High-intensity interval training for reducing cardiometabolic syndrome in healthy but sedentary populations.

HIIT compared with a non-exercise control likely increases cardiorespiratory fitness measured with VO2max.

• Mean difference of 5.98 mL/min/kg (95% CI 4.66 to 7.30)
• Based on 16 studies, 517 participants
• Rated as moderate-certainty evidence
• Evidence was downgraded for inconsistency, imprecision, and risk of bias

HIIT compared with a non-exercise control reduces waist circumference.

• Mean difference of -3.56 cm (95% CI -6.14 to -0.98)
• Based on 8 studies, 270 participants
• Rated as high-certainty evidence
• This was the highest certainty rating among all outcomes assessed

HIIT compared with a non-exercise control likely results in little to no difference in waist-to-hip ratio.

• Mean difference of -0.01 (95% CI -0.03 to 0.01)
• Based on 6 studies, 224 participants
• Rated as moderate-certainty evidence

HIIT compared with a non-exercise control may result in little to no difference in circulating triglycerides.

• Standardised mean difference (SMD) of -0.22 (95% CI -0.62 to 0.17)
• Based on 9 studies, 262 participants
• Rated as low-certainty evidence, precluding firm conclusions

The evidence for the effect of HIIT versus non-exercise control on systolic blood pressure is very uncertain.

• Mean difference of -5.22 mmHg (95% CI -12.27 to 1.84)
• Based on 7 studies, 215 participants
• Rated as very low-certainty evidence, precluding firm conclusions

HIIT may result in a slight increase in cardiorespiratory fitness (VO2max) compared with MICT.

• Mean difference of 1.39 mL/min/kg (95% CI 0.44 to 2.34)
• Based on 37 studies, 1115 participants
• Rated as low-certainty evidence
• The improvement was smaller in magnitude than the HIIT versus non-exercise comparison (1.39 vs 5.98 mL/min/kg)

HIIT compared with MICT likely results in little to no difference in waist circumference.

• Mean difference of 0.06 cm (95% CI -1.49 to 1.62)
• Based on 15 studies, 407 participants
• Rated as moderate-certainty evidence

HIIT compared with MICT likely results in little to no difference in waist-to-hip ratio.

• Mean difference of 0.00 (95% CI -0.01 to 0.02)
• Based on 5 studies, 155 participants
• Rated as moderate-certainty evidence

HIIT compared with MICT may result in little to no difference in systolic blood pressure.

• Mean difference of -0.56 mmHg (95% CI -3.02 to 1.90)
• Based on 18 studies, 515 participants
• Rated as low-certainty evidence, precluding firm conclusions

HIIT compared with MICT may result in little to no difference in circulating triglycerides.

• SMD of 0.00 (95% CI -0.28 to 0.27)
• Based on 18 studies, 526 participants
• Rated as low-certainty evidence, precluding firm conclusions

No studies reported all-cause mortality or adverse events in either comparison.

• All-cause mortality was a pre-specified main outcome but was not reported in any of the 58 included RCTs
• There were no reports of adverse events across any included studies
• The authors noted uncertainty about whether studies actively monitored for adverse events
• This gap was identified as a limitation of the existing evidence base

The review included 58 RCTs with 2075 participants, with studies commonly having few participants and risk of bias concerns.

• 35 studies evaluated HIIT versus MICT, and 11 evaluated HIIT versus no exercise
• 12 studies evaluated HIIT versus MICT versus another control and were included in both comparisons where possible
• Evidence was downgraded for inconsistency, imprecision, and risk of bias, mainly due to lack of detail on randomisation and no available protocol or trial registration
• All studies examined supervised HIIT interventions; no studies investigated unsupervised HIIT