The effect of high-intensity interval training versus moderate-intensity continuous training on liver fat : a systematic review and meta-analysis

Aim: This systematic review aimed to determine the effect of aerobic exercise interventions, including high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT), on liver fat in adults. A secondary aim was to investigate the interaction between total weekly exercise volume and exercise-related energy expenditure and change in liver fat. Methods: Relevant databases were searched to December 2020 for randomised trials, comparing HIIT to control, MICT to control, or HIIT to MICT. Studies were excluded if they did not implement ≥2 weeks intervention or assess liver fat using magnetic resonance-based techniques. Weighted mean differences and 95% confidence intervals (CI) were calculated. Regression analyses were undertaken to determine the interaction between weekly exercise volume in minutes and kcal with change in liver fat %. Results: The search returned 28,262 studies of which 19 were included involving 745 participants. Both HIIT and MICT elicited moderate reductions in liver fat % when compared to control (HIIT:-2.85%, 95%CI:-4.86 to -0.84, p=0.005, I2=0%, n=114, low certainty evidence; MICT:-3.14%, 95%CI:-4.45 to -1.82, p<0.001, I2=5.2%, n=533, moderate certainty evidence). There was no difference between HIIT or MICT (-0.34%, 95%CI:-2.20 to 1.52, p=0.721, I2=0%, n=177, moderate certainty evidence). Neither total exercise volume in minutes (β=0.0002, SE=0.0017, Z=0.13, p=0.89) nor exercise-related energy expenditure in kcal (β=0.0001, SE=0.0002, Z=-0.63, p=0.52) were related to changes in liver fat %. Conclusion: HIIT elicits comparable improvements in liver fat to MICT despite often requiring less energy and time commitment. Further studies should be undertaken to assess the relative importance of aerobic exercise prescription variables, such as intensity, on liver fat.