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The Effect of Dietary Nitrate Supplementation on Endurance Exercise Performance in Healthy Adults: A Systematic Review and Meta-Analysis

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Background Recent research into the use of dietary nitrates and their role in vascular function has led to it becoming progressively more popular amongst athletes attempting to enhance performance. Objective The objective of this review was to perform a systematic review and meta-analysis of the literature to evaluate the effect of dietary nitrate (NO3−) supplementation on endurance exercise performance. An additional aim was to determine whether the performance outcomes are affected by potential moderator variables. Data sourcesRelevant databases such as Cochrane Library, Embase, PubMed, Ovid, Scopus and Web of Science were searched for the following search terms ‘nitrates OR nitrate OR beetroot OR table beet OR garden beet OR red beet AND exercise AND performance’ from inception to October 2015. Study selectionStudies were included if a placebo versus dietary nitrate-only supplementation protocol was able to be compared, and if a quantifiable measure of exercise performance was ≥30 s (for a single bout of exercise or the combined total for multiple bouts). Study appraisal and synthesisThe literature search identified 1038 studies, with 47 (76 trials) meeting the inclusion criteria. Data from the 76 trials were extracted for inclusion in the meta-analysis. A fixed-effects meta-analysis was conducted for time trial (TT) (n = 28), time to exhaustion (TTE) (n = 22) and graded-exercise test (GXT) (n = 8) protocols. Univariate meta-regression was used to assess potential moderator variables (exercise type, dose duration, NO3− type, study quality, fitness level and percentage nitrite change). ResultsPooled analysis identified a trivial but non-significant effect in favour of dietary NO3− supplementation [effect size (ES) = −0.10, 95 % Cl = −0.27 to 0.06, p > 0.05]. TTE trials had a small to moderate statistically significant effect in favour of dietary NO3− supplementation (ES = 0.33, 95 % Cl = 0.15–0.50, p < 0.01). GXT trials had a small but non-significant effect in favour of dietary NO3− supplementation in GXT performance measures (ES = 0.25, 95 % Cl = −0.06 to 0.56, p > 0.05). No significant heterogeneity was detected in the meta-analysis. No statistically significant effects were observed from the meta-regression analysis. Conclusion Dietary NO3− supplementation is likely to elicit a positive outcome when testing endurance exercise capacity, whereas dietary NO3− supplementation is less likely to be effective for time-trial performance. Further work is needed to understand the optimal dosing strategies, which population is most likely to benefit, and under which conditions dietary nitrates are likely to be most effective for performance.
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The effect of dietary nitrate supplementation on endurance exercise performance in healthy adults: A
Systematic Review and Meta-Analysis
Nicholas F. McMahona, Michael D. Leveritta, Toby G. Paveyb
aSchool of Human Movement and Nutrition Sciences, University of Queensland, St. Lucia, QLD, Australia
bSchool of Exercise & Nutrition Sciences, Queensland University of Technology, Kelvin Grove, QLD,
Australia
Running Head: Dietary Nitrate Supplementation and Endurance Exercise Performance
Corresponding author:
Nicholas McMahon
School of HMNS
University of Queensland E-mail: n.mcmahon2@uq.edu.au
Key Points
Findings from this meta-analysis highlight the positive ergogenic effect of dietary nitrate supplementation on endurance
exercise capacity.
Further randomised controlled trials are required to determine the true ergogenic effect of dietary nitrate supplementation o n
exercise performance.
Abstract
BACKGROUND: Recent research into the use of dietary nitrates and their role in vascular function has led to it becoming
progressively more popular amongst athletes attempting to enhance performance. OBJECTIVE: The objective of this review
was to perform a systematic review and meta-analysis of the literature to evaluate the effect of dietary nitrate (NO3-)
supplementation on endurance exercise performance. An additional aim was to determine whether the performance outcomes
are affected by potential moderator variables. DATA SOURCES: Relevant databases such as Cochrane Library, Embase,
PubMed, Ovid, Scopus and Web of Science were searched for the following search terms ‘nitrates OR nitrate OR beetroot OR
table beet OR garden beet OR red beet AND exercise AND performance’ from inception to October 2015. STUDY SELECTION:
Studies were included if a placebo versus dietary nitrate-only supplementation protocol was able to be compared, and if a
quantifiable measure of exercise performance was ≥ 30 seconds (for a single bout of exercise or the combined total for multiple
bouts). STUDY APPRAISAL AND SYNTHESIS: The literature search identified 1038 studies, with 47 (76 trials) meeting the
inclusion criteria. Data from the 76 trials was extracted for inclusion in the meta-analysis. A random-effects meta-analysis was
conducted for time trial (TT) (n = 28), time to exhaustion (TTE) (n = 22), and graded-exercise test (GXT) (n = 8) protocols.
Univariate meta-regression was used to assess potential moderator variables (exercise type, dose duration, NO3- type, study
quality, fitness level, and percentage nitrite change). RESULTS: Pooled analysis identified a trivial, but non-significant effect in
favour of dietary NO3- supplementation (effect size (ES) = -0.10, 95% Cl = -0.27-0.06, p > 0.05). TTE trials had a small to moderate
statistically significant effect in favour of dietary NO3- supplementation (ES = 0.33, 95% Cl = 0.15-0.50, p < 0.01). GXT trials had a
small, but non-significant effect in favour of dietary NO3- supplementation in GXT performance measures (ES = 0.25, 95% Cl = -
0.06-0.56, p > 0.05).
No significant heterogeneity was detected in the meta-analysis. No statistically significant effects were observed from the meta-
regression analysis. CONCLUSION: Dietary NO3- supplementation is likely to elicit a positive outcome when testing endurance
exercise capacity; whereas, dietary NO3- supplementation is less likely to be effective for time-trial performance. Further work is
needed to understand the optimal dosing strategies, which population is most likely to benefit, and under which conditions
dietary nitrates are likely to be most effective for performance.
1. Introduction
Through dietary manipulation, a number of different macronutrient and micronutrients have been identified as having the
capacity to enhance exercise performance [1]. These nutritional ergogenic aids allow athletes to reach beyond the abilities
achieved from training alone, and could be the difference between victory or defeat. For this reason, exploring and evaluating
the efficacy of nutritional ergogenic aids is a valuable process to undertake [2]. Recent research into the use of dietary nitrates
and their role in vascular function has led to it becoming progressively more popular amongst athletes attempting to enhance
performance. Other physiological processes that might be altered to provide an ergogenic effect due to nitrate ingestion include
skeletal muscle contractility and mitochondrial efficiency, glucose homeostasis, and respiration [3].
