1/27
Lithuanian University of Health Sciences
MEDICAL ACADEMY: FACULTY OF NURSING
Department of sports medicine
THE EFFECTIVENESS OF BEETROOT JUICE
SUPPLEMENTATION ON EXERCISE PERFORMANCE IN
SPORTS:
A SYSTEMATIC LITERATURE REVIEW
Author/Student: Kilian Petry Supervisor: Assoc. prof. Alma Kajėnienė
2/27
Table of Contents
1. SUMMARY ... 3
2. ABBREVIATION LIST ... 4
3. INTRODUCTION ... 4
4. AIM AND OBJECTIVES ... 5
5. LITERATURE REVIEW ... 5
6. RESEARCH METHODOLOGY AND METHODS ... 6
6.1. SEARCH STRATEGY AND STUDY SELECTION ... 6
6.2. ELIGIBILITY CRITERIA (PICO) ... 7
6.3. QUALITY ASSESSMENT OF STUDIES ... 7
7. RESULTS AND DISCUSSIONS ... 8
7.1. THE EFFECTS OF BEETROOT SUPPLEMENTATION ON SPORTS PERFORMANCE ... 21
7.1.1. VO2MAX AND VO2AVERAGE CONSUMPTION ... 22
7.1.2. TIME TRIAL, MEAN POWER OUTPUT AND DISTANCE COVERED ... 22
7.1.3. POSSIBLE RESPONDERS ... 23
7.2. MOST EFFECTIVE PROTOCOL FOR BEETROOT SUPPLEMENTATION IN SPORTS ... 23
7.3. POSSIBLE SIDE EFFECTS FROM BEETROOT SUPPLEMENTATION. ... 24
7.3.1. BEETURIA ... 24
7.3.2. GI DISTURBANCE ... 24
8. CONCLUSIONS ... 24
9. PRACTICAL RECOMMENDATION ... 25
3/27
1. Summary
Authors Name: Kilian Petry
Title: The Effectiveness of Beetroot Juice Supplementation on Exercise Performance in Sports.
Research aim: The aim of this literature review was to analyse recent studies (2014-2019) for the effects
of beetroot juice supplementation in sports.
Objectives: 1) To evaluate the effects of beetroot juice supplementation on sports performance; 2) To
find the most effective protocol for beetroot juice supplementation in sports; 3) To evaluate the possible side effects from beetroot juice supplementation.
Methodology: The database Pub Med was used to conduct this literature review, including studies and
articles in English, not older than 5 years. The eligibility of the studies was determined by the PICO approach. Inclusion criteria: Population - The Subjects were healthy, aged between 19-44 years, both sexes. The athletic fitness varied from well-trained athletes to elite athletes. Intervention - All selected studies included supplementation with beetroot juice with different dosages. An exercise performance test was conducted after acute, chronic or acute + chronic BRJ supplementation. Comparison - placebo group. Outcome - changes in either mean power output (W), time trials (s), VO$max consumption,
VO$average consumption or distance covered (m). All possible side effects mentioned and most appropriate dosing. Exclusion criteria: Population - Recreationally active and inactive subjects. Intervention - Combinations with other supplements. Comparison - Case reports, nitrate supplementation in other forms than BRJ. Outcome - Case studies, meta-analysis, no full text articles, literature reviews, effects on blood pressure, pulse, effects on diseases.
Results: The study subjects were healthy, aged between 19-44 years, including both sexes. The athletic
fitness of the subject included everyone from well-trained athletes to elite athletes, as specified by the study authors. A total of seventeen studies with 178 subjects (149 males, 29 females) were screened for possible sports performance effects, it was shown that in 130 subjects beetroot juice supplementation did not have any positive effect on mean power output, time trials, VO$max, VO$average oxygen consumption or distance covered (m). A positive effect was noted in 48 subjects on time trials, VO$max,
VO$average consumption or distance covered, however no subject improved in mean power output. The quality of studies was ranked good in four, moderate in twelve and poor in one study.
Conclusion:1) Beetroot juice was ineffective in 12 out of 17 studies in improving physical performance
of trained athletes. However, positive effects on time trials, VO$max and VO$average were found on
4/27
2. Abbreviation List
ANOVA: Analysis of variance BRJ: Beetroot juice
CI: Confidence interval CL: Confidence level EC: Experimental condition FiO2: Fraction of inspired oxygen m: minutes
MPO: Mean power output NOS: Nitric oxide synthase NO&' : Nitrate
NO$': Nitrite
NO: Nitric oxide PL: Placebo s: seconds TT: Time trial
VO$max: Maximal oxygen consumption
VO$average: average oxygen consumption W: Watt
3. Introduction
5/27
4. Aim and objectives
The aim of this literature review was to analyse recent studies (2014-2019) for the effects of beetroot juice supplementation in sports.
Objectives:
1. To evaluate the effects of beetroot juice supplementation on sports performance 2. To find the most effective protocol for beetroot juice supplementation in sports. 3. To evaluate the possible side effects of beetroot juice supplementation.
