Effects of Beetroot Juice Supplementation on Cardiorespiratory Endurance in Athletes. A Systematic Review

e nutritional supplementation to enhance the effects of training and achieve improvements in their athletic performance. Beetroot juice increases levels of nitric oxide (NO), which serves multiple functions related to increased blood ow, gas exchange, mitochondrial biogenesis and ef ciency, and strengthening of muscle contraction. These biomarker improvements indicate that supplementation with beetroot juice could have ergogenic effects on cardiorespiratory endurance that would bene t athletic performance. The aim of this literature review was to determine the effects of beetroot juice supplementation and the combination of beetroot juice with other supplements on cardiorespiratory endurance in athletes. A keyword search of DialNet, MedLine, PubMed, Scopus and Web of Science databases covered publications from 2010 to 2016. After excluding reviews/meta-analyses, animal studies, inaccessible full-text, and studies that did not supplement with beetroot juice and adequately assess cardiorespiratory endurance, 23 articles were selected for analysis. The available results suggest that supplementation with beetroot juice can improve cardiorespiratory endurance in athletes by increasing ef ciency, which improves performance at various distances, increases time to exhaustion at submaximal intensities, and may improve the cardiorespiratory performance at anaerobic threshold intensities and maximum oxygen uptake (VO2max). Although the literature shows contradictory data, the ndings of other studies lead us to hypothesize that supplementing with beetroot juice could mitigate the ergolytic effects of hypoxia on cardiorespiratory endurance in athletes. It cannot be stated that the combination of beetroot juice with other supplements has a positive or negative effect

at anaerobic threshold intensities and maximum oxygen uptake (VO2max). Although the literature shows contradictory data, the ndings of other studies lead us to hypothesize that supplementing with beetroot juice could mitigate the ergolytic effects of hypoxia on cardiorespiratory endurance in athletes. It cannot be stated that the combination of beetroot juice with other supplements has a positive or negative effect on cardiorespiratory endurance, but it is possible that the effects of supplementation with beetroot juice can be undermined by interaction with other supplements such as caffeine. Keywords:nutrition; sport; exercise; nitric oxide; physical activity 1. Introduction Cardiorespiratory endurance is de ned as a health-related component of physical tness that relates to the ability of the circulatory and respiratory systems to supply fuel during sustained physical activity and to eliminate fatigue products after supplying fuel [1]. Cardiorespiratory endurance is a performance factor in all sports in which adenosine triphosphate (ATP) is resynthesized, mainly by aerobic metabolism or oxidative processes that produce energy. In these sports, the expended effort typically lasts longer than ve minutes, primarily depending on the metabolic level of the oxidative Nutrients2017,9, 43; doi:10.3390/nu9010043

Nutrients2017,9, 43 2 of 18 processes involved [2]. Factors that limit performance in this type of endurance patterns include maximum oxygen uptake (VO2max), ventilatory thresholds ( rst and second ventilatory threshold) and energy ef ciency or economy [3–5]. In competitive sports, 0.5%–1.5% improvements in performance are considered a critical difference [6]. In order to enhance the effects of training and improve performance, athletes often turn to nutritional supplements [7]. According to the American College of Sports Medicine (ACSM), adequate selection of nutrients and supplements, adjusting intake according to the exercise performed, is necessary for optimal performance in athletes [8]. However, not all supplements have been shown to produce a positive effect on performance. The Australian Institute of Sport [9], classi ed supplements to which athletes have access, with the goal of categorizing nutritional supplements based on the level of evidence for impact on an athlete s performance (Table). However, the effectiveness of supplements also depends on dosage and type of effort, because the potential ergogenic effect may differ by the speci c type of sport [10]. Table 1. Classification of nutritional supplements, based on performance effect. Adapted from Australian Institute of Sport [9] and Burke [11]. Category Sub-Categories Supplements High level of evidence Will improve athletic performance with adequate dosing and speci c types of effort -alanine Sodium bicarbonate Caffeine Creatinine Beetroot juice Moderate level of evidence May improve performance, under speci c dosing and effort conditions, although additional research is needed Fish oils Carnitine Curcumin Glucosamine Glutamine HMB Quercetin Vitamins C and E Tart cherry juice Low level of evidence No demonstrated bene cial effectsSupplements not found in other categories Prohibited supplements May result in positive doping tests and therefore are prohibited Substances on the list published annually by theWorld Anti-Doping Agency(WADA) Beetroot juice is used as a supplement because of its high inorganic nitrate (NO3 ) content, a compound found naturally in vegetables and in processed meats, where it is used as a preservative [12]. Once ingested, the NO3 is reduced to nitrite (NO2 ), by anaerobic bacteria in the oral cavity by the action of nitrate reductase