Green leafy and root vegetables constitute the primary dietary source of nitrate (NO3-). Vegetables with a very high
NO3- concentration (> 250mg/100g) include spinach, rocket, cress, lettuce, celery, radish, Swiss chard, chervil, and red beetroot
[4]. Once ingested, dietary NO3- is reduced to nitric oxide (NO) via the NO3--nitrite-NO pathway, increasing the level of NO in the
blood and tissues [5]. NO is a potent signalling molecule that plays a key role in vasodilation by relaxing smooth muscle and
subsequently improving blood circulation.
The first study to observe the benefits of dietary NO3- ingestion on exercise was performed by Larsen et al. [6]. The
randomised double-blind placebo-controlled crossover study involved nine, well-trained male subjects performing progressive
work rate cycling after chronic sodium NO3- supplementation (0.1 mmol∙kg-1/day for three days). NO3- ingestion resulted in a
significantly lower oxygen (O2) cost of exercise at work rates ranging from 45-80% peak oxygen uptake (𝑉
̇O2peak), without an
increase in blood lactate concentration, resulting in enhanced exercise efficiency. The amount of NO3- supplemented by Larsen
et al. [6] resembled the amount found in 150 - 250g of NO3--rich vegetables. Their findings were unexpected because it is
generally considered that the O2 cost of exercise at a given work rate was a fixed quantity among individuals, particularly during
cycling [7, 8].
The observation by Larsen et al. [6] instigated further studies investigating the effect of dietary NO3- supplementation
on exercise performance. Reported physiological changes include reduced blood pressure [9, 10], enhanced muscle
deoxyhemoglobin kinetics [11], reduced adenosine triphosphate (ATP) utilisation and phosphocreatine (PCr) degradation
resulting in enhanced muscle contractile efficiency [9], reduced O2 cost of submaximal exercise [12-14], and improved exercise
performance [15, 9, 16, 12]. However, a number of studies have found dietary NO3- supplementation to have no effect on
performance [17-22]. The variability in findings may be due to different study designs, protocols, or participant characteristics
but this has not been systematically evaluated.
A previous systematic review and meta-analysis conducted in 2013 examined the effects of dietary NO3-
supplementation on endurance exercise performance [23]. After examining 17 studies, the meta-analysis concluded that dietary
NO3- supplementation had a minor benefit on time trial (TT) performance (ES = 0.11, p > 0.05, n = 9); moderate effect on time to
exhaustion (TTE) trials (ES = 0.79, p < 0.01, n = 3); and a slight benefit on graded-exercise test (GXT) performance (ES = 0.23, p >
0.05, n = 7). Due to the small number of studies, Hoon et al. [23] concluded that more research was necessary to determine the
overall effect of dietary NO3- supplementation on endurance performance.
The purpose of this systematic review and meta-analysis was to update, critically evaluate, and summarise the
methodological quality of the literature on dietary NO3- supplementation and endurance exercise performance. A secondary aim
was to determine whether the performance outcomes are affected by potential moderator variables such as exercise type and
duration, protocol, dose duration and amount, NO3- type, subject’s level of fitness, and change in nitrite (NO2-). The results may
help to further our understanding of the influence dietary NO3- supplementation has on performance, with the purpose of
providing clear usage recommendations to augment participant performance.
2. Methods
We conducted and reported this systematic review in accordance with the guidelines outlined in the PRISMA (Preferred
Reporting Items for Systematic Reviews and Meta-Analyses) statement [24].
2.1 Search strategy
The following databases were systematically searched, and limited to English language: Cochrane Library, Embase, PubMed,
Ovid, Scopus and Web of Science, from inception to October 2015. The following search terms and Medical Subject Headings
(MeSH) were used to source pertinent peer-reviewed journals: nitrates (MeSH) OR nitrates (All Fields) OR nitrate (All Fields) OR
beetroot (All Fields) OR table beet (All Fields) OR garden beet (All Fields) OR red beet (All Fields) AND exercise (MeSH) OR
exercise (All Fields) AND performance (All Fields). The search was supplemented by manually cross-matching reference lists, key
author searches, and citation searching of all retrieved papers to potentially identify additional studies.
2.2 Inclusion and Exclusion Criteria
Selection criteria for all relevant articles was determined by two researchers (NM and ML). Only full-text primary source articles
published in peer-reviewed journals utilising a randomised placebo-controlled crossover design were included. Other specific
eligibility criteria were: (i) participants had to be healthy, human adolescents or adults (age ≥ 16 years); (ii) studies had to
evaluate dietary NO3- supplementation such as nitrate-rich vegetable sources or beetroot juice; (iii) studies evaluating multiple
supplements were included only if the placebo vs. nitrate-only supplementation protocol was able to be compared; (iv) studies
had to include an outcome of a quantifiable measure of exercise performance lasting ≥ 30 seconds (for a single bout of exercise
or the combined total for multiple bouts).
2.3 Data Extraction and Analysis
Two researchers (NM and ML) independently assessed the retrieved title and abstract with clearly irrelevant studies excluded.
Full papers of abstracts potentially eligible for inclusion were then screened (NM and ML). Differences in opinion were resolved
through discussion and consensus with a third reviewer (TP).
2.4 Data Extraction
Data was extracted using a standardised form. The primary outcome measures in this review were changes in exercise
performance after dietary NO3- supplementation. Data on participant characteristics (sex, age, training status, and maximal rate
of oxygen uptake (𝑽
̇O2max/peak - when reported), intervention protocol (dose and delivery method), study methodology, exercise
protocol (type, duration and exercise assessment), percentage difference between NO3- and placebo, significant performance
effect, and trial results were extracted systematically by one researcher (NM) and substantiated by a second (ML). The effect of
dietary NO3- supplementation was calculated at the end of the exercise assessment, as [(meannitrate meanplacebo) ÷ meanplacebo x
100]. If a study included an additional NO3- protocol or exercise assessment it was extracted separately and included as another
trial. A time trial was defined as a timed race over a specified course or distance. A time to exhaustion trial was defined as a
single step increment in work rate that is continued until exhaustion. A graded-exercise performance test was defined as a
multiple step or continuous ramp incremental test until exhaustion.