5. Literature review
Beetroot juice is a source of inorganic nitrate (NO&') which is converted by anaerobic bacteria
in the mouth to the bioactive NO$' [6]. Some of the nitrite is then further reduced to nitric oxide in the
stomach, subsequently leading to an overall increase of nitrite and NO in the blood plasma [7]. During low oxygen levels in tissues, the reduction of NO$' to NO is facilitated by a hypoxic and acidic
6/27
6. Research methodology and methods
The systematic literature review has been conducted to examine the effectiveness of beetroot supplementation on exercise performance, finding the best protocol and evaluating possible side effects.
6.1. Search strategy and study selection
This literature review was conducted by screening Pub Med for studies and articles/journals in English and not older than 5 years, from 2014 to 2019.
The search strategy included following keywords: beet or beetroot or beetroot juice or nitrate or nitrite, and endurance or exercise or sport or athlete, and performance or effectiveness or efficiency or efficacy, and dosage or intake protocol or protocol, and supplement or supplementation or nutrition or dietary supplement or dietary, and placebo, not cancer, not hypertension, not dementia.
The study selection is represented in Fig.1. After manually screening, 17 studies ([16] A and B) qualified for this literature review.
7/27
6.2. Eligibility Criteria (PICO)
Inclusion criteria:
Population: The Subjects were all healthy with no diseases or comorbidities, aged between 19-44
years and including both sexes. The athletic fitness of the subject included everyone from well-trained athletes to elite athletes, as specified by the studies’ authors.
Intervention: All selected studies included supplementation with beetroot juice, either
acute (2-3 hours prior exercising), chronic (over several days) or acute + chronic and with different dosages. An exercise performance test was conducted after acute, chronic or acute + chronic BRJ supplementation.
Comparison: All selected studies were compared to a placebo group.
Outcome: Sports performances were measured by evaluating changes in either mean power
output(W), time trials(s), VO$max consumption, VO$average consumption or distance covered (m). All possible side effects mentioned and most appropriate dosing.
Exclusion criteria:
Population: Recreationally active and inactive subjects.
Intervention: Combinations with other supplements which made a clear comparison between BRJ and
PL not possible.
Comparison: Case reports, nitrate supplementation in other forms than BRJ
Outcome: Case studies, meta-analysis, no full text articles, literature reviews, effects on (blood
pressure, pulse), effects on diseases.
6.3. Quality assessment of studies
8/27
Table 1. Study quality score key: Total score (14, 100%)
1. What study design was used and, and how were participants allocated?
2 = mixed design, randomized and counterbalanced between conditions
1 = repeated measures/within subjects only OR mixed design but not randomized. 0 = between groups only
2. Was the assigned intervention concealed before allocation?
2 = adequate 1 = unclear
0 = inadequate/impossible
3. Were the outcome assessors blinded to treatment status?
2 = Effective action taken to blind assessors
1 = Small or moderate chance of unbinding of assessors 0 = Not mentioned or not possible
4. Were the inclusion and exclusion criteria clearly defined?
2 = clearly defined 1 = inadequately defined 0 = not defined
5. Were the intervention and control group comparable at entry?
2= good comparability of groups, or confounding adjusted for in analysis/within group design
1= confounding small; mentioned but not adjusted for 0= large potential for confounding, or not discussed
6. Were the interventions clearly defined?
2= clearly defined interventions were applied
1= clearly defined interventions were applied but the application was not standardized 0= intervention and/or application were poorly or not defined
7. Were the outcome measures used clearly defined?
2 = clearly defined
1 = adequately defined/recorded 0 = not adequately defined/recorded
7. Results and discussions
This literature review was aiming to evaluate the performance enhancing properties of BRJ, possible side effects and the most effective dosing. Given the PICO criteria, the results important for this literature review were the following: mean power output (W), time trials (s), VO$max, VO$average oxygen consumption and distance covered (m), therefore subsequently excluding other results from the selected studies. Regarding the population, the subjects were all healthy with no diseases or comorbidities, aged between 19-44 years and including both sexes. The athletic fitness of the subject included everyone from well-trained athletes to elite athletes, as specified by the study authors.
9/27 It had a positive effect according to five studies on VO$max, TT and distance covered [16, 18-20]. However, the study quality were moderate, moderate, poor, moderate and moderate respectively. Two studies found a possibly harmful effect on MPO [9, 21]. Previous studies with recreationally active subjects have shown a positive ergogenic effect [22, 23]. The amount of capillaries around the skeletal muscle tissue is known to be increased by endurance training [18]. Therefore muscle oxygenation is greater in well trained athletes compared with recreational individuals [24]. Increased O2 supply enhances the NOS activity, which would explain the limitations of positive effects of nitrate supplementation in trained athletes [25]. Moreover well trained athletes showed a higher plasma nitrate concentration limiting the bioavailability of dietary nitrate [26].