theWorld Anti-Doping Agency(WADA) Beetroot juice is used as a supplement because of its high inorganic nitrate (NO3 ) content, a compound found naturally in vegetables and in processed meats, where it is used as a preservative [12]. Once ingested, the NO3 is reduced to nitrite (NO2 ), by anaerobic bacteria in the oral cavity by the action of nitrate reductase enzymes [13] and then to nitric oxide (NO) in the stomach [14]. This physiological mechanism depends on the entero-salivary circulation of inorganic nitrate without involving NOS activity. Once in the acidic stomach, nitrite is instantly decomposed to convert to NO and other nitrogen oxides performing determinant physiological functions (Figure). Nitrate and remaining nitrite is absorbed from the intestine into the circulation, which can become bioactive NO in tissues and blood [14] under physiological hypoxia. NO induces several physiological mechanisms that in uences O2utilization during contraction skeletal muscle. Physiological mechanisms for NO2 reduction are facilitated by hypoxic conditions, therefore, NO (vasodilator) is produced in those parts of muscle that are consuming or in need of more O2. This mechanism would allow local blood ow to adapt to O2requirement providing within skeletal muscle an adequate homogeneous distribution. This physiological response could be positive in terms of muscle function, although it would not explain a reduced O2cost during exercise [15]. Another probable mechanism is related to NO2 and NO as regulators of cellular O2utilization [15].

Nutrients2017,9, 43 3 of 18Nutrients 2017, 9, 43 3 of 18 Figure 1. Pathway of nitric oxide (NO) production from beetroot juice supplementation. Nitrate (NO 3 − ) is reduced to nitrite (NO 2 − ) by anaerobic bacteria in the oral cavity and then to NO in the stomach. NO 3 − and remaining NO 2 − are absorbed from the intestine into the circulation, which can become bioactive NO in tissues and blood. NO induces several physiological functions improving skeletal muscle function and, consequently, increasing cardiorespiratory performance. NO induces several physiological mechanisms that influences O 2 utilization during contraction skeletal muscle. Physiological mechanisms for NO 2 − reduction are facilitated by hypoxic conditions, therefore, NO (vasodilator) is produced in those parts of muscle that are consuming or in need of more O 2. This mechanism would allow local blood flow to adapt to O 2 requirement providing within skeletal muscle an adequate homogeneous distribution. This physiological response could be positive in terms of muscle function, although it would not explain a reduced O 2 cost during exercise [15]. Another probable mechanism is related to NO 2 − and NO as regulators of cellular O 2 utilization [15]. In addition, a potent signaling molecule that affects cell function in many body tissues, NO is endogenously produced by synthesizing nitric oxide from L‐arginine oxidation. The molecule has important hemodynamic and metabolic functions [16,17], being a major vasodilator that can increase blood flow to muscles [18] and promote oxygen transfer in the muscle. Additional physiological benefits of NO include improved mitochondrial efficiency and glucose uptake in muscle [19] and enhanced muscle contraction and relaxation processes [20]. Other researchers have reported that NO can act as an immunomodulator [21] and stimulates gene expression and mitochondrial biogenesis [22]. Given the positive effects of beetroot juice, which are induced by means of NO, this supplement has been proposed as part of the therapeutic approach in people with chronic obstructive pulmonary disease [23], hypertension [24], heart failure [25] and insulin resistance [26]. Figure 1. Pathway of nitric oxide (NO) production from beetroot juice supplementation. Nitrate

