High‐intensity interval training vs. moderate interval training: the lack of equalisation, an ongoing problem

Dear Editor: Initially, we highlighted the systematic review and meta‐ analysis by Wewege et al., ‘The effects of high‐intensity interval training vs. moderate‐intensity continuous training on body composition in overweight and obese adults’, which evaluated the effects of intervention programs involving high‐intensity interval training (HIIT) and moderate‐intensity continuous training (MICT) on body weight and composition of overweight and obese adults (1). Body weight and body composition of overweight and obese adults are worldwide public health problems (2), and the preventive and therapeutic roles of physical exercise should be considered (3). In this meta‐analysis, no significant differences were observed between HIIT and MICT for the effects on body mass and body fat. Body mass was not significantly reduced in any of the interventions (HIIT: −0.17 [95% confidence interval: −0.36 to 0.03 kg]; MICT: −0.18 [95% CI: −0.37 to 0.02 kg]). However, the effectiveness of the HIIT and MICT training programswas evaluated using an intra‐group analysis. Pre‐ and post‐intervention results were compared without a control group (non‐exercise), which could contribute to the reduction of the risk of bias, allowing for a better understanding of the real effects of the trainings. In only 7 out of the 13 studies included in the meta‐ analysis, some equalisation by workload or energy expenditure was observed (1). This is extremely relevant in the analysis of the results because the process of body mass regulation is closely associated with energetic balance (energy intake/energy expenditure) (4). This was demonstrated in other studies, including Kong et al. (5), where MICT was compared with HIIT. Kong et al. found an energy expenditure approximately 102% higher for MICT than HIIT (HIIT: 12,919 ± 2,159 kJ; MICT: 26,125 ± 2,986 kJ) as well as an approximately 124%higher weekly training volume for MICT (HIIT: 66 ± 4 min; MICT: 148 ± 12 min). Sawyer et al. (6) verified that MICT sessions resulted in approximately 33% higher energy expenditure than HIIT sessions (HIIT: 753 kJ; MICT: 1,004 kJ), which is a possible source of bias in the results. Our concern is that the main argument in favour of HIIT is the short duration of training sessions. The central idea of the HIIT proposal is not ‘to train in less time’ but ‘to train for a longer period at high‐intensity’. Some researchers have suggested that the energy expenditure in the ‘short training session’ could not be high. For example, a session in a cycle ergometer that consisted of a 5‐min warm‐up at 50–60% of the HRmax + 10 × 1 min at 90–95% of the HRmax: 1 min at approximately 25–50 W + a 5‐min cool‐down at 50–60% of the HRmax resulted in an energy expenditure of only 753 kJ in obese men and women, which is equivalent to an oxidation of approximately 23 g of fat (6). We emphasise that we do not want to question the benefits of HIIT, but we want to warn about serious methodological failures that have been constantly repeated, especially in studies that have analysed the effect of HIIT on weight loss, which limit a correct comparison between HIIT and MICT results in overweight and obese adults. Therefore, future studies involving obese patients should adopt iso‐caloric protocols to compare the different types of training. Review studies need to consider sub‐analyses that include only iso‐caloric protocols. In their study, Wewege et al. (1) compared exercise modes (treadmill running vs. cycle ergometer) and observed a large effect on body fat (kg) and a small effect on body mass for HIIT and MICT treadmill running protocols. However, no significant effects were identified for HIIT or MICT cycle ergometer protocols on these measures. Nevertheless, the interpretation of these results has a high bias because the protocols (treadmill vs. cycle ergometer) were not equalised. For example (considering the information presented by Wewege et al. (1) in Table 2), compared with the cycle ergometer protocols, HIIT running in treadmill protocols had 25.8%more follow‐up time (running: 11.7 weeks; cycle ergometer: 9.3 weeks) and 55.6% more exercise time per week (running: 117.9 min/week; cycle: 75.8 min/week), with 107.2% more exercise time during the entire program (running: 1,471 min; cycle ergometer: 710 min). In the MICT group, compared with the cycle ergometers, running on a treadmill had 25.8% more follow‐up time (running: 11.7 weeks; cycle ergometer: 9.3 weeks) and 10.5% more exercise time per week (running: 166.8 min/ week; cycle: 151.0 min/week), with 53.3% more exercise time during the entire program (running: 2,004 min; cycle ergometer: 1,307 min). The superiority in the effectiveness of the treadmill compared to the cycle ergometer, as found by Wewege et al., can be easily associated with the longer exercise exposure time in the treadmill protocols. obesity reviews doi: 10.1111/obr.12583