Table 2: Studies methods and Results Ref. Participants and energy system used Experimen tal conditions Protocol Method of
evaluation Side effects Results
[26] N = 8♂ Elite 1500m runners Age: 23.8 ± 5yrs VO$max: 80 ± 5ml/kg/ min EC 1:BRJ (6.5-mmol NO&', 70-mL Beet It Sport; James White Drinks, 3 Ipswich, UK) EC2: PL nitrate-free BRJ EC 1-2: Day 1 (acute) and day 8 (Chronic): 210ml of BRJ (19.5mmol NO&') or PL 2.5hours prior testing Day 2-7: 140ml of BRJ or PL with lunch (13.0 mmol NO&') Respiratory parameter Submaxima l treadmill run: 7min at 50% VO$max ; 7min at 65% VO$max; 5min at 80% VO$max Performanc e Test: 1500m time trial No informa tion Submaximal treadmill run: no significant differences in VO$ consumption at any time between any
supplementation conditions. 1500-m individual time trial(s): no significant differences; BRJ (acute) 250.7 ± 4.3 vs. PL (acute) 250.4 ± 7.0, or during BRJ (chronic) 250.5 ± 6.2 vs. PL (chronic) 251.4 ± 7.6 2 potential responders during the submaximal treadmill test and
10/27 -At 80% VO$max, VO$ decreased by 211 mL (acute) and no change for chronic. Second: -At 50% VO$max, VO$ decreased by 42mL (acute) and 157-mL (chronic), -At 65% VO$max, VO$ decreased by 96mL (acute) and 210mL (chronic) -At 80% VO$max, VO$ decreased by 25mL (acute) and 66mL (chronic) 1500m TT: First: 5.8s (acute) and 7s (chronic) faster; Second: 5.0s (acute) and 0.5s (chronic) faster compared with
PL supplementation. [27] N = 8 ♂ trained cyclists Age: 34 ± 7yrs VO$max 65 ± 4.2ml kg min EC1: BC 90% Beet Essence, Green Foods Corp., CA, USA; 10% Black Cherry Kool - Aid, Kraft Foods Group Inc., IL, USA ,gelatine capsule) EC2: PL EC1-EC2: 3day loading phase 5mmol NO&' at 8pm + acute dose with an additional 5mmol NO&' 1h prior TT or PL 4km all out TT on a mounted cycle ergometer No side
effects difference on mean No significant power output(W): BRJ 388±54 vs. PL 386±55 or time to completion of TT(s) BR: 337.41±17.11vs. PL 338.10±18.04 [28] N = 10 ♂ trained cyclists Age: 28±7yrs VO$max:61.0 ± 7.4 ml/kg/min EC 1: BRJ (Beet It, James White Drinks Ltd, Ipswich, EC 1-EC2: 2.5 hours before the test Cycling test in Hypobaric Hypoxic condition at 3500m altitude Not mentio ned No significant difference on VO$ consumption between BRJ and PL at any intensity stage p=0.13 and p=0.17
11/27 UK) 12.8 mmol/ day EC 2: PL 5 bouts each 5min with 4min rest in between. 25%/40%/5 0%/60%/70 % of VO$max [29] N=12 ♂ well trained triathletes N=8 at national level N=4 international level Age: 39.3 ± 7.5yrs VO$max: 54.8 ± 3.1ml/min/kg No smoking No supplements taken in three months before EC1= BRJ 70ml 6.5mmol NO&' BRJ (Beet-It-Pro Elite Shot,James White Drinks, Ipswich UK) EC2: PL EC1-EC2: 3hours before the test Cycle ergometer test Endurance test Ventilatory Threshold 1 (aerobic) 30min without rest Time trial Ventilatory Threshold 2 (anaerobic) approx.15m in Well tolerate d, some shows beeturia und red stools No significant differences between BRJ and PL in VT1 and VT2: VO$ consumption (p=0.241) [30] N = 14 females trained, competitive cyclists and triathletes Age: 31 ± 7yrs EC1: BRJ + caffeine 5 mg×kg-1 70ml 7.3mmol NO&' BRJ (Beet it Sport, James White Drinks, Ipswich UK) EC2: PL By the manufacture r with the nitrate content removed 0.01mmol + Caffeine 5 mg×kg-1 EC1-EC4: BRJ or PL depleted nitrate BRJ were consumed 2.5hours prior testing Caffeine or PL was consumed 1 hour prior testing Racing bicycle seated on a motor-braked turbo trainer 20km TT against a resistance designed to replicate outdoor level gradient conditions. No informa tion Power output: No significant difference between
12/27 EC3: BRJ + PL EC4: PL+PL [9] N=28 male trained cyclists 20.3 years ± 1.4 yrs max. aerobic power: 365 ± 35W EC1: 70ml BRJ 4.1mmol Beet-it, James White Drinks, Ipswich, UK NO&' EC2:105ml 6.1mmol EC3:70ml BRJ 4.1mmol EC4: PL at all times EC1: 70ml BRJ 150min before the onset of TT1 EC2: 70ml BRJ 75min before TT1 EC3: 70ml BRJ before the onset of TT1 and an additional 35ml BRJ 75min before TT2 (top up) EC4: PL at all times 4 separate occasions 2 bouts of 4min cycling time trials TT1 and TT2 separated by 75min 2 subject withdre w from the study one because of intolera nce to the BRJ the other because of an injury during the TT D No effect on Mean power under any
conditions. Possibly harmful effect on power in TT2 compared to placebo with an average change of -0.3 ± 1.7% EC1: TT1 402 ± 47W TT2 396 ± 46W EC2: TT1 403 ± 52W TT2 396 ± 54W EC3: TT1 400 ± 48W TT2 396 ± 45W EC4: TT1 396 ± 57W TT2 397 ± 56W [20] N=10 males highly trained rowers Age 20.6 ± 2.5 yrs 2000m rowing best time: 6min