expression and mitochondrial biogenesis [22]. Given the positive effects of beetroot juice, which are induced by means of NO, this supplement has been proposed as part of the therapeutic approach in people with chronic obstructive pulmonary disease [23], hypertension [24], heart failure [25] and insulin resistance [26]. Figure 1. Pathway of nitric oxide (NO) production from beetroot juice supplementation. Nitrate (NO 3 ) is reduced to nitrite (NO 2 ) by anaerobic bacteria in the oral cavity and then to NO in the stomach. NO 3 and remaining NO 2 are absorbed from the intestine into the circulation, which can become bioactive NO in tissues and blood. NO induces several physiological functions improving skeletal muscle function and, consequently, increasing cardiorespiratory performance. In addition, a potent signaling molecule that affects cell function in many body tissues, NO is endogenously produced by synthesizing nitric oxide fromL-arginine oxidation. The molecule has important hemodynamic and metabolic functions [16,17], being a major vasodilator that can increase blood ow to muscles [18] and promote oxygen transfer in the muscle. Additional physiological bene ts of NO include improved mitochondrial ef ciency and glucose uptake in muscle [19] and enhanced muscle contraction and relaxation processes [20]. Other researchers have reported that NO can act as an immunomodulator [21] and stimulates gene expression and mitochondrial biogenesis [22]. Given the positive effects of beetroot juice, which are induced by means of NO, this supplement has been proposed as part of the therapeutic approach in people with chronic obstructive pulmonary disease [23], hypertension [24], heart failure [25] and insulin resistance [26]. These ndings re ect the importance of supplementation with NO3 or nitrate salts to increase the bioavailability of NO in order to in uence muscle function improving exercise performance, mainly in aerobic metabolism [27]. Therefore, supplementation with beetroot juice may have an ergogenic effect in athletes [9], especially with respect to cardiorespiratory endurance. However, the assumption that the beetroot juice supplementation improves performance in cardiorespiratory endurance under

in aerobic metabolism [27]. Therefore, supplementation with beetroot juice may have an ergogenic effect in athletes [9], especially with respect to cardiorespiratory endurance. However, the assumption that the beetroot juice supplementation improves performance in cardiorespiratory endurance under

Nutrients2017,9, 43 4 of 18 hypoxic conditions, and the combination of beetroot juice supplementation with other supplements, as caffeine, has a positive effect on cardiorespiratory endurance is controversial. The objective of the present literature review was to analyze the effects of beetroot juice supplementation on cardiorespiratory endurance in several conditions (normoxia, hypoxia and beetroot juice with other supplements) and determine the appropriate dosage to enhance the potential ergogenic effects on performance. The focus of the article is mainly on the in uence of beetroot juice of the acute and chronic responses on trained endurance athletes. 2. Methodology A keyword search for articles published in English or Spanish since 2010 was carried out in the DialNet, MedLine, PubMed, Scopus and Web of Science databases on 8 June 2016. The search terms included beet, beetroot, nitrate, nitrite, supplement, supplementation, nutrition, “sport nutrition” and “ergogenic aids”. The 210 selected articles included at least one of those search terms, in combination with endurance, exercise, sport or athlete. Exclusion criteria were the following: literature reviews and meta-analyses, animal studies, population other than endurance athletes, and inadequate assessment of cardiorespiratory endurance, speci cally de ned as< VO2maxtesting or no test lasting more than 5 min to determine how long the subject can maintain the lowest intensity at which VO2maxwas achieved [28]. Therefore, 23 articles were selected for the present review (Figure).Nutrients 2017, 9, 43 4 of 18 These findings reflect the importance of supplementation with NO 3 − or nitrate salts to increase the bioavailability of NO in order to influence muscle function improving exercise performance, mainly in aerobic metabolism [27]. Therefore, supplementation with beetroot juice may have an ergogenic effect in athletes [9], especially with respect to cardiorespiratory endurance. However, the assumption that the beetroot juice supplementation improves performance in cardiorespiratory endurance under hypoxic conditions, and the combination of beetroot juice supplementation with other supplements, as caffeine, has a positive effect on cardiorespiratory endurance is controversial. The objective of the present literature review was to analyze the effects of beetroot juice supplementation on cardiorespiratory endurance in several conditions (normoxia, hypoxia and beetroot juice with other

