High-intensity interval training and cardiac autonomic control in individuals with metabolic syndrome: A randomised trial☆
Introduction
Individuals with the metabolic syndrome (MetS) have been recently reported to be at higher risk of sudden cardiac death (SCD) relative to individuals without the condition [1]. There is also evidence to suggest that impaired cardiac autonomic function (CAF), characterized by an imbalance between sympathetic and parasympathetic nervous activity [2] is one of the major contributing factors towards increased risk of SCD [3]. Moreover, impaired CAF has also been put forth as an underlying cause of MetS [4]. This is perhaps due to the over-activity of the sympathetic nervous system influencing the increased magnitude of insulin resistance, which has been postulated to be the central mediating factor towards the emergence of other MetS components [5]. This notion is supported by studies, which suggest, that altered CAF or increased sympathetic activity may precede [6] or predict [7] the onset of insulin resistance. Collectively, these studies indicate that CAF improvement should be targeted to reduce or prevent the development of MetS, and thus risk of SCD.
The analysis of heart rate variability (HRV) is a feasible, non-invasive, and reproducible method to measure CAF [8]. HRV refers to the variation in time interval between consecutive heartbeats, corresponding to the balance between parasympathetic and sympathetic influences on the sinoatrial (SA) node [9]. Increased HRV therefore reflects the ability of the autonomic nervous system and SA node to dynamically respond to environmental changes, generally indicating a healthy heart. Whereas reduced HRV has been associated with poorer cardiovascular health [10], [11], [12], [13] and increased risk of SCD [3].
Higher cardiorespiratory fitness (CRF) has also been associated with decreased risk of SCD [14], suggesting that aerobic exercise may be a potent stimulus to improve cardiac autonomic modulation. Indeed, meta-analyses have shown that aerobic exercise has the capacity to improve CAF depicted as improved HRV in healthy individuals [15] and patients with coronary heart failure [16]. However, the current aerobic exercise recommendation of 150 min/week of moderate-intensity continuous training (MICT) has been difficult for the majority of adults to adhere to, with ‘lack of time’ as the most commonly reported barrier [17]. Interestingly, a high-volume high-intensity interval training protocol (4HIIT, 4 × 4 min intervals at ~ 90% maximum/peak heart rate [HRpeak/max], separated by 3 min active recovery) performed for 120 min/week (including warm-up and cool down) is a more effective stimulus for improving CRF compared to MICT in individuals with the MetS [18]. Moreover, a lower volume HIIT protocol (1HIIT, 1 × 4 min interval at ~ 90%HRmax) has also been demonstrated to be effective in improving CRF, despite only requiring less than half of the total weekly time commitment (57 min/week) [19]. It could therefore be inferred from these studies that a low-volume of HIIT may be sufficient to induce CAF improvement. However, no studies have yet to examine the minimum volume of HIIT to elicit a beneficial effect on CAF. This is important because understanding more about the dose response of exercise to various health-related cardiovascular outcomes is necessary for optimising exercise prescription guidelines.
The primary aim of this study was to therefore investigate the impact of different volumes of HIIT and MICT on CAF, depicted as the change in HRV, in patients with the MetS. The changes in MetS risk factors, CRF, and anthropometric measures were also assessed as secondary outcome measures. It was hypothesised that both HIIT volumes would improve CAF and all secondary outcome measures, more than MICT.
Section snippets
Methods
This is a sub-study of the ‘Exercise in prevention of Metabolic Syndrome (EX-MET)’ international multicenter trial (ClinicalTrials.gov NCT01676870), which reports the change in CAF modulation following a 16-week training program in participants with MetS (men and women age ≥ 30 years), diagnosed according to the International Diabetes Federation criteria [20]. These participants were recruited from January 2013 to August 2015 as part of the EX-MET trial. The HRV indices to represent CAF change
Results
Fifty-six participants had a complete data set for HRV (Fig. 1). Baseline demographics and medication intake (Table 1) were similar between groups. Participants in the MICT, 4HIIT, and 1HIIT completed 89%, 89%, and 90% of the prescribed training sessions (between groups, p = 0.75), respectively. In participants included in this sub-study, there were no reported physical injuries that were directly related to the exercise prescribed.
Discussion
The main finding was that there were no significant differences for the effects of exercise dose on CAF indices. However, a combination of higher exercise intensity and volume (4HIIT) had the greatest within group effect. It should be highlighted that this finding was derived from linear and non-linear HRV measures, which have all been shown to be independently associated with risk of sudden cardiac death [3] and mortality after myocardial infarction [12]. Thus given the increased
Conclusion
Although there were no statistically significant between-group differences in CAF indices, a combination of a greater exercise volume and intensity, as seen in 4HIIT, showed the greatest within-group effect. This suggests that 4HIIT relative to the other training interventions employed in the present study, may provide more cardio-protection by improving CAF in individuals with MetS. This is a clinically significant finding given that patients with MetS have been reported to be more susceptible
Conflict of interest
The authors declare no conflict of interest associated with this manuscript.
Acknowledgements
JSR wrote the manuscript, collected and analyzed/interpreted the data and reviewed/edited and approved the final draft of the manuscript. JSR and JSC designed the study. JSC, LCD, FB, KSB, GIM, KAD, MPW, SEK, and RGF also substantially contributed to the conception and design, analysis and interpretation of data and reviewed/edited and approved the final draft of the manuscript. JSC is the guarantor of this study.
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Funding: Funding for this study was provided by the Norwegian University of Science and Technology and from an unrestricted research grant from the Coca-Cola Company.
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This author takes responsibility for all aspects of the reliability and freedom from bias of the data presented and their discussed interpretation.