Nutrients, Vol. 14, Pages 5119: Beetroot Juice Produces Changes in Heart Rate Variability and Reduces Internal Load during Resistance Training in Men: A Randomized Double-Blind Crossover

1. IntroductionBeetroot juice (BJ) and its influence on sport performance has been studied [1], showing a great interest in the sport nutrition field and the sport performance in different disciplines, and not least, being also interesting on general population health reducing different cardiovascular and coronary diseases [1,2]. Mainly, the effects of BJ and its interest on the field of health, sport nutrition and sport performance are mostly based by the principal active principle present of BJ supplementation, which is the dietary inorganic nitrate (NO3−) [3]. After its ingestion, the NO3− goes through a complex digestion and convoluted metabolic pathways. It is first absorbed by the salivary circulation and subsequently reduced to nitrite (NO2) by the action of nitrate reductase facultative anaerobic bacteria present at the dorsal surface of the tongue [4]. Then, at the stomach, NO2 is decomposed into nitric oxide (NO) and finally reaches the plasma and systemic circulation [5,6].In sport performance, the physiological effects of BJ supplementation and NO could be summed up in a dilation of the vascular endothelium [7,8], causing a vasodilator effect and reducing the blood pressure (BP) [9,10]. In addition, these physiological effects are interesting for sport performance because it could increase muscle blood flow [11], alter and improve lactate removal in the exercise [12]. In addition, the intake of NO3− effects has been proved in different endurance sports, where cardiovascular system has an important role in performance [1,7,13], showing a lower oxygen consumption (VO2) during exercise [1] with an improvement in adenosine triphosphate (ATP) synthesis [14].Nevertheless, in strength sports or resistance training (RT), BJ supplements have received less attention [3]. However, research has shown BJ as an effective ergogenic aid in RT, observing an increase in muscle strength, explosive force and muscular endurance due to better ATP utilization, increased blood volume, and therefore, oxygen during exercise [15,16,17,18,19].In the cardiovascular system, NO regulates various functions such as contractile force, myocardial relaxation, mitochondrial respiration and coronary perfusion [20], and it clearly has a cardioprotective role in pathologies such as myocardial infarction and heart failure [21]. In animal models of cardiac ischemia–reperfusion, treatment with NO2 produced a clear cardioprotective effect [22,23,24,25]. However, the results of human studies are not so obvious [26].Normally, a healthy heart is not a regular metronome with the same interval between heartbeats, but it experiences variations of the normal rhythm of the heart. To measure and control these different oscillations of the period between consecutive heartbeats, heart rate variability (HRV) is a proved valuable tool [27]. HRV can be used as a mirror of the cardiorespiratory control system and parasympathetic function of the autonomic nervous system (ANS), being a useful signal for understanding the status of the ANS [28]. According to a meta-analysis [29], HRV data obtained in men and women cannot be treated equally and that studies need to characterize these differences. In autonomic control of the heart indexed by HRV measures, it has been observed that women showed a significantly lower mean RR interval, showing greater vagal activity [30].HRV has a direct relationship between sport performance and physical activity and their physiological effects, decreasing with stress activities such as exercise and when respiratory increases [31]. Recently, HRV has been considered useful to determine the internal load of physical activity [32], evaluating the modulation of the sympathetic and parasympathetic system, specifically, to know the activation of the parasympathetic system in the athlete’s recovery [33,34,35,36]. Using HRV, certain data obtained in his measurement, such as the Root Mean Square of the Successive Differences between adjacent RR intervals (RMSSD), could be one of the best reliable measures of parasympathetic activity. A measurement in a short period of time is sufficient [37]. In this way, Naranjo Orellana et al. have determined the measure of internal load based on the recovery of the RMSSD-Slope on 30 min post-exercise to monitor the effect of workloads and fatigue caused by exercise [32].Although an electrocardiogram, measured by measurement instruments such as the Holter system, would be the reference method for assessing HRV [38], it not suitable for daily measurement and sport. In this regard, new technologies such as wearable devices [39,40,41,42] and smartphone applications [43,44] have taken the spotlight, becoming the reference method for measuring RR intervals. Chest straps such as the Polar H10 (Polar Inc., Kempele, Finland) can be considered the gold standard for RR interval assessments if intense activities with strong body movements are investigated. Polar H10 has demonstrated the validity of for the detection of RR intervals in a wide range of physical activities and sports [39]. HRV parameters and several nonlinear parameters can be further interpreted and analyzed using advanced HRV analysis software such as Kubios HRV (University of Eastern Finland, Kuopio, Finland) [45]. This software supports several input data formats for electrocardiogram data and beat-to-beat RR interval data.Despite the knowledge of the ergogenic component of BJ and the changes produced in BP after its consumption pre-exercise, certain physiological post-exercise recovery effects have not been evaluated during RT, especially in relation to HR and HRV, which is a critical period in which various modifications occur, including changes in autonomic modulation [46], which can promote an environment conducive to the development of abnormal alteration in both BP, HR and HRV [46,47]. Thus, due the possible effects of BJ supplementation in addition to RT in cardiovascular system, the purpose of this study was to investigate the possible effects of BJ acute supplementation during RT in BP and the variations in HR, HRV and internal load during RT measured by changes in HRV in man. We hypothesized that the BJ consumption could produce changes in the HRV and produce a reduction in the internal load during RT. 4. DiscussionThis study aimed to investigate the possible effects of acute BJ supplementation on BP and variations in HR, HRV, and internal load during RT. The results of the study showed that BJ supplementation before training (120 min before) could produce changes in HRV, reducing RMSSD during exercise, but showing no changes in BP or RMSSD after exercise. Therefore, it seems that BJ could reduce the internal load measured through the RMSSD-Slope during RT, thus obtaining an improvement in RT performance, mainly in muscular endurance, as previously published [52].Although there was no effect compared to placebo, BJ has the potential to decrease BP, vascular resistance and myocardial oxygen demand in both recovering and exercising subjects [64]. Vanhatalo et al. (2010) conclude that BJ reduces only SBP [65], whereas Webb et al. (2008) indicated that reductions in both systolic and diastolic BP were observed, respectively, 2.5 and 3 h post BJ supplementation [66]. Furthermore, systolic BP remained decreased after 24 for hours post ingestion, while diastolic BP returned toward baseline [67]. As a whole, these data invite us to think that BJ or nitrate-rich supplementation diet is more suitable to changes in systolic BP than diastolic. The result of this decline in BP can attenuate the O2 cost of an increment in work rate by 20% especially the younger one is. In fact, Stanaway et al. (2019) have described that SBP was reduced in young and older adults following BR supplementation, while DBP was reduced only in the older ones [68].A possible explanation for the results of the present study may be the reactive oxygen and nitrate species (RONS) produced by RT [69]. Due to the antioxidant effect of NO3− and betalains [70,71,72], it could be theorized that the lack of effect in our study is due to the oxidation of these components of the BJ carried out by the RONS produced during the RT. There is some controversy in the mechanism by which BJ reduce BP, because some reviews [9] and studies [65,68] suggest that this effect is due to NO3− content, but a recent systematic review and metanalysis carried out by Bahadoran et al. (2017) [73] highlights that this effect could be NO3− independent and may be related to other bioactive components such as betalains or other antioxidants [74,75].RT has been probed to decrease acute parasympathetic modulation regardless of the age [76]. In the present study, it was shown how the BJ consumption modified the maximum HR and decreased RT-intensive exercise-mediated RMSSD in trained young men, reducing internal load compared with a control group. Similar to this, Benjamin et al. (2020) found that beetroot extract accelerates the return of parasympathetic modulation during recovery after an RT protocol in healthy adult men [77]. Although changes in HRV during aerobic exercise have been studied, an increase in HRV was observed during the day [78,79]. Carrijo et al. (2021) conclude that a single dose of BJ, independent of NO3− content, does not change aerobic exercise-mediated responses in HRV indexes in time, frequency, and non-linear domains in hypertensive postmenopausal women [80], which is aspect not studied in trained men during RT.In this regard, the BJ group had a significantly lower RMSSD during exercise than the placebo group. Exercise enhances the activity of the sympathetic nervous system [81,82] while reducing the vagal tone especially as the HR and exercise workload increase [83]. With RMSSD being a measurement of parasympathetic activation [84], besides the fact that the BJ group accumulated a greater total number of repetitions [52] and achieved a significantly higher HR than the placebo group, it could explain the reduced RMSSD during exercise in this group. Anyway, there were no differences between groups in post-exercise RMSSD, and the BJ group showed an increase in RMSSD-Slope between groups, which would mean a decrease in the internal training load in the BJ group [32,37]. A possible explanation for this could be the effect of NO on the autonomous nervous system, which could inhibit the sympathetic activity while increasing the vagal outflow [85,86] and thus enhance the recovery after exercise.Despite the fact that, to our knowledge, no other clinical trials have studied the effects of BJ during RT in HRV. The relationship between HRV, sport and the utilization of some sport supplements has been analyzed before. In this regard, other commonly used supplements such us creatine limits the parasympathetic modulation of the RT exercise [87], whereas caffeine has contradictory results by increasing the parasympathetic modulation after anaerobic exercise [88] but delaying parasympathetic recovery after aerobic exercise [89]. Other not as common supplements such as the black thai ginger show a similar effect to caffeine in the response of autonomic nervous system to anaerobic exercise [90]. In this sense, studies that combine different supplements are required.To our knowledge, this was the first study to evaluate the acute effects of BJ on HRV and internal load during resistance training. The results should be interpreted with caution. The main limitation of this study is the fact that only one measure of the HRV was realized in both groups and, with HRV being a very variable between days, it can bias the results of this trial. We suggest that future research on this topic should replicate HRV measurements not only to know the acute effects but also chronic ones. Another possible limitation to the study is the exercise order selection. Due to the back-squat exercise, which was performed previous to the bench press, and because of the back-squat involving a larger muscle volume [91,92], the accumulated neuromuscular fatigue and metabolic by-products [93] could cause different interpretation of the results if HRV was measured after back-squat instead of bench press.

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