Key Takeaway
HIIT and SIT significantly increase fat oxidation during exercise (MD = 0.08 g/min, p < 0.001), with meaningful effects emerging after 4 or more weeks of training.
Summary
This systematic review and meta-analysis investigated whether high-intensity interval training (HIIT) and sprint interval training (SIT) improve fat oxidation rates during exercise. Fat oxidation is an important metabolic marker linked to body composition, metabolic health, and endurance performance. The review pooled data from 18 studies to quantify the effect.
The meta-analysis found a significant pooled increase in fat oxidation of 0.08 g/min (p < 0.001) following HIIT/SIT interventions compared to controls. Subgroup analyses revealed that training regimens lasting 4 weeks or longer produced statistically significant improvements, while shorter protocols did not reach significance. This suggests a minimum training duration is needed for metabolic adaptations that enhance fat utilization.
These findings provide strong evidence that HIIT and SIT shift substrate metabolism toward greater fat reliance during exercise, which has implications for weight management and metabolic health. The results support these protocols as effective strategies for improving the body's capacity to oxidize fat, a key adaptation for both athletic performance and cardiometabolic health.
Methods
Systematic review and meta-analysis of 18 studies comparing HIIT/SIT interventions to control conditions. Fat oxidation was measured during submaximal exercise using indirect calorimetry. Random-effects models were used to pool mean differences. Subgroup analyses examined training duration (< 4 weeks vs. ≥ 4 weeks), protocol type (HIIT vs. SIT), and participant characteristics.
Key Results
- Significant pooled increase in fat oxidation: MD = 0.08 g/min (p < 0.001)
- Training programs ≥ 4 weeks showed significant effects on fat oxidation
- Programs < 4 weeks did not reach statistical significance
- Both HIIT and SIT protocols contributed to improved fat oxidation
- Effects observed across different populations including sedentary and active individuals
Limitations
- Relatively small number of included studies (n = 18)
- Heterogeneity in HIIT/SIT protocols, intensities, and modalities across studies
- Fat oxidation measured during exercise only (not resting fat oxidation)
- Most studies were short-term (4-12 weeks), limiting long-term conclusions
- Potential confounding from dietary differences between studies
- Indirect calorimetry has inherent measurement limitations at high intensities