2.5 Quality assessment
The studies were assessed for quality using the Physiotherapy Evidence Database (PEDro) scale [25]. PEDro scale items and
operational definitions of each item are given in the Electronic Supplementary Material Appendix S1. The PEDro scale was used
because of its ability to objectively and reliably assess a randomised controlled trial’s (RCT) internal validity [25]. Each article was
independently analysed by two reviewers (NM and ML) using the 11-item checklist to yield a maximum score of 10. The kappa
value signifying the level of agreement between reviewers was k = 0.94. Differences in opinion concerning the scoring of an
article were settled via discussion with a third reviewer (TP).
2.6 Statistical analysis
Data synthesis was descriptive, with detailed tabular summaries presented. For the primary outcomes of TT performance (n =
28), TTE (n = 22), and GXT (n = 8), we were able to consistently extract data across studies to allow a quantitative summary using
a meta-analysis (where the performance outcome could be measured in seconds). Trials that could not be measured in seconds
were excluded from the meta-analysis due to the quantitative differences [6, 10, 14, 32, 41, 49, 53, 55, 70]. Despite the
difference in physiological stressors between the hypoxic and normoxic trials, a sub-analysis was not undertaken due to the
small number of hypoxic trials (n = 6; TT = 3, TTE = 3). We compared absolute changes and calculated a standardised mean
difference (95% confidence intervals) for each study.
Heterogeneity was investigated by reviewing study populations, methods, and interventions, and by using the χ2 test
for homogeneity and the I2 statistic. A random effects model for the meta-analysis was used unless statistical heterogeneity was
identified 2 test, p 0.05, or I2 50%). The random effects model was applied because of the considerable variability in several
experimental factors (e.g., test and dose duration, dose amount) across trials. However, random and fixed effects models
produced the same results. Hedges’ g was used to determine potential bias due to the reasonably small sample sizes prevalent
across the studies [26]. Effect sizes were interpreted using Cohen’s definitions of trivial (< 0.2), small (0.2-0.3), moderate (0.5)
and large (> 0.8) [27]. Analysis was conducted using Review Manager 5.0 (Nordic Cochrane Centre, Copenhagen, Denmark).
The level of agreement between reviewers evaluating the study quality was assessed using Cohen’s kappa statistics
using SPSS for Windows, Version 23.0 (Armonk, NY: IBM Corp.). The kappa values were interpreted using the ranges suggested
by Landis and Koch [28] of < 0.00 = poor, 0.00 0.20 = slight, 0.21 0.40 = fair, 0.41 0.60 = moderate, 0.61 0.80 = substantial,
0.81 1.00 = almost perfect.
Eight trial features were identified as potential moderator variables. The analysis included dichotomous data of exercise
type (cycling or other), test duration (≥ 10 mins or < 10 mins), dose duration (acute (< 6 hours) or chronic (repeated doses 6
hours apart)), NO3- type (beetroot juice or other), NO3- dose (< 6.5 mmol or ≥ 6.5 mmol), and study quality (< 9 or ≥ 9 (assessed
using the PEDro scale)). Fitness level (𝑉
̇O2max) and percentage nitrite change were analysed using continuous data. Univariate
meta-regression was used to assess the association between each potential moderating variable and TT and TTE performance
outcomes. Univariate meta-regression was not used for trials utilising graded-exercise performance tests as there were fewer
than 10 studies.
As outcomes were continuous, we assessed for publication bias using Egger’s test and by visual inspection of funnel
plots, with a p-value of > 0.10 considered statistically significant (publication bias was not assessed for the GXT, for which there
were fewer than 10 studies) [29, 30].
3. Results
The bibliographic search yielded 1,038 articles (Figure 1) for preliminary screening of titles and abstracts, with 62 full-text
articles retrieved, and 47 identified as meeting the inclusion criteria.
Figure 1:
Flowchart of study selection. PEDro = Physiotherapy evidence database scale
3.1 Study characteristics
The characteristics of each study and the physiological changes are summarised in Table 1. Multiple studies utilised
more than one category of participants: dose-response trials [31-33]; different distances [16]; different exercise intensities [34-
37]; acute (< 6 hours) or chronic (repeated doses ≥ 6 hours apart) [22, 10]; hypoxia vs normoxia [38, 39]; sex [40, 41]; different
exercise protocols [42]; or level of fitness [14]. Consequently, these studies were reported as two or more trials, raising the total
number of cross-over trials to 76 across 47 publications, each with a NO3- and placebo condition.
The studies were published between 2007 [6] and 2015 [43]. Three types of performance assessments were utilised
across the studies, with 38 examining the effect of dietary NO3- supplementation vs. placebo on exercise time/distance (TT
summarised in Table 1), 22 trials using a TTE protocol (TTE summarised in Table 2), and 16 included a graded-exercise
performance test (GXT summarised in Table 3). Sixty-one trials showed improved performance after dietary NO3-
supplementation, 29 of which were statistically significant (p < 0.05), and in one study, decreased performance was observed
following NO3- supplementation [44]. Following dietary NO3- supplementation, 20 of the 22 TTE trials showed a mean
improvement in performance (16 of which showed significant improvements), as did 27 of the 38 TTs and 14 of the 16 GXTs (of
which 7 and 5 were significant improvements, respectively).
Cycling was the most common method of exercise, utilised in 44 of the 76 trials. Fourteen opted for treadmill running
[14, 18, 19, 22, 35, 36, 42, 74], 6 utilised field running [14, 43, 58], 3 used kayaking [13, 41], 3 used rowing [33, 48], 3 used
resistance training in the form of knee extensions [9, 36, 45], and 1 each for underwater diving [46], walking [49], and arm/leg
crank [51]. Eight trials investigated exercise performance in hypoxic conditions [45, 46, 12, 47, 38, 42, 39]. Exercise duration
ranged from 1.5 to 137 minutes. All studies included a NO3- and placebo group.
3.2 Characteristics of subjects
In total, 581 participants (494 males, 87 females) participated in the included studies. The mean ages ranged from 16.7 [48] to
64 [49] years. Fifty-nine trials had male only participants, 4 trials had exclusively women subjects [40, 41, 50, 43], and 13 trials
had both sexes [51, 10, 45, 46, 18, 11, 49, 44, 35]. The number of participants involved in the trials ranged from 5 [41] to 28 [32],
with a mean sample size of 10.8 ± 4.