17s ± 10s EC1: 2x 70ml bottles of BRJ (8,4mmol NO&') EC2: 1x 70ml bottle of BRJ (4,2mmol NO&') and 1x 70ml PL EC3: 2x 70mlPL BRJ and PL by: Beet-it, James White Drinks, Ipswich, UK EC1,2,3: 2 hours before testing 2000m time trial on Rowing ergometer No major side effects; One subject describ ed slight GI sympto ms immedi ately after beverag e consum ption 2000m TT (min:s): EC3: PL 6:23.5 ± 9.0 vs. EC2: Single 6:23.4 ± 8.7 vs. EC1: Double 6:21.9 ± 9.0 EC1 compared to EC3 was possibly
beneficial with a mean difference of
-1.6 ± -1.6s ± 90% CL
EC2 compared to EC3 was likely trivial with a mean difference of -0.2 ±
13/27 [14] N=12 male cyclists trained athletes Age: 26.6 ± 4.4yrs VO$max: 65.8 ± 5.5ml kg min EC1: 1 bottle of BRJ (70ml, 6.5mmol NO&') every morning for 2 days On 3rd day 2 bottles of BRJ(140ml, 13mmol NO&') EC2: Same as EC1 but PL (1 bottle 70ml nitrate depleted BRJ) BRJ and PL provided by: Beet-it Sport Shot, James White Drinks, Ipswich, UK EC1-2: 2 hours before testing Cycling TT performanc e in hot (34.9°C) or euthermic (21.1°C) condition TT was divided into 5 x 20% work periods (i.e., period 1 = 0–20%, period 2 = 20–40%. . . period 5 = 80–100%). No informa tion TT performance (min:s) results in Hot
environment BRJhot: 56:50±05:08 min vs. PLhot: 58:30±04:48 (p=0.178) BRJeut: 53:09 ± 04:35min vs. PLeut: 54:01 ± 04:05min (p=0.380) Potential responders: 5 subjects increased TT performance in
15/27 [5] N=9 male competitive cyclists Age: 26 ± 8yrs VO$max: 63 ± 4ml/kg/min EC1: 70ml BRJ (4 mmol NO&') EC2: 70ml PL NO&' depleted (James White Drinks, Ipswich, UK) EC1-2: 70ml daily for 8 days, on day of a trial 2h prior scheduled lab appointme nt. 4km TT on cycle ergometer One withdre w due to illness N=8 No significant differences on: 4km TT time (s): PL 344.8 ± 14.0 vs. BRJ 343.6 ± 14.3 4km TT MPO (W): PL 375 ± 40 vs. BRJ 380 ± 41 VO$max (l/min) PL 4.64 ± 0.34 vs. BRJ 4.70 ± 0.55 [18] N=8 n=5 males Age: 22 ± 3 yrs, VO$max: 71.5 ± 4.7 mL·kg-1·min-1 N=3 females Age: 21 ± 1 yrs VO$max: 58.4±2.5 mL·kg-1·min-1 Well trained cross country skiers EC1: 140 ml BRJ (13m mol NO&') EC2: PL (0mmol NO&') BRJ and PL provided by: (Beet It Sport, James White Drinks Ltd., Ipswich, UK) EC1-2:2.5 hours prior warm up and testing On treadmill will roller-skis 2 submaximal bouts correspondi ng to 60% and 75% of subjects VO$max and a 1000m TT speed 11.5km/h for females, 12.5/13.5k m/h for males (speed based on pilot testing) in normoxic (20.9% O2) and hypoxic (16.8% O2) condition, each trial was separated by at least 72hours No informa tion given 1st Submaximal
exercise bout was performed at 63 ± 4% of VO$max. No significant effect of supplementation on VO$ consumption (p > 0.05). 2nd Submaximal
16/27 [16] Study A: N=6 National level male Age: 24.7 ± 3 yrs VO$max: 57.15 ± 2.77 ml/kg/min Study B: N=5 international level female kayak athletes Age: 25 ± 2.8 yrs VO$max: 47.8 ± 3.7 ml/kg/min Study A: EC1: 70ml BRJ (4.8mmol NO&') EC2: 70ml PL (nitrate depleted) Study B: EC1: 2x 70 ml BRJ (9.6 mmol nitrate) EC2: PL: 2x 70 ml nitrate- depleted equivalents BRJ and PL provided by: Beet-It Stamina Shot, James White drinks Ltd. Ipswich, UK Study A: EC1-2: 2.5 hour prior testing Study B: EC1-EC2: two 70 ml BRJ shots or PL Study A: 4 min all-out maximal effort was completed, with power output (W) and distance covered (m) being recorded Study B: 500 m TT paddle No side
17/27 [32] N=8 well trained cyclists Age: 25 ± 8yrs VO$max: 64 ± 5ml/kg/min EC1: 140ml BRJ (8mmol NO&') EC2: 140ml PL BRJ and PL: Provided by: James White Drinks, Ipswich, UK EC1-2: 140ml Daily for 3 days Time of consumpti on day 3: 90min prior scheduled arrival at the laboratory 2 separate trials in Hot(35℃ 60% relative humidity) 4km cycling TT All subjects experie nced beeturia , no other side effects
Mean power output: In relation to PL, BRJ showed unclear effects for 4km performance time (mean ± 95% CL: -0.1 ± 0.9%) and MPO (0.2 ± 2.5%) [33] N=9 endurance runners N=7 males Age: 30.4 ± 6.3 yrs VO$max: 59.0 ± 2.9 ml/kg/min N=2 females Age: 31.5 ± 9.2 yrs VO$max: 53.1 ± 11.4ml/kg/min Caffeine supplement ation was excluded EC1: 70ml BRJ (7.3mmol nitrate) EC2: 70ml of PL (nitrate depleted BRJ) BRJ and PL provided by: Beet It;
James White Drinks, Ltd., Ipswich, UK EC1-EC2: 2.5hours before warming up for the trials. Submaxima l treadmill running test at 70% and 80% of subjects VO$max, 5min each. Maximal treadmill running test. 1-km TT No informa tion given Submaximal test: No significant differences between
EC1 and EC2 for absolute or relative
VO$
1-km TT time(s): No significant difference between
EC1 and EC2: EC1: 200 ± 23 vs.
