The effects of short term high-intensity interval training on energy metabolism and irisin, preptin and adropin

High-intensity interval training (HIIT) induces similar or even superior adaptations compared to continuous endurance training. Indeed, just 6 HIIT sessions over 2 weeks significantly improves maximal oxygen consumption (VO 2 max), endurance performance and submaximal exercise fat oxidation. Whether even faster adaptations can be achieved with HIIT is not known. Thus, the primary aim of this study is to determine whether 2 sessions of HIIT per day, separated by 3 h, every other day for 5 days (double HIIT, HIIT-D, n = 15 participants) can increase VO 2max, submaximal exercise fat oxidation and endurance capacity as effectively as 6 sessions of HIIT over 2 weeks (single HIIT, HIIT-S, n = 13). We also aimed to determine the effects of this exercise model on irisin, preptin and adropin molecules known to affect metabolism, and their relations with VO 2 max, endurance performance and submaximal exercise substrate oxidation. Each training session consisted of 10*60 s of cycling at 100% of VO 2max interspersed with 75 s of low-intensity cycling at 60 W. Pre- andpost-training assessments included VO 2max, time to exhaustion at ~80% of VO 2max and 60-mincycling trials at ~67% of VO 2max. The blood samples were collected pre and post the HIIT programs and immediately before and after the 2nd and 6th HIIT sessions. In order to reveal the effect of exercise interventions and the difference between groups, ANOVA was used for repeated measurements, and Pearson correlation analysis was used to determine the relation between variables. Similar increases (p < 0.05) in VO 2max (HIIT-D 7.7% vs. HIIT-S 6.0%, p > 0.05) and endurance capacity (HIIT-D 80.1%, HIIT-S 79.2%, p > 0.05) were observed in the two groups. Submaximal exercise CHO oxidation was reduced in both groups after exercise training (HIIT-D 9.2%, p < 0.05 vs. HIIT-S 18.8%, p < 0.05) while submaximal exercise fat oxidation was significantly increased in HIIT-D (15.5%; p = 0.05) but not in HIIT-S (9.3%; p > 0.05). The single and double HIIT programs did not affect resting irisin, preptin or adropin levels (p>0.05). However, irisin was increased immediately after the 2 and HIIT both in the HIIT-S and HIIT-D groups (p<0.05) with no difference between the groups (p>0.05). Adropin also increased significantly after the 6 th HIIT only in HIIT-D group, while preptin markedly decreased only in the HIIT-S group immediately after the 2 nd HIIT (p<0.05). Furthermore, irisin was positively associated with fat oxidation (r=0.390) and negatively associated with CHO oxidation (r=- 0.431) during the submaximal exercise (p<0.05). Also, irisin (r= 0.427), preptin (r= 0.516) and adropin (r= 0.450) were positively correlated with aerobic capacity (p<0.05). Preptin also showed positive correlation with exercise exhaustion time (r= 0.382; p<0.05). As a result, the study findings reveal that 6 HIIT sessions over 5 days was as effective in increasing VO 2max and endurance capacity and was more effective in improving submaximal exercise fat oxidation than six HIIT sessions over 2 weeks. It also showed that irisin, preptin and adropin molecules were associated with aerobic capacity and likely to be induced by an acute HIIT intervention, yet resting levels of these molecules were not affected by short-term HIIT programs over 2 weeks or 5 days.