𝑉
̇O2max values were reported in 53 trials, with values ranging between 28.2 and 81.1 mL kg-1 min-1. Porcelli et al. [14]
implemented 2 trials with participants classified in the “low aerobic fitness” group (28.1- 44.1 mL kg-1 min-1). The remaining 51
trials included participants with a 𝑉
̇O2max > 45 mL kg-1 min-1, and ranged from physically active and well-trained, right up to
elite international level athletes.
3.3 Nitrate administration
The trials utilised a variety of dietary NO3- supplementation types. The majority opted for beetroot juice (n=58; 76%) as the
source of NO3- delivery, 6 used NO3- water [14], 4 used sodium NO3- [6, 51-53], 3 utilised pomegranate extract [35], 3 used
potassium NO3- [19, 54, 55], and 1 trial each for NO3- gel [56] and beetroot portions [18]. There was a large variability in the
amount of NO3- given per dose, with doses ranging from 4.1mmol [32] 19.5mmol per day [22].
The intervention period ranged from 30 minutes to 15 days prior to testing. Forty trials had an acute invention protocol,
whereas 36 trials utilised a chronic dietary NO3- supplementation protocol.
3.4 Methodological quality of studies
The mean PEDro score was 8.8 ± 1.1 out of 10. All 47 studies reviewed scored a moderate to high score of 7 and above. Thirty-
nine of the 47 studies reported blinding of both the assessors and participants, and received a perfect 10 score, 4 studies scored
8 out of 10 as they failed to blind therapists and assessors thus opting for a single-blind crossover study design [13, 32, 56, 41],
and the remaining 4 studies scored 7 out of 10 due to a lack of allocation concealment and single-blind crossover studies [10, 12,
20, 21]. Overall, the study quality was deemed to be good to excellent.
Table 1 Summary of studies examining the effect of NO3- on time trial performance.
Reference
PEDro
score
Sample
size and
sex
Fitness level
(𝑉
̇O2max/peak, mL·kg·min1
[mean ± SD])
Exercise protocol
Percentage NO3-/
NO2-
change
Trial result
(mean ± SD)
% Difference
Lansley et al.
2011 [16]
10
9
M
Well-trained cyclists
(𝑉
̇O2peak 56 ± 5.7)
4km TT
Cycle ergometer
139% NO2-*
TT
N: 376.2 ± 21 s
P: 387 ± 25.2 s
2.79*
Lansley et al.
2011 [16]
10
9
M
Well-trained cyclists
(𝑉
̇O2peak 56 ± 5.7)
16.1km TT
Cycle ergometer
139% NO2-*
TT
N: 1614 ± 108 s
P: 1662 ± 126 s
2.89*
Bescós et al.
2012 [53]
10
13
M
Cyclists and triathletes
40-min TT
Cycle ergometer
79% NO2-*
TT
N: 26.4 ± 1.1 km
P: 26.3 ± 1.2 km
0.38
Bond et al.
2012 [48]
10
14
M
Well-trained junior rowers
6x500m maximal TT
Rowing ergometer
Not reported
TT (1-6)
N: 89.4 ± 3.2s
P: 90.19 ± 2.9s
0.88
Cermak et al.
2012 [57]
10
13
M
Well-trained cyclists and
triathletes
(𝑉
̇O2max 58 ± 2)
10km TT
Cycle ergometer
1906.67% NO3-*
TT
N: 953 ± 75.7 s
P: 965 ± 75.7 s
1.24*
Cermak et al.
2012 [17]
10
20
M
Well-trained cyclists and
triathletes
(𝑉
̇O2max 60 ± 1)
~1h cycling at 75% Wmax
(energy expenditure
based) TT
Cycle ergometer
96% NO2-*
TT
N: 3930 ± 295.2 s
P: 3900 ± 295.2 s
-0.77
Murphy et al.
2012 [18]
10
11
Both
Recreationally fit
5km TT
Treadmill
Not measured
TT
N: 1541 ± 380 s
P: 1581 ± 382 s
2.53
Peacock et al.
2012 [19]
10
10
M
Junior-elite cross-country skiers
(𝑉
̇O2max 69.6 ± 5.1)
5km TT treadmill
127% NO2-*
TT
N: 1005 ± 53 s
P: 996 ± 49 s
-0.9
Wilkerson et
al. 2012 [20]
7
8
M
Well-trained cyclists
(𝑉
̇O2max 63 ± 8)
50 mile TT
Cycle ergometer
25% NO2-*
TT
N: 8202 ± 336 s
P: 8274± 384 s
0.87
Christensen et
al. 2013 [21]
7
10
M
Elite cyclists
(𝑉
̇O2max 72.1 ± 4.5)
~400kcal (15-20min)
cycling TT
297% NO3-*
TT:
N: 1100 ± 163 s
P: 1117 ± 167 s
1.52
Kelly et al.
2013 [49]
10
12
Both
Older participants (> 60 yrs.)
6-min walk test TT
418% NO2-*
TT
N: 682 ± 89 m
P: 667 ± 86 m
2.25
Muggeridge
et al. 2013
[13]
8
8
M
Trained kayakers
(𝑉
̇O2max 49 ± 6.1)
1km TT
kayak ergometer
32% NO2-*
TT
N: 276 ± 14.1 s
P: 277 ± 14.1 s
0.36
Boorsma et al.
2014 [22]
10
8
M
Elite 1500m runners
(𝑉
̇O2max 80 ± 5)
1500m TT on indoor
track
(Chronic > acute*)
TT
N: 250.7 ± 4.3 s
P: 250.4 ± 7 s
-0.12
Boorsma et al.
2014 [22]
10
8
M
Elite 1500m runners
(𝑉
̇O2max 80 ± 5)
1500m TT on indoor
running track
(Chronic > acute*)
TT
N: 250.5 ± 6.2 s
P: 251.4 ± 7.6 s
0.36
Hoon et al.
2014 [32]
8
28
M
Trained cyclists
4-min TT
Cycle ergometer
22% NO2-*
TT1
N150: 402 ± 47 W
P: 396 ± 57 W
1.52
Hoon et al.
2014 [32]
8
28
M
Trained cyclists
4-min TT
Cycle ergometer
70% NO2-*
TT1
N75: 403 ± 52 W
P: 396 ± 57 W
1.77
Hoon et al.
2014 [32]
8
28
M
Trained cyclists
4-min TT
Cycle ergometer
38% NO2-*
TT1
N-Top: 400 ± 48 W
P: 396 ± 57 W
1.01
Hoon et al.