EC2: 198 ± 29 (P>0.05)
Table 3: Quality assessment results
STUDIES Q1 Q2 Q3 Q4 Q5 Q6 Q7 SCORE QUALITY RATING
Boorsma et al. [1]
Beetroot juice
supplementation does not improve performance of elite 1500-m runners.
2 2 2 1 2 2 2 13 GOOD
18/27
Single and Combined Effects of Beetroot Crystals and Sodium Bicarbonate on 4-km Cycling Time Trial Performance.
Carriker et al. [2]
Effect of Acute Dietary Nitrate Consumption on Oxygen Consumption During Submaximal Exercise in Hypobaric Hypoxia. 2 2 2 2 2 2 2 14 GOOD Garnacho-Castano, M.V., et al. [3]
Effects of a single dose of beetroot juice on cycling time trial performance at ventilatory thresholds intensity in male triathletes.
2 2 2 2 2 2 2 14 GOOD
Glaister, M., et al. [6]
Effects of dietary nitrate, caffeine, and their combination on 20-km cycling time trial performance. 2 2 2 0 2 2 2 12 MODERATE Hoon M.W., et al. [4] Nitrate supplementation and high-intensity performance in competitive cyclists. 2 2 1 0 2 2 2 13 GOOD Hoon, M.W., et al. [29]
The effect of variable doses of inorganic
19/27 nitrate-rich beetroot juice on simulated 2,000-m rowing perfor2,000-mance in trained athletes. Kent, G.L., et al. [7] Dietary nitrate
supplementation does not improve cycling time-trial performance in the heat.
1 2 2 0 2 2 2 11 MODERATE
Lowings, S., et al. [8]
Effect of Dietary Nitrate Supplementation on Swimming Performance in Trained Swimmers.
2 2 2 0 1 2 2 11 MODERATE
McQuillan, J.A., et al. [9]
The Effect of Dietary Nitrate Supplementation on Physiology and Performance in Trained Cyclists.
2 2 2 1 1 2 1 11 MODERATE
McQuillan, J.A., et al. [10]
Dietary Nitrate Fails to Improve 1 and 4 km Cycling Performance in Highly Trained Cyclists.
20/27 Study A Peeling, P., et
al. [13]
Beetroot Juice Improves On-Water 500 M Time-Trial Performance, and Laboratory-Based Paddling Economy in National and International-Level Kayak Athletes. 1 1 0 0 1 2 1 6 POOR
Study B Peeling, P., et al. [13]
Beetroot Juice Improves On-Water 500 M Time-Trial Performance, and Laboratory-Based Paddling Economy in National and International-Level Kayak Athletes. 1 2 2 0 1 2 1 9 MODERATE
Shannon, O.M., et al. [28] Dietary nitrate supplementation enhances high-intensity running performance in moderate normobaric hypoxia, independent of aerobic fitness. 2 2 2 0 2 2 2 12 MODERATE
McQuillan, J.A., et al. [11]
The Effect of Nitrate Supplementation on Cycling Performance in
21/27
7.1. The effects of beetroot supplementation on sports performance
A total of seventeen studies with 178 subjects (149 males, 29 females) were screened for possible sports performance effects and are represented in table 2. It was shown that in twelve studies N=130 (109 males, 21 females) beetroot juice supplementation did not have any positive effect on MPO, TT, VO$max, VO$average oxygen consumption or distance covered (m). The quality of studies was ranked good in four [9, 26, 28, 29] and moderate in eight [5, 14, 21, 27, 30-33] studies. The results are represented in table 3. Positive effects were noted in five studies N=48 (43 males, 5 females) on TT, VO$max, VO$average consumption or distance covered, however no subject improved in MPO. The study quality was moderate in four [16, 18-20] and poor in one [16]. The results are represented in table 3. Cycling was, with eight out of the seventeen studies, the most common method of exercise employed [5, 9, 14, 21, 27, 28, 30, 32], followed by running with three studies [19, 26, 33], two for kayakers[16] and one each for triathletes [29], swimmers [31], rowers [20], cross country skiers [18]. The athletic fitness of the subjects was expressed by thirteen studies in VO$max and ranged from 54.8 ± 3.1 ml/kg/min [29] to 80 ± 5 ml/kg/min [26] for males and from 47.8 ± 3.7 ml/kg/min Study B [16] to 53.1 ± 11.4 ml/kg/min [33] for females. During the analysis of the results it appeared that none of the studies showed a positive effect in a normoxic condition on TT, MPO or distance covered in trained athletes with a VO$max of > 58 ml/kg/min. Nevertheless, possible responders with a VO$max of > 58 ml/kg/min were identified and are further discussed in section 7.1.3.The absence of positive effects can be explained due to the higher NOS activity of highly trained athletes, as well as the reduction of type II fibres in endurance trained athletes may annihilate the effects of nitrate supplementation [21, 25]. Therefore, positive effects on distance covered, 500m TT and VO$max, observed in the studies of highly trained kayakers by Peeling et al. study A and B [16] might be explained due to the sport specific increase of type II fibres in the upper body [21]. The amount of nitrate consumed by the subjects varied between 4 mmol of nitrate [21] and 19.5 mmol of nitrate [26].
