Effects of high-intensity interval training (HIIT) and sprint interval training (SIT) on fat oxidation during exercise: a systematic review and meta-analysis

Correspondence to Professor Zeljko Pedisic, Institute for Health and Sport, Victoria University, Melbourne, VIC 3011, Australia; [e-mail]

WHAT IS ALREADY KNOWN

• High-intensity interval training (HIIT) and sprint interval training (SIT) seem to increase fat oxidation during exercise (FatOx), but it is unclear what is the magnitude of their effect on FatOx and whether they are more effective for improving FatOx than moderate-intensity continuous training (MICT).

WHAT ARE THE NEW FINDINGS

• HIIT and SIT are effective for improving FatOx even when applied for only 4 weeks, with the mean effect ranging from 0.05 g/min for short training regimens to 0.13 g/min for training regimens lasting more than 12 weeks.

• Compared with MICT, HIIT and SIT are only slightly more effective in improving FatOx (on average 0.03 g/min), but they usually require almost two times lower time commitment.

• Significantly greater improvements in FatOx following HIT and SIT can be expected among individuals with overweight or obesity, compared with individuals with ‘normal’ weight.

Introduction

In recent years, effects of high-intensity interval training (HIIT) and sprint interval training (SIT) on cardio-metabolic outcomes have been widely investigated.^{1 2} A large number of adults engage in this type of exercise to improve their health and fitness.³ Both HIIT and SIT involve short bursts of high-intensity exercise interspersed with rest intervals.^4–6 HIIT is usually performed at intensities below or at maximal oxygen uptake (VO_2max) or peak power output (PPO), while SIT requires ‘all-out’ efforts at supramaximal work rates.^{4 5} For brevity, when referring to both HIIT and SIT hereafter, we used the term ‘interval training’, unless stated otherwise. It should be noted that there are other modes of interval training, such as Fartlek, CrossFit style exercise and various high-intensity functional training workouts. However, they are likely to elicit different acute responses compared with HIIT and SIT, and they were therefore beyond the scope of this review.

Studies performed among athletes and in the general population show that HIIT and SIT induce various adaptations such as improvements in VO_2max, body composition, mitochondrial biogenesis, exercise performance and fat oxidation during exercise (FatOx).^7–10 Increased whole-body FatOx with a concomitant reduction of carbohydrate utilisation during exercise or at rest is a hallmark adaptation observed following endurance training.