2014 [32]
8
28
M
Trained cyclists
4-min TT
Cycle ergometer
Not reported
TT2
N150: 396 ± 46 W
P: 397 ± 56 W
-0.25
Hoon et al.
2014 [32]
8
28
M
Trained cyclists
4-min TT
Cycle ergometer
Not reported
TT2
N75: 396 ± 54 W
P: 397 ± 56 W
-0.25
Hoon et al.
2014 [32]
8
28
M
Trained cyclists
4-min TT
Cycle ergometer
Not reported
TT2
N-Top: 396 ± 45W
P: 397 ± 56 W
-0.25
Hoon et al.
2014 [33]
10
10
M
Highly trained
2000m TT
Rowing ergometer
Not reported
TT
N: 383.4 ± 8.7s
P: 383.5 ± 9s
0.03
Hoon et al.
2014 [33]
10
10
M
Highly trained
2000m TT
Rowing ergometer
Not reported
TT
N: 381.9 ± 9s
P: 383.5 ± 9s
0.42
Kokkinoplitis
and Chester
2014 [70]
10
7
M
Healthy
5 x 6-sec sprints
interspersed with 30-
sec recovery
Treadmill
Not measured
TT
N: 4133.5 ± 674.4
W
P: 3938.3 ± 603.1
W
4.96
Lane et al.
2014 [40]
10
12
M
Competitive cyclists
(𝑉
̇O2peak 71.6 ± 4.6)
43.83km TT
Cycle ergometer
Not reported
TT
N: 3845.03 ±
196.15 s
P: 3813.39 ±
170.09 s
-0.91
Lane et al.
2014 [40]
10
12
F
Competitive cyclists
(𝑉
̇O2peak 59.9 ± 5.1)
29.35km TT
Cycle ergometer
Not reported
TT
N: 3101.06 ±
159.51
P: 3100.10 ±
151.71
-0.03
Muggeridge
et al. 2014
[47]
10
9
M
Trained cyclists
(𝑉
̇O2peak (at altitude) 51.9 ± 5.8)
16.1km TT
Cycle ergometer
242% NO2-*
TT
N: 1664 ± 42 s
P: 1702 ± 45 s
2.23*
Muggeridge
et al. 2014
[56]
8
9
M
Trained cyclists and triathletes
(𝑉
̇O2max 53.1 ± 4.4)
16.1km TT
Cycle ergometer
61.6% NO2-*
TT
N: 1455 ± 47 s
P: 1469 ± 52 s
0.95
Peeling et al.
2014 [41]
8
6
M
National-level kayakers
(𝑉
̇O2peak 57.15 ± 2.8)
4-min TT
kayak ergometer
Not measured
TT
N: 989 ± 31 mtrs
P: 982 ± 36 mtrs
-0.71
Peeling et al.
2014 [41]
8
5
M
International-level kayakers
(𝑉
̇O2peak 47.8 ± 3.7)
500m TT Kayak
Not measured
TT
N: 114.6 ± 1.5 s
P: 116.7 ± 2.2 s
1.8*
Porcelli et al.
2014 [14]
10
8
M
Participants with a low fitness
level
(𝑉
̇O2peak range 28.2-44.1)
3km TT on a running
track
Not reported
TT
N: 886 ± 74 s
P: 910 ± 82 s
2.64*
Porcelli et al.
2014 [14]
10
7
M
Participants with a moderate
fitness level
(𝑉
̇O2peak range 45.5-57.1)
3km TT on a running
track
Not reported
TT
N: 723 ± 90 s
P: 734 ± 93 s
1.5*
Porcelli et al.
2014 [14]
10
6
M
Participants with a high fitness
level
(𝑉
̇O2peak range 63.9-81.1)
3km TT on a running
track
Not reported
TT
N: 627 ± 30 s
P: 629 ± 28 s
0.32
Sandbakk et
al. 2014 [54]
10
9
M
Junior-elite cross-country skiers
(𝑉
̇O2max 69.3 ± 5.8)
5km TT on indoor
running track
120.1% NO2-*
TT
N: 1016 ± 52s
P: 1005 ± 47s
-1.09
Arnold et al.
2015 [42]
10
10
M
Well-trained competitive
runners
(𝑉
̇O2peak 66 ± 7)
10km TT
Treadmill
675% NO2-*
TT
N: 2862 ± 233 s
P: 2874 ± 265 s
0.42
Buck et al.
2015
[43]
10
13
F
Team-sport trained
Simulated team-game
circuit.
With 6 x 20-m
repeated-sprint set
performed at the start,
half-time and end
Running
891% NO3-*
TT (sprints)
N: 69.84 ± 4.94 s
P: 69.97 ± 4.17 s
0.19
Glaister et al.
2015 [50]
10
14
F
Well-trained cyclists and
triathletes
(𝑉
̇O2max 52.3 ± 4.9)
20km TT
Cycle ergometer
223.7% NO2-*
TT
N: 2119.8 ± 90 s
P: 2122.2± 102 s
0.11
MacLeod et
al. 2015 [39]
10
11
M
Trained cyclists
(𝑉
̇O2peak 67.5 ± 5.8)
10km TT
(normoxia)
Cycle ergometer
441% NO3-*
TT
N: 961 ± 54 s
P: 954 ± 47 s
-0.73
MacLeod et
al. 2015 [39]
10
11
M
Trained cyclists
(𝑉
̇O2peak 67.5 ± 5.8)
10km TT
(hypoxia)
Cycle ergometer
441% NO3-*
TT
N: 1018 ± 52 s
P: 1023 ± 49 s
0.49
* = significantly different from placebo (as reported within studies; p < 0.05)
𝑉
̇O2max = maximal oxygen uptake 𝑉
̇O2peak = peak oxygen uptake PEDro = physiotherapy evidence database scale TT = time trial N = NO3- P = placebo
BR = beetroot juice M = male F = female s = seconds W = watts km∙h-1 = kilometres per hour Wmax = maximal power
N-Top = NO3- top up = increase
Table 2 Summary of studies examining the effect of NO3- on time to exhaustion performance.
Reference
PEDro
score
Sample
size and
sex
Fitness level
(𝑉
̇O2max/peak, mL·kg·min1
[mean ± SD])
Exercise protocol
Percentage NO3-/
NO2-
change
Trial result
(mean ± SD)
% Difference
Bailey et al.
2009 [15]
10
8
M
Recreationally fit
(𝑉
̇O2max 49 ± 5)
SI TTE
Cycling ergometer
96% NO2-*
TTE
N: 675 ± 203 s
P: 583 ± 145 s
15.78*
Bailey et al.