the Heat in Well-Trained Cyclists.
Oskarsson, J. and K. McGawley. [12]
No individual or combined effects of caffeine and beetroot-juice supplementation during submaximal or maximal running.
22/27
7.1.1.
VO
$max and VO
$average consumption
One study, conducted by Shannon et al. [19] in a normobaric hypoxic condition, simulating an altitude of 2500m (FIO2 ~ 15.0 %), demonstrated a significant difference on V̇O2 consumption with a reduction at 45% of V̇O2 max p= 0.014 and at 65% of VO$max p= 0.002. Interesting to note was that
similar changes on the oxygen consumption and VO$max were observed in normobaric normoxic conditions in other studies [16, 18]. Those findings are normally associated with reduced exercise performance [34]. Paradoxically it was noted that the 1500 time trial (s) treadmill running test conducted by Shannon et al. [19] was improved by 3.2%. As well as the distance covered of kayakers in a 4 minutes all out test were improved by 0.7%, Study A [16]. Strong evidence is suggesting that whole body oxygen consumption could be reduced by an increased mitochondrial efficiency and due to a decreased proton leakage. Although additional studies are needed to investigate the possibility of dietary nitrate to shift the ATP synthesis to a nonoxidative pathway [35]. Notwithstanding it is important to mention that a reduction on VO$max and on VO$average consumption was not proven for five studies [5, 26, 28, 29, 33].
7.1.2. Time trial, mean power output and distance covered
23/27
7.1.3. Possible responders
A finding which is important to note was that in the study of Kent, G.L., et al. [14], five potential responders were identified. Those subjects increased their cycling TT time by a minimum of 1.8%, however those potential responders had a significantly lower VO$max compared to non responders. Further, two additional responders were found in the study by Boorsma, R.K. et al. [26].
The responders had a significantly reduced VO$max and at the same time improved their 1500m cycling TT time (p<0.05). Due to discrepancies in individual factors such as genetics, microbiome, habitual diet as well as the mode of use may cause responses to vary widely and are therefore a possible explanation for those findings of possible responders [36].
7.2. Most effective protocol for beetroot supplementation in sports
The results were contradicting themselves which made an effective protocol difficult to obtain. A supplementation with a nitrate content between 4 and 8 mmol of nitrate was most commonly used. The duration and time of ingestion varied between acute supplementation (2.5-3 hours prior testing) and chronic + acute (3-8days and 1-2.5 hours prior testing) [5, 9, 16, 20, 21, 26, 27, 29, 30, 32, 33].
Positive effects were only noted in the studies by Peeling, P., et al. (A and B) [16] and Hoon, M.W., et al.[20], that were of poor, moderate and moderate quality respectively (table3). Another set of studies supplemented in an acute fashion (2-3 hours prior testing) with > 12.5mmol of nitrate [14, 18, 19, 28, 31]. No positive effects were found in the study by Lowings, S., et al. [31], Carriker, C.R., et al. [28] and Kent, G.L., et al. [14], the latter study however showed a positive effect on five potential responders. The most effective ingestion scheme was seen in a study by Shannon et al. [19] in which subjects received 140 ml of BRJ containing 15.2 mmol of nitrate 3 hours prior testing. Twelve male runners (Age: 24.4 ± 4.3 years, VO$max: 62.1 ± 9.3 ml/kg/min) performed in a hypoxic condition (FIO2 ~ 15.0
%) a 1500m TT on a treadmill. It was shown that the athletes could improve their time (s) in the TT by 3.2%. However, no improvement was seen in a study by Nyback, L., et al [18], investigating five males (Age: 2 ± 3 years, VO$max 71.5 ± 4.7 ml/kg/min) and three females (Age: 21 ± 1 years, VO$max 58.4 ± 2.5 ml/kg/min) cross country skiers. The probands received the same amount of BRJ but with a slightly lower nitrate content (140ml, 13mmol NO&') 2,5 hours prior testing. The method of evaluation was a
1000m TT with roller skis in a hypoxic and normoxic condition. No significant difference in time (s) to complete the TT compared to the placebo group but a reduction in the oxygen consumption was achieved in the normoxic condition (BRJ: 4.56 ± 0.95 l/min vs. PL: 4.68 ± 0.97 l/min) but not in the hypoxic condition (FIO2 ~16.8 %). These results are conflicting with those of Shannon et al. [19]. Beetroot juice
24/27
7.3. Possible side effects from beetroot supplementation.
A total of 86 subjects (81 males and 5 females) were screened for side effects. It was shown that the most common side effect was beeturia affecting seventeen subjects, the second most common side effect was GI disturbances, affecting two subjects. Apart from that, no further side effects were noted.
