We read the review article by Guo et al1 with great interest, which addressed the role of different exercise training (ET) modalities in the treatment of heart failure with preserved ejection fraction (HFpEF). The prevalence of HFpEF has increased, and few effective treatment options are currently available. ET has been considered as an adjuvant therapy in the management of HFpEF.2 Although there is consistent evidence that ET improves cardiorespiratory fitness, physical capacity, and quality of life, the impact of ET on diastolic function in patients with HFpEF remains controversial.3–5 The study by Guo et al1 provides valuable information showing that, in addition to increasing functional capacity and quality of life, ET reduces the early diastolic mitral inflow to the annular velocity ratio (E/e'),1 which is considered an important indicator of diastolic function.6 This finding corroborates previous reviews that also observed improvement in E/e' and other indicators of diastolic function with ET in patients with HFpFE.3,4 Furthermore, a review by Guo et al1 reveals a trend of reduction in left atrial volume indexed promoted by ET in patients with HFpEF,1 considered a predictor of cardiovascular risk in patients with diastolic dysfunction.6 Regarding the effect of ET on cardiorespiratory fitness, the review by Guo et al1 demonstrates that ET is able to significantly increase the peak of oxygen consumption (VO2peak) in patients with HFpEF, which also corroborates the findings of previous reviews.3,7 In addition, the study by Guo et al1 found that only moderate-intensity continuous training (MICT) significantly increased the VO2peak compared with high-intensity interval training (HIIT). This result is in disagreement with previous reviews that showed superiority or similarity of HIIT compared with MICT in increasing cardiorespiratory fitness,8,9 even in patients with cardiovascular diseases.9 Pattyn et al9 found that HIIT was superior to MICT in increasing VO2peak in patients with coronary artery disease {MD [95% confidence interval (CI)], 1.25 (0.49–2.02)]}and with HFrEF [MD (95% CI), 1.46 (0.10–2.82)], but no significant difference between HIIT and MICT was observed in patients with HFpEF [MD (95% CI), 0.37 (−1.59 to 2.32)]. However, it is important to mention that only 2 studies with patients involving HFpEF were included in the analysis. In the subgroup analysis performed by Guo et al,1 3 studies compared HIIT and MICT in patients with HFpFE.10–12 Pooled analysis revealed that only MICT significantly increased VO2peak [SMD (95% CI), 0.42 (0.05–0.54)]. Differently from what was found by Guo et al,1 our analyses did not reveal a significant difference between pre-MICT and post-MICT [SMD (95% CI), 0.29 (−0.04 to 0.62)], pre-HIIT and post-HIIT [SMD (95% CI), 0.35 (−0.01 to 0.71)], and between HIIT and MICT [SMD (95% CI), 0.94 (−0.49 to 2.36)] in improving VO2peak (Fig. 1). In addition, of the 3 studies included in the analysis,10–12 2 of them found superiority for HIIT10,11 and 1 of them found no difference between HIIT and MICT in increasing VO2peak.12FIGURE 1.: A, Forest plot for within-group and (B) between-group effects of HIIT and MICT interventions on peak oxygen uptake (VO2peak).Another point that should be highlighted is the difference in the doses of HIIT and MICT used in these studies (Table 1). In the study by Angadi et al,10 the dose of MICT was 25% lower than that of HIIT (∼5.850 ± 170 vs. ∼7.800 ± 636, respectively), which could partially explain the superiority of HIIT compared with MICT in improving VO2peak. On the other hand, in the study by Donelli da Silveira et al,11 the doses of MICT and HIIT were equivalent (∼8.610 ± 806 vs. ∼9.165 ± 997, respectively) and HIIT was superior to MICT in VO2peak. In the study by Mueller et al,12 the MICT protocol had a dose 47.7% higher than the HIIT protocol (∼13.000 ± 1414 vs. ∼8.802 ± 738, respectively), which could explain the greater increase, although not statistically significant, in VO2peak after MICT compared with HIIT. This means that the effect of HIIT and MICT on VO2peak seems to be dose-dependent. In fact, Pattyn et al9 found superiority of HIIT compared with MICT in increasing VO2peak when studies were grouped into isocaloric groups [MD (95% CI), 2.08 (0.91–3.25)]; however, there was no difference between HIIT and MICT on VO2peak for the no isocaloric group of studies [MD (95% CI), 0.21 (−0.48 to 0.90)]. Interestingly, even with an almost 50% higher dose of MICT compared with HIIT, the changes in VO2peak were similar,12 reinforcing the time-efficient nature of HIIT, because this modality is capable of promoting the same changes in VO2peak with almost half the ET dose in patients with HFpEF. TABLE 1. - Characteristics of Studies Included in the Meta-Analysis References Age (y) Number of Subjects Length/Frequency/Mode ET HIIT MICT Mueller et al 12 69.5 ± 8.8 N = 116 (76 female) 12/48 wk Total duration: 38 min Total duration: 40 min HIIT = 58 HIIT: 3×/wk Warm-up: 10 min at 60%–70% HRmax MICT: 60%–70% HRmax MICT = 58 MICT: 5×/wk HIIT: 4 × 4 min at 90%–98%HRmax, interspersed by 3 min at 60%–70% HRmax Stationary cycle ergometer Cool down: 3 min at 60%–70% HRmax Dose (F × I × T): ∼8.802 ± 738 Dose (F × I × T): ∼13.000 ± 1.414 Donelli da Silveira et al 11 60.0 ± 9.5 N = 19 (12 females) 12 wk Total duration: 38 min Total duration: 47 min HIIT = 10 3×/wk Warm-up: 10 min at 60%–70% HRpeak MICT: 47 min at 50%–60% HRpeak MICT = 9 Running HIIT: 4 × 4 min at 85%–95% of HRpeak interspersed by 3 min at 60–70 HRpeak Cool down: 3 min at 60%–70% HRpeak Dose (F × I × T): ∼8.610 ± 806 Dose (F × I × T): ∼9.165 ± 997 Angadi et al 10 70.0 ± 8.3 N = 15 (12 females) 4 wk First wk First wk HIIT = 9 3×/wk Total duration: 16 min Total duration: 15 min MICT = 6 Running HIIT: 8 × 2 min at 80%–85% HRpeak, interspersed by 2 min at 50% HRpeak MICT: 60% HRpeak From second to fourth wk Second wk Total duration: 38 min Total duration: 30 min Warm-up: 10 min at 50% HRpeak MICT: 70% HRpeak HIIT: 4 × 4 min at 85%–90% HRpeak, interspersed by 3 min at 50% HRpeak Cool down: 5 min at 50% HRpeak Dose (F × I × T): ∼7.800 ± 170 Dose (F × I × T): ∼5.850 ± 636 HRmax, maximum heart rate; HRpeak, peak heart rate; F × T × I, frequency × intensity × time. Finally, we emphasize the contributions that the review by Guo et al1 bring to this topic, but we disagree with the findings related to the superiority of MICT in improving VO2peak in patients with HFpEF, based on the evidence presented and the analyses we conducted. Based on this, it seems more reasonable to say that there is insufficient evidence regarding the superiority of either MICT or HIIT in improving VO2peak in patients with HFpEF. However, it is possible to state that HIIT could be considered a time-efficient modality in improving cardiorespiratory fitness in patients with HFpEF. Further well-conducted studies with dose-equalized protocols are needed to elucidate this unresolved issue.