2010 [9]
10
7
M
Recreationally fit
2-legged HI (30% MVC)
knee-extension TTE
137% NO2-*
TTE
N: 734 ± 290 s
P: 586 ± 212 s
25.26*
Lansley et al.
2011 [36]
10
9
M
Physically active
(𝑉
̇O2max 55 ± 7)
SI run TTE
Treadmill
104% NO2-*
TTE
N: 522 ± 108 s
P: 456 ± 90 s
14.47*
Vanhatalo et
al. 2011 [45]
10
9
Both
Recreationally fit
Knee extension TTE
50% NO2-*
TTE
N: 477 ± 200 s
P: 393 ± 169 s
21.37*
Engan et al.
2012 [46]
10
12
Both
Well-trained apnea divers
Apnea TTE
Not measured
TTE
N: 278 ± 64 s
P: 250 ± 58 s
11.2*
Breese et al.
2013 [11]
10
9
Both
Recreationally active
(𝑉
̇O2max M: 48.4 ± 6, F: 46.4 ± 9)
SI TTE
Cycling ergometer
435% NO2-*
TTE
N: 635 ± 258 s
P: 521 ± 158 s
21.88*
Handzlik and
Gleeson 2013
[68]
10
14
M
Well-trained
(𝑉
̇O2max 63 ± 10)
Cycling (80% VO2max)
TTE
Cycle ergometer
Not reported
TTE
N: 1240 ± 994 s
P: 1003 ± 480 s
23.63
Kelly et al.
2013 [34]
10
9
M
Habitually active
(𝑉
̇O2max 54.5 ± 7.5)
TTE (60% peak power)
Cycle ergometer
208.7% NO2-*
TTE
N: 696 ± 120 s
P: 593 ± 68 s
17.37*
Kelly et al.
2013 [34]
10
9
M
Habitually active
(𝑉
̇O2max 54.5 ± 7.5)
TTE (70% peak power)
Cycle ergometer
156.3% NO2-*
TTE
N: 452 ± 106 s
P: 390 ± 86 s
15.9*
Kelly et al.
2013 [34]
10
9
M
Habitually active
(𝑉
̇O2max 54.5 ± 7.5)
TTE (80% peak power)
Cycle ergometer
181.2% NO2-*
TTE
N: 294 ± 50 s
P: 263 ± 50 s
11.79*
Kelly et al.
2013 [34]
10
9
M
Habitually active
(𝑉
̇O2max 54.5 ± 7.5)
TTE (100% peak power)
Cycle ergometer
227.6% NO2-*
TTE
N: 182 ± 37 s
P: 166 ± 20 s
9.64
Wylie et al.
2013 [31]
10
10
M
Recreationally active
SI TTE
Cycle ergometer
Not reported
TTE
N: 508 ± 102 s
P: 470 ± 81 s
8.09
Wylie et al.
2013 [31]
10
10
M
Recreationally active
SI TTE
Cycle ergometer
Not reported
TTE
N: 570 ± 153 s
P: 498 ± 113 s
14.46*
Wylie et al.
2013 [31]
10
10
M
Recreationally active
SI TTE
Cycle ergometer
Not reported
TTE
N: 552 ± 117 s
P: 493 ± 114 s
11.97*
Kelly et al.
2014 [38]
10
12
M
Physically active
(𝑉
̇O2peak 58.3 ± 6.3)
SI cycling TTE
(hypoxia)
Cycle ergometer
242% NO2-*
TTE
N: 214 ± 43 s
P: 197 ± 28 s
8.63*
Kelly et al.
2014 [38]
10
12
M
Physically active
(𝑉
̇O2peak 58.3 ± 6.3)
SI cycling TTE
(normoxia)
Cycle ergometer
557% NO2-*
TTE
N: 412 ± 139 s
P: 431 ± 124 s
-4.41
Martin et al.
2014 [44]
10
16
Both
Moderately trained - team sport
(𝑉
̇O22max M: 57.4 ± 8, F: 47.2 ±
8)
8-sec sprints
interspersed with 30-
sec active rest TTE
Cycle erogometer
Not reported
HIIST
N: 104 ± 40 s
P: 120 ± 48 s
-13.33
Thompson et
al. 2014 [69]
10
16
M
Recreationally active
(𝑉
̇O2max 47.3 ± 6.3)
1x TTE (~90% VO2max)
Cycle ergometer
79% NO2-*
TTE
N: 185 ± 122 s
P: 160 ± 109 s
15.63*
Trexler et al.
2014 [35]
10
19
Both
Highly active
(𝑉
̇O2max 51.3 ± 9.4)
TTE (90% PV)
Treadmill
Not reported
TTE
N: 387.9 ± 199.2 s
P: 346 ± 162.5 s
12.11*
Trexler et al.
2014 [35]
10
19
Both
Highly active
(𝑉
̇O2max 51.3 ± 9.4)
TTE (100% PV)
Treadmill
Not measured
TTE
N: 170.8 ± 66.3 s
P: 159.3 ± 62.3 s
7.22*
Trexler et al.
2014 [35]
10
19
Both
Highly active
(𝑉
̇O2max 51.3 ± 9.4)
TTE (110% PV)
Treadmill
Not measured
TTE
N: 108.8 ± 45.1 s
P: 104.4 ± 40.1 s
4.21
Aucouturier
et al. 2015
[59]
8
12
M
Healthy
(𝑉
̇O2peak 46.6 ± 3.4)
15-sec sprints
interspersed with 30-
sec recovery
Cycle ergometer
108% NO2-*
TTE
N: 1176 ± 486 s
P: 984 ± 360 s
19.51*
* = significantly different from placebo (as reported within studies; p < 0.05)
𝑉
̇O2max = maximal oxygen uptake 𝑉
̇O2peak = peak oxygen uptake PEDro = physiotherapy evidence database scale TTE = time to exhaustion N = NO3- P = placebo
BR = beetroot juice M = male F = female s = seconds W = watts km∙h-1 = kilometres per hour HI = high-intensity SI = severe-intensity
PV = peak velocity MVC = maximal voluntary contraction HIIST = high-intensity interval sprint training = increase
Table 3 Summary of studies examining the effect of NO3- on graded exercise performance.