7.3.1.Beeturia
Beeturia is the passing of red or pink urine and in some cases red or pink faeces [37]. Beeturia itself is harmless and explained due to the excretion of betalain pigments such as betanin. Studies suggest that betanin degradation is dependent on stomach acidity and decolorized in the presence of hydrochloric acid [38].
7.3.2.GI disturbance
Two subjects were complaining about gastrointestinal discomfort, however, the nature of the complaints was not specified. Beetroot juice is classified as a FODMAP food (Fermentable Oligo-, Di- and Mono- saccharides and Polyols) and evidence is pointing out that FODMAP’s can induce abdominal symptoms, such as: diarrhoea, constipation, bloating, pain and nausea [39].
8. Conclusions
According to the selected studies analysed in this literature review we can conclude that: 1. Beetroot juice appeared to be ineffective in well trained and elite athletes in twelve out of seventeen
studies. In athletes with a lower VO$max and higher amount of type II muscle fibres compared to endurance athletes, an improvement in time trials and distance covered was observed as well as a reduction of VO$max and VO$average with an unchanged mean power output.
Those findings are in accordance with the possible responders, which had a significantly lower VO$max compared to the rest of the study participants.
2. A dose of 15.2 mmol of nitrate 3 hours prior exercise appeared to be the most effective protocol in this literature review.
3. Beetroot juice does not have any serious side effects. Beeturia and GI disturbances, which subsided after stopping the supplementation, were the only side effects found during this literature review.
25/27
9. Practical recommendation
In this literature review a positive effect of beetroot juice supplementation could not consequently improve the physical performance of trained athletes, however beetroot juice might induce a positive ergogenic effect on VO$max, VO$average consumption as well as on time trial times. If an athlete decides to supplement with BRJ a dose of 15.2 mmol of nitrate, 3 hours prior exercising is recommended. However, due to possible side effects, it is advised to start with a dose of 4 mmol of nitrate and gradually increase it, so the gastrointestinal tract can accommodate to the substance.
10.
References
1. Knapik, J.J., et al., Prevalence of Dietary Supplement Use by Athletes: Systematic Review and
Meta-Analysis. Sports Med, 2016. 46(1): p. 103-23.
2. Paton, C.D.H., W.G., Performance Enhancement at the Fifth World Congress on Sport
Sciences;. 1999. Volume 3.
3. Santamaria, P., Nitrate in vegetables: toxicity, content, intake and EC regulation. Journal of the Science of Food and Agriculture, 2006. 86(1): p. 10-17.
4. Sport, A.I.o., ABCD Classification System. 2016.
5. McQuillan, J.A., et al., The Effect of Dietary Nitrate Supplementation on Physiology and
Performance in Trained Cyclists. Int J Sports Physiol Perform, 2017. 12(5): p. 684-689.
6. Duncan, C., et al., Chemical generation of nitric oxide in the mouth from the enterosalivary
circulation of dietary nitrate. Nat Med, 1995. 1(6): p. 546-51.
7. Lundberg, J.O., E. Weitzberg, and M.T. Gladwin, The nitrate-nitrite-nitric oxide pathway in
physiology and therapeutics. Nat Rev Drug Discov, 2008. 7(2): p. 156-67.
8. Vanhatalo, A., P.G. Winyard, and A.M. Jones, The nitrate-nitrite-nitric oxide pathway: Its role
in human exercise physiology AU - Bailey, Stephen J. European Journal of Sport Science,
2012. 12(4): p. 309-320.
9. Hoon, M.W., et al., Nitrate supplementation and high-intensity performance in competitive
cyclists. Appl Physiol Nutr Metab, 2014. 39(9): p. 1043-9.
10. Umans, J.G. and R. Levi, Nitric oxide in the regulation of blood flow and arterial pressure. Annu Rev Physiol, 1995. 57: p. 771-90.
11. Dominguez, R., et al., Effects of Beetroot Juice Supplementation on a 30-s High-Intensity
Inertial Cycle Ergometer Test. Nutrients, 2017. 9(12).
12. Mosher, S.L., et al., Ingestion of a Nitric Oxide Enhancing Supplement Improves Resistance
26/27 13. Stamler, J.S. and G. Meissner, Physiology of nitric oxide in skeletal muscle. Physiol Rev, 2001.
81(1): p. 209-237.
14. Kent, G.L., et al., Dietary nitrate supplementation does not improve cycling time-trial
performance in the heat. J Sports Sci, 2018. 36(11): p. 1204-1211.