juice supplementation improves performance in cardiorespiratory endurance under hypoxic conditions, and the combination of beetroot juice supplementation with other supplements, as caffeine, has a positive effect on cardiorespiratory endurance is controversial. The objective of the present literature review was to analyze the effects of beetroot juice supplementation on cardiorespiratory endurance in several conditions (normoxia, hypoxia and beetroot juice with other supplements) and determine the appropriate dosage to enhance the potential ergogenic effects on performance. The focus of the article is mainly on the influence of beetroot juice of the acute and chronic responses on trained endurance athletes. 2. Methodology A keyword search for articles published in English or Spanish since 2010 was carried out in the DialNet, MedLine, PubMed, Scopus and Web of Science databases on 8 June 2016. The search terms included beet, beetroot, nitrate, nitrite, supplement, supplementation, nutrition, “sport nutrition” and “ergogenic aids”. The 210 selected articles included at least one of those search terms, in combination with endurance, exercise, sport or athlete. Exclusion criteria were the following: literature reviews and meta‐analyses, animal studies, population other than endurance athletes, and inadequate assessment of cardiorespiratory endurance, specifically defined as <VO 2max testing or no test lasting more than 5 min to determine how long the subject can maintain the lowest intensity at which VO 2max was achieved [28]. Therefore, 23 articles were selected for the present review (Figure 2). Figure 2. Flowchart of article selection. 3. Results and Discussion The selected studies on the effects of beetroot juice supplementation on cardiorespiratory endurance are summarized in Table 2. Figure 2.Flowchart of article selection. 3. Results and Discussion The selected studies on the effects of beetroot juice supplementation on cardiorespiratory endurance are summarized in Table.

Nutrients2017,9, 43 5 of 18 Table 2.Summary of studies that have evaluated the performance or metabolic responses after supplementation protocol with beet juice. Reference Participants Experimental Conditions Supplementation Protocol Variables Results [12] M (n: 5) and W (n: 6), trained athletes EC1: beet juice, EC2: placebo EC1: beet juice (8 mmol nitrate) (90 min before) Test 5 km: Performance, HR, RPE Performance: Last mile 1.1: faster EC1 vs. EC2 (5%), RPE: 1 mile: lower in EC1 vs. EC2 [27] M, competitive cyclists (n: 9) EC1: beet juice, EC2: placebo EC1: 500 mL beet juice (6.2 mmol nitrate) (120 min before) Tests 4 km and 16 km: respiratory parameters, performance Performance in test 4 km: Time: lower in EC1 vs. EC2 (6.27 0.35 vs. 6.45 0.42 min), Power: Higher in EC1 vs. EC2 (292 44 vs. 279 51 W), W/VO 2: Higher in EC1 vs. EC2 (93 17 vs. 83 9 W/L/min), Performance test of 16 km: Time: lower in EC1 vs. EC2 (26.9 1.8 vs. 27.7 2.1 min), Power: Higher in EC1 vs. EC2 (247 44 vs. 233 43 W), W/VO 2: Higher in EC1 vs. EC2 (69 3 vs. 64 6 W/L/min) [29] M, national level athletes kayak (n: 5) EC1: beet juice, EC2: placebo Study A: EC1: 140 mL beet juice (4.8 mmol nitrate) (150 min before), Study B: EC1: 140 mL beet juice (9.6 mmol nitrate) (150 min before) Study A: kayaking incremental test: Test 10 min (10 min + 5 min LT1 LT2) + 4 min test: respiratory parameters, lactate, performance (test 4 min), HR, RPE. Study B: Test 500 m Study A: 4 min test: VO 2: decreases in EC1 vs. EC2 (46.87 2.56 vs. 47.83 2.77 mL/kg/min), Economy: improved EC1 vs. EC2 (189.67 8.17 vs. 193.90 8.17 mL/kg/km). Study B: Test 500 m: Time: improved EC1 vs. EC2 (114.6 + 1.5 s vs. 116.7 + 2.2 s), Rowing often partially 100–400 m: increases in EC1 vs.EC2 (108 + 2 vs. 105 + 2 strokes), Partial speed 100–400 m: increases in EC1 vs. EC2 (4.40 + 0.03 vs. 4.30 + 0.05 m/s) [30] M, trained