Reference
PEDro
score
Sample
size and
sex
Fitness level
(𝑉
̇O2max/peak, mL·kg·min1
[mean ± SD])
Exercise protocol
Percentage NO3-/
NO2-
change
Trial result
(mean ± SD)
% Difference
Larsen et al.
2007 [6]
10
9
M
Well-trained cyclists or
triathletes
(𝑉
̇O2peak 55 ± 3.7)
Incremental TTE
Cycle ergometer
82% NO2-*
Maximal work
capacity
N: 360.6 ± 32.8 W
P: 358.9 ± 32.3 W
0.47
Larsen et al.
2010 [51]
10
9
Both
Recreationally fit
(𝑉
̇O2max 3.72 ± 0.33 L∙kg∙min−1)
Combined arm + leg
crank (separate
ergometers)
Incremental TTE
133% NO2-*
TTE
N: 563 ± 90.1 s
P: 524 ± 93.7 s
7.44
Vanhatalo et
al. 2010 [10]
7
8
Both
Recreationally fit
(𝑉
̇O2max 47 ± 8)
Incremental TTE
Cycle ergometer
36% NO2-*
TTE
N: 325 ± 71 W
P: 322 ± 68 W
0.93
Vanhatalo et
al. 2010 [10]
7
8
Both
Recreationally fit
(𝑉
̇O2max 47 ± 8)
Incremental TTE
Cycle ergometer
Not reported
TTE
N: 328 ± 68 W
P: 323 ± 67 W
1.55
Vanhatalo et
al. 2010 [10]
7
8
Both
Recreationally fit
(𝑉
̇O2max 47 ± 8)
Incremental TTE
Cycle ergometer
46% NO2-*
TTE
N: 331 ± 68 W
P: 323 ± 68 W
2.48*
Bescós et al.
2011 [52]
10
11
M
Cyclists and triathletes
(𝑉
̇O2peak 65.1 ± 6.2)
Incremental TTE
Cycle ergometer
15.77% NO2-*
TTE
N: 416 ± 106.1 s
P: 409 ± 89.5 s
1.71
Lansley et al.
2011 [36]
10
9
M
Physically active
(𝑉
̇O2max 55 ± 7)
Incremental knee
extension TTE
104% NO2-*
Knee TTE
N: 510 ± 48 s
P: 492 ± 54 s
3.66*
Masschelein
et al. 2012
[12]
7
15
M
Physically active
(𝑉
̇O2peak 61.7 ± 2.1)
Incremental TTE
Cycle ergometer
39% NO2-*
TTE
N: 597 ± 85.2 s
P: 568 ± 89.1 s
5.11*
Wylie et al.
2013 [58]
10
14
M
Team sport trained
(𝑉
̇O2max 52 ± 7)
Yo-Yo IR1 TTE test
395% NO2-*
TTE
N: 1704 ± 304 s
P: 1638 ± 288 s
4.03*
Porcelli et al.
2014 [14]
10
8
M
Participants with a low aerobic
fitness
(𝑉
̇O2peak range 28.2-44.1)
Incremental TTE
treadmill
Not reported
Peak speed
N: 14.5 ± 0.8 km∙h-1
P: 14.4 ± 1.2 km∙h-1
0.69*
Porcelli et al.
2014 [14]
10
7
M
Participants with a moderate
aerobic fitness
(𝑉
̇O2peak range 45.5-57.1)
Incremental TTE
treadmill
Not reported
Peak speed
N: 17.7 ± 1.9 km∙h-1
P: 17.4 ± 1.9 km∙h-1
1.72*
Porcelli et al.
2014 [14]
10
6
M
Participants with a high aerobic
fitness
(𝑉
̇O2peak range 63.9-81.1)
Incremental TTE
treadmill
Not reported
Peak speed
N: 20.0 ± 0.9 km∙h-1
P: 20.0 ± 1.4 km∙h-1
0
Arnold et al.
2015 [42]
10
10
M
Well-trained competitive
runners
(𝑉
̇O2peak 66 ± 7)
Incremental step TTE
Treadmill
675% NO2-*
TTE
N: 402 ± 80 s
P: 393 ± 62 s
2.29
Bailey et al.
2015 [37]
10
7
M
Recreationally active
SI step test (35 rpm)
Cycle ergometer
179% NO2-*
TTE
N: 344 ± 74 s
P: 341 ± 99 s
0.88
Bailey et al.
2015 [37]
10
7
M
Recreationally active
SI step test (115 rpm)
Cycle ergometer
179% NO2-*
TTE
N: 362 ± 137 s
P: 297 ± 79 s
21.89*
Carpentier et
al. 2015 [55]
10
13
M
Healthy
(𝑉
̇O2peak 46.8 ± 1.1)
Incremental step TTE
(85% VO2max)
Cycle ergometer
Not measured
TTE
N: 178 ± 15 W
P: 179 ± 15 W
0.56
* = significantly different from placebo (as reported within studies; p < 0.05)
𝑉
̇O2max = maximal oxygen uptake 𝑉
̇O2peak = peak oxygen uptake PEDro = physiotherapy evidence database scale TTE = time to exhaustion N = NO3- P = placebo
BR = beetroot juice M = male F = female s = seconds W = watts km∙h-1 = kilometres per hour SI = severe-intensity
IR1 = intermittent recovery test level 1 = increase rpm = revolutions per minute
Page 22 of 38
3.5 Meta-Analysis
3.5.1 Time trial performance
Following data pooling from 28 trials, the standardised mean difference was -0.10 (95% Cl -0.27 - 0.06),
providing a trivial, but non-significant effect in favour of dietary NO3- supplementation in TT performance
measures (p > 0.05) as shown in Fig. 2. There was no heterogeneity displayed among these studies (I2 = 0%; Q
= 7.46, df = 27, p = 1.00), utilising a random effects analysis
Figure 2: Effect size forest plot for the effect of dietary NO3- supplementation on time trial
performance (means ± 95% confidence intervals). ES effect size, SD standard deviation, CI confidence
interval, SMD standardised mean difference, ergo ergometer, kcal kilo calorie
3.5.2 Time to exhaustion
The standardised mean difference from 22 trials was 0.33 (95% Cl 0.15 - 0.50), indicating a small to moderate
statistically significant effect in favour of dietary NO3- supplementation in TTE performance measures (p <
0.01) as shown in Fig. 3. There was no heterogeneity displayed among these studies (I2 = 0%; Q = 9.82, df = 21,
p = 0.98) utilising a random effects analysis.
