with mental illnesses. Despite the rapid interest in HIIT, there is a lack of clarity in the totality of the evidence for which outcomes and under what regimes HIIT is effective and safe, both in populations with and without mental illnesses.
Methods
A review of the literature was undertaken, comprising a) a meta-review investigating HIIT in all populations and all health outcomes to identify the global health benefits of HIIT; b) a traditional systematic review of all individual interventions of HIIT in all mental disorders (including schizophrenia), to see specifically what has been done in mental health populations. Firstly, major databases were searched for systematic reviews (with/without meta-analyses) of randomised & non-randomised trials that compared HIIT to a control in any human population. Findings were summarised narratively. Secondly, major databases were searched for intervention studies investigating HIIT among people with mental illnesses. Findings were summarised narratively, and a preliminary meta-analysis was undertaken.
Results
Regarding our first search, 33 systematic reviews (including 25 meta-analyses) were retrieved encompassing healthy subjects and people with physical health complications. Evidence suggested HIIT improved cardiorespiratory fitness (21/23, 91% systematic reviews), arterial compliance and vascular function (3/5, 60% systematic reviews), cardiac function (2/3, 67%), muscle mass (2/3, 67%), quality of life (2/4, 50%) exercise capacity (2/3, 67%) and inflammatory markers (3/5, 60% reviews), versus control. Improvements in muscular structure, anxiety and depression, and blood pressure were seen, compared to pre-training. Additionally, no acute injuries were reported, and mean adherence rates surpassed 80% in most systematic reviews. Regarding our second search, 12 intervention studies, (including 7 RCTs), were included encompassing Major Depressive Disorder, Schizophrenia-Spectrum Disorders, Bipolar Disorder, Substance Use Disorder, Panic Disorder and ADHD. Evidence suggested HIIT improved cardiorespiratory fitness (5/8 intervention studies, 63%), anthropometric variables (3/4, 75%), mental health outcomes (9/12, 75%), cardiovascular fitness (5/9, 56%), physical fitness (1/1, 100%) and motor skills (1/1, 100%), compare to pre-training. The preliminary meta-analysis of pre-post changes found HIIT increased High-Density-Lipoprotein (SMD:0.373 [95%CI 0.18; 0.57], p=0.0002) and possibly reduced general psychopathology (SMD:-1.58 [95%CI -3.35; 0.18], p=0.08) in people with schizophrenia-spectrum disorders. HIIT reduced depression severity (Standardised mean difference (SMD):-1.36 [95%CI-1.63;-1.089], p<0.0001) and possibly improved VO2max (SMD:0.18 [95%CI -0.02; 0.37], p=0.08) in people with depression. No acute injuries were reported, mean adherence to HIIT sessions ranged from 64–94%, and dropout ranged from 0–50%.
Discussion
HIIT appears to be associated with multiple benefits in healthy subjects and people with physical health complications. HIIT may also improve a range of physical and mental health outcomes among people with mental illnesses including schizophrenia. Nonetheless, high-quality well-powered trials are needed to reaffirm these findings.
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