vs. 193.90 8.17 mL/kg/km). Study B: Test 500 m: Time: improved EC1 vs. EC2 (114.6 + 1.5 s vs. 116.7 + 2.2 s), Rowing often partially 100–400 m: increases in EC1 vs.EC2 (108 + 2 vs. 105 + 2 strokes), Partial speed 100–400 m: increases in EC1 vs. EC2 (4.40 + 0.03 vs. 4.30 + 0.05 m/s) [30] M, trained cyclists-triathletes (n: 13) EC1: beet juice, EC2: placebo EC1: beet juice 140 mL (8 mmol nitrate) (6 days) Test 30 min at 45% MAP + 30 min at 65% MAP + test 10 km: respiratory parameters, lactate, glucose, performance (test to exhaustion at 80% VO 2max), HR, RPE Respiratory parameters VO 2at 45% MAP: lower in EC1 vs. EC2 (1.93 0.05 vs. 2.0 0.07 L/min), VO 2at 65% MAP: lower in EC1 vs. EC2 (2.94 0.10 vs. 3.1 0.09 L/min), Performance (test 10 km): improvement in EC1 vs. EC2 (953 21 vs. 965 21 s) [31] M, trained athletes (n: 13) EC1: beet juice, EC2: placebo EC1: 280 mL of beet juice (6.5 mmol nitrate) for 7 days. EC2: Control Test 20 min (10 min to 10 min 50% + 70% VO 2max): respiratory parameters Respiratory parameters: 70% VO 2max: oxygen consumption decrease in EC1 (3%) [32] M, trained cyclists (n: 8) EC1: beet juice, EC2: placebo EC1: 500 mL beet juice (6.2 mmol nitrate) (150 min before) Test 50 miles: respiratory parameters, lactate, performance Performance: last 10 miles: lower time in EC1 vs. EC2, W/VO 2: higher in EC1 vs. EC2 (67.4 5.5 vs. 65.3 4.8 W/L/min) [33] M, trained athletes (n: 16) EC1: beet juice, EC2: placebo EC1: 450 mL beet juice (5 mmol nitrate) (115 min before) Test 40 min [20 min at 50% VO 2max + 20 min at 70% VO 2max] + time to exhaustion at 90% VO 2max: respiratory parameters, performance (test to exhaustion at 90% VO 2max), lactate, HR, RPE Respiratory parameters: RER: greater in EC1 vs. EC2 at 50% VO 2max(0.89 0.03 vs. 0.86 0.06) and test to exhaustion (1.04 0.06 vs. 1.01 0.06), Performance (test to exhaustion at 90% VO 2max):

20 min at 70% VO 2max] + time to exhaustion at 90% VO 2max: respiratory parameters, performance (test to exhaustion at 90% VO 2max), lactate, HR, RPE Respiratory parameters: RER: greater in EC1 vs. EC2 at 50% VO 2max(0.89 0.03 vs. 0.86 0.06) and test to exhaustion (1.04 0.06 vs. 1.01 0.06), Performance (test to exhaustion at 90% VO 2max): time increases in EC1 vs. EC2 (185 122 s vs. 160 109 s), Max lactate: Higher in EC1 vs. EC2 (8.80 2.10 vs. 7.90 2.30 mmol/L)

This review analyzes the effects of beetroot juice supplementation on cardiorespiratory endurance in athletes.