15. Hernandez, A., et al., Dietary nitrate increases tetanic [Ca2+]i and contractile force in mouse
fast-twitch muscle. J Physiol, 2012. 590(15): p. 3575-83.
16. Peeling, P., et al., Beetroot Juice Improves On-Water 500 M Time-Trial Performance, and
Laboratory-Based Paddling Economy in National and International-Level Kayak Athletes. Int J
Sport Nutr Exerc Metab, 2015. 25(3): p. 278-84.
17. Altman, D.G., et al., The revised CONSORT statement for reporting randomized trials:
explanation and elaboration. Ann Intern Med, 2001. 134(8): p. 663-94.
18. Nyback, L., et al., Physiological and performance effects of nitrate supplementation during
roller-skiing in normoxia and normobaric hypoxia. Nitric Oxide, 2017. 70: p. 1-8.
19. Shannon, O.M., et al., Dietary nitrate supplementation enhances high-intensity running
performance in moderate normobaric hypoxia, independent of aerobic fitness. Nitric Oxide,
2016. 59: p. 63-70.
20. Hoon, M.W., et al., The effect of variable doses of inorganic nitrate-rich beetroot juice on
simulated 2,000-m rowing performance in trained athletes. Int J Sports Physiol Perform, 2014. 9(4): p. 615-20.
21. McQuillan, J.A., et al., Dietary Nitrate Fails to Improve 1 and 4 km Cycling Performance in
Highly Trained Cyclists. Int J Sport Nutr Exerc Metab, 2017. 27(3): p. 255-263.
22. Thompson, C., et al., Influence of dietary nitrate supplementation on physiological and muscle
metabolic adaptations to sprint interval training. J Appl Physiol (1985), 2017. 122(3): p.
642-652.
23. Bailey, S.J., et al., Inorganic nitrate supplementation improves muscle oxygenation, O(2)
uptake kinetics, and exercise tolerance at high but not low pedal rates. J Appl Physiol (1985),
2015. 118(11): p. 1396-405.
24. McConell, G.K., et al., Skeletal muscle nNOSμ protein content is increased by exercise training
in humans. American Journal of Physiology-Regulatory, Integrative and Comparative
Physiology, 2007. 293(2): p. R821-R828.
25. Jungersten, L., et al., Both physical fitness and acute exercise regulate nitric oxide formation in
healthy humans. J Appl Physiol (1985), 1997. 82(3): p. 760-4.
26. Boorsma, R.K., J. Whitfield, and L.L. Spriet, Beetroot juice supplementation does not improve
27/27 27. Callahan, M.J., et al., Single and Combined Effects of Beetroot Crystals and Sodium
Bicarbonate on 4-km Cycling Time Trial Performance. Int J Sport Nutr Exerc Metab, 2017. 27(3): p. 271-278.
28. Carriker, C.R., et al., Effect of Acute Dietary Nitrate Consumption on Oxygen Consumption
During Submaximal Exercise in Hypobaric Hypoxia. Int J Sport Nutr Exerc Metab, 2016. 26(4): p. 315-22.
29. Garnacho-Castano, M.V., et al., Effects of a single dose of beetroot juice on cycling time trial
performance at ventilatory thresholds intensity in male triathletes. J Int Soc Sports Nutr, 2018. 15(1): p. 49.
30. Glaister, M., et al., Effects of dietary nitrate, caffeine, and their combination on 20-km cycling
time trial performance. J Strength Cond Res, 2015. 29(1): p. 165-74.
31. Lowings, S., et al., Effect of Dietary Nitrate Supplementation on Swimming Performance in
Trained Swimmers. Int J Sport Nutr Exerc Metab, 2017. 27(4): p. 377-384.
32. McQuillan, J.A., et al., The Effect of Nitrate Supplementation on Cycling Performance in the
Heat in Well-Trained Cyclists. Int J Sports Physiol Perform, 2018. 13(1): p. 50-56.
33. Oskarsson, J. and K. McGawley, No individual or combined effects of caffeine and
beetroot-juice supplementation during submaximal or maximal running. Appl Physiol Nutr Metab,
2018. 43(7): p. 697-703.
34. Larsen, F.J., et al., Dietary nitrate reduces maximal oxygen consumption while maintaining
work performance in maximal exercise. Free Radic Biol Med, 2010. 48(2): p. 342-7.
35. Nair, K.S., B.A. Irving, and I.R. Lanza, Can dietary nitrates enhance the efficiency of
mitochondria? Cell Metab, 2011. 13(2): p. 117-8.
36. Maughan, R.J., et al., IOC consensus statement: dietary supplements and the high-performance
athlete. British Journal of Sports Medicine, 2018. 52(7): p. 439.
37. Eastwood, M.A. and H. Nyhlin, Beeturia and colonic oxalic acid. Qjm, 1995. 88(10): p. 711-7. 38. Watts, A.R., et al., Beeturia and the biological fate of beetroot pigments. Pharmacogenetics,
1993. 3(6): p. 302-11.
39. Gibson, P.R. and S.J. Shepherd, Evidence-based dietary management of functional
gastrointestinal symptoms: The FODMAP approach. J Gastroenterol Hepatol, 2010. 25(2): p.
