Our study identified that beetroot extract accelerates the recovery of cardiovascular and autonomic parameters in recovery to exercise by the following observations:
a) The means HR, SBP, DBP, and MAP values were reduced quicker in the beetroot protocol immediately after exercise;
b) HF indices (typical of vagal modulation) recovered earlier after exercise cessation in the beetroot vs. placebo protocol.
The antioxidant properties of beetroot have been presented as substances capable of optimizing cardiovascular responses and the autonomic response to physical exercise28. Throughout physical exertion, the metaboreflex is activated, decreasing baroreflex sensitivity at the brainstem level. Metaboreceptors are activated due to high cellular metabolism and metabolite accumulation. Sympathetic activity is increased by the stimulation of unmyelinated afferent fibers, increasing HR, cardiac output, and BP. Upon cessation of exercise, metabolites are progressively removed and metaboreflex activation decreases, restoring baroreflex activity and increasing cardiac interval variability29. Consequently, the ability to hasten the removal of metabolites and assist the resumption of parasympathetic modulation of heart rate can be attributed to the antioxidant effect of beetroot compounds28,30,31.
Additionally, NO3 appears to contribute to the slowing of cardiac activity following the stress triggered by physical exercise by increasing NO concentrations in the circulation. One of the benefits of substantially increasing NO in the body is improving endothelial function32. The endothelium plays a vital role in cardiovascular physiology and pathophysiology, and its ability to maintain cardiac homeostasis is chiefly dependent on NO production32. These aspects validate the quickening of cardiovascular parameters' recovery after the consumption of beetroot compounds.
Another pathway prejudiced by NO's action is the solitary nucleus pathway that receives afferent nerves from the arterial baroreflex and plays a key role in the baroreceptor reflex. Hence, it can influence blood pressure through baroreceptors33. Therefore, founded on the fundamental role that NO plays in the homeostasis of the cardiovascular system, the consumption of beetroot extract can cause deviations in the baroreflex, increasing HRV and decreasing the HR and BP of individuals.
Previous studies have demonstrated promising results from beetroot compounds on the behavior of vagal HRV indices in response to exercise. Bond et al.7 presented 500 ml of beetroot juice (~750 mg nitrate) to 13 African-American women and, after 120 minutes, subjected them to exercise sessions on a stationary bicycle at 40% and 80% intensity. Vo2 Max. SDNN index values were higher before and during exercise with beetroot juice than during placebo treatment.
These results comply with the effects detected in a study previously published by Benjamin et al.15. Twelve healthy male adults were evaluated over two days in randomized protocols (beetroot extract 600 mg in capsule and placebo 600 mg starch in capsule). Next, the subjects remained seated for 120 minutes at rest, followed by a strength exercise for the lower limbs at an intensity of 75% of 1RM. Afterwards, they were once more at rest for 60 minutes. It was conceivable to perceive that in the group that ingested the beetroot extract, there was an acceleration of the recovery of the SDNN, HF, and RMSSD indices.
Notay et al.6 established that the application of 70 ml of nitrate-rich SB (6.4 mmol NO3-) in 14 volunteers (7 women) 165 to 180 minutes before exercise was capable of cutting sympathetic activity before and during physical activity.
Consistent with the previously cited studies allied with our findings, we endorse that 600mg of beetroot extract was able to improve the recovery of vagal heart rhythm modulation. This outcome was confirmed by accelerating the recovery of the SDNN7, RMSSD15 indices and endorsed by the HF index.
The improvement in these indices specifies a quicker reactivation of the vagus nerve in the post-exercise period in the beetroot extract protocol compared to the placebo. It is imperative to confirm that a slow post-exercise autonomic recovery, analyzed by HRV indices that assess vagal modulation, is linked with an increased cardiovascular risk34.
To provide complementary information regarding the influence of beetroot extract on the ANS, we similarly estimated the hemodynamic parameters of BP and HR as secondary consequences.
Before, it was illustrated that beetroot extract recovered HR faster in the post-exercise period15. The results of this study strengthened this evidence. Yet, Bond et al.7 did not observe any influence of beetroot juice on HR. The HR recovery in the post-exercise period is influenced by the reactivation of the parasympathetic nervous system. Considering that the reduction in vagal activity after exercise is linked with the risk of mortality, this allowed the HR recovery after a bout of exercise a steadfast predictor of mortality9.
In the hemodynamic parameters of BP, we revealed that the beetroot extract accelerated the recovery of SBP, DBP, and MAP values compared to placebo. Earlier studies substantiate our findings15. The improvement in BP can be attributed to the increased anti-inflammatory activity of compounds in beetroot and increased nitric oxide bioavailability and peripheral vasodilation6.
In the study led by Carrijo et al.35, the effects of different nitrate concentrations in beetroot juice on the HRV of hypertensive postmenopausal women were compared. HRV was assessed for 20 minutes after sitting at rest, 120 minutes after drinking one of the drinks, and after performing 40 minutes of aerobic exercise at 65% and 70% of the HR reserve on a treadmill. For subsequent analysis, HRV was logged for 90 minutes after exercise for time, frequency and non-linear domains.
These authors reported non-significant effects of beetroot juice on HRV indexes, HR35, and BP36. Apparently, this is evidence contrary to that presented in the research literature6,7,15. Yet, the low amount of NO3 provided in the study may contribute to the study results by Carrijo et al.35. The authors justified that, in spite of a probable increase in NO3 bioavailability, fluctuations in autonomic function were only induced by exercise.
Another account for the lack of influence of beetroot juice is that there may have been a clearance of nitrate and nitrite during the exercise itself 36. Still, we cannot be sure as data on NO3 and NO2 concentrations after exercise was not presented by Carrijo et al.35.
Our findings offers essential points of interest for clinical and sports nutrition studies and contribute to the health professionals’ performance and bring to light alternatives for new therapies and interventions. Our study established aerobic exercise and physically challenged subjects to test beetroot extract. We encourage other studies to replicate our experiments, as we understand that just as important when presenting unprecedented results is reassessing the reproducibility of findings in the identical population and during other conditions. Based on the data presented, we recap that beetroot extract proved efficient in improving mortality predictors.
Our study has some strengths since the interventions were randomized and the participants and researchers blinded. While the sample number was small, it exceeded the calculated sample size. Moreover, the prevailing "large effect size” of the values that were considered to be significantly different supports our findings.
Other studies with beetroot extract ought to measure plasma NO2 and NO3 concentrations and inflammatory markers to understand precisely which biological and molecular aspects are related to the effects achieved in this study and in other similar studies15. We highlight that complementary research with clinical populations must be enforced before these effects are considered. At the beginning of the study, we had problems locating subjects with a BMI<25kg/m², and thus, we redefined our criteria to values up to 29.9kg/m². We understand that this is a limitation; nevertheless, almost half of the world population will be overweight in the coming years, which will undoubtedly increase the external validity of these results. Studies with female participants are similarly encouraged. At this time, we excluded females owing to the difficulty of encouraging women to participate in the study and difficulties controlling the menstrual period and its interference with HRV.