Page 23 of 38
Figure 3: Effect size forest plot for the effect of dietary NO3- supplementation on time to exhaustion
performance (means ± 95% confidence intervals). ES effect size, SD standard deviation, CI confidence
interval, SMD standardised mean difference, ergo ergometer, PV peak velocity, HIIST high-intensity
interval sprint training, SI severe-intensity, HI high-intensity, PP peak power
3.5.3 Graded-exercise performance test
The standardised mean difference from 8 trials was 0.25 (95% Cl -0.06 - 0.56), providing a small, but non-
significant effect in favour of dietary NO3- supplementation in GXT performance measures (p > 0.05) as shown
in Fig. 4. There was no heterogeneity displayed among these studies (I2 = 0%; Q = 0.90, df = 7, p = 1.00) utilising
a random effects analysis.
Figure 4: Effect size forest plot for the effect of dietary NO3- supplementation on graded-exercise
test performance (means ± 95% confidence intervals). ES effect size, SD standard deviation, CI
confidence interval, SMD standardised mean difference, ergo ergometer, rpm revolutions per
minute, IR1 intermittent recovery test level 1
Page 24 of 38
Publication bias was assessed by visual inspection of the funnel plot of standard error verses ES for
both TT (Fig. 5) and TTE (Fig. 6), with minor asymmetrical inverted distributions prominent for both plots. For
both TT performance and TTE, there was evidence of publication bias, Egger’s test <0.02 and <0.001
respectively, suggesting small study bias.
3.5.4 Meta-regression analyses
There was no statistically significant effects observed from the meta-regression analysis. Data from the
analyses of moderator variables are presented in Tables 4 and 5. A positive trend towards significance (p =
0.11) was seen in trials implementing a chronic dosage regime in the TTE protocol.
Table 4 Time trial univariate meta-regression
Trial feature
Classification
Number
of trials
SMD (95% CI)
Z-value
p-value
Dichotomous outcomes
Exercise type
Other
Cycling
15
13
-0.013 (-0.35, 0.32)
0.08
0.94
Test duration
< 10 mins
≥ 10 mins
9
19
0.12 (-0.24, 0.49)
0.66
0.51
Dose duration
Acute
Chronic
19
9
0.02 (-0.33, 0.37)
0.11
0.92
NO3- type
Other
Beetroot
7
21
-0.10 (-0.53, 0.33)
0.47
0.64
NO3- dose
< 6.5 mmol
≥ 6.5 mmol
11
16
0.23 (-0.12, 0.58)
1.29
0.20
Continuous outcomes
Fitness level
𝑉
̇O2max
23
0.008 (-0.010, 0.026)
0.87
0.39
% NO2- change
11
-0.0001 (-0.0016,
0.0015)
0.10
0.92
SMD = standardised mean difference, NO2- = nitrite, CI = confidence interval
Page 25 of 38
Table 5 Time to exhaustion univariate meta-regression
Trial feature
Classification
Number
of trials
SMD (95% CI)
Z-value
p-value
Dichotomous outcomes
Exercise type
Other
Cycling
7
15
0.05 (-0.31, 0.41)
0.27
0.78
Test duration
< 10 mins
≥ 10 mins
16
6
0.25 (-0.17, 0.67)
1.18
0.24
Dose duration
Acute
Chronic
10
12
0.29 (-0.064, 0.67)
1.59
0.11
NO3- type
Other
Beetroot
3
19
0.22 (-0.20, 0.64)
1.03
0.30
NO3- dose
< 6.5 mmol
≥ 6.5 mmol
8
11
0.17 (-0.23, 0.57)
0.84
0.40
Continuous outcomes
Fitness level
𝑉
̇O2max
16
0.003 (-0.045, 0.046)
0.01
0.99
% NO2- change
13
-0.0008 (-0.0025,
0.0009)
0.95
0.34
SMD = standardised mean difference, NO2- = nitrite, CI = confidence interval
3.6 Adverse Events
Information on adverse events was reported in 6 of the 47 studies. Bailey et al. [15], Bailey et al. [9], Vanhatalo
et al. [10] and Wylie et al. [31] reported beeturia (red urine) and red stools. Hoon et al. [32] reported the
withdrawal of one subject due to a beetroot juice intolerance. Hoon et al. [33] reported slight gastrointestinal
symptoms immediately after beetroot juice ingestion across the exercise trials, while another reported minor
discomfort before one trial. Both occurrences were resolved prior to performance tests. Peeling et al. [41]
measured NO3- ingestion and its effects on gut sensation. The results showed a lower level of gut distress after
a double dose beetroot juice (~9.6 mmol of NO3-) when compared to the placebo protocol. No major adverse
events were reported across the 47 studies.
Page 26 of 38
Figure 5: Funnel plot of Hedges g effect size versus study standard error - outcome: time trial.
Figure 6: Funnel plot of Hedges g effect size versus study standard error - outcome: time to
exhaustion.
4. Discussion
The primary aim of this study was to perform a systematic review and meta-analysis to determine the efficacy
of dietary NO3- supplementation on endurance exercise performance. The pooled analysis for nitrate’s
Page 27 of 38
influence on TTE showed a significantly greater ES when compared to a placebo control. However, the small
effects of dietary NO3- supplementation on TT and GXT performance were not statistically significant. The
main conclusion of this meta-analysis was the differing effects dietary NO3- supplementation had on TT and
TTE protocols.
The findings of this meta-analysis are similar to that of a previous meta-analysis of the impact of
dietary NO3- supplementations on exercise performance. In Hoon et al.’s [23] meta-analysis TT protocols had
an ES of -0.11 (n = 9) compared to an ES of -0.12 (n = 24) in the present study. In addition, Hoon et al.’s [23]
meta-analysis of GXT protocols had an ES of 0.23 (n = 7) compared to an ES of 0.25 (n = 8) in the present study.
Hoon et al. [23] also found that dietary NO3- supplementation has a statistically significant effect on TTE
protocols (ES = 0.79; n = 3). Similarly, the results of the current meta-analysis suggest dietary NO3-
supplementation is more likely to affect TTE protocols (ES = 0.33; n = 22). The larger number of trials in the
current meta-analysis reinforce the findings reported by Hoon et al. [23] and strengthens the evidence for
dietary NO3- supplementation. This review and quantitative analysis provides an important contribution to the
literature and suggests that there is clear evidence that dietary NO3- supplementation can boost aerobic
exercise capacity measured by TTE protocols.
This enhanced exercise performance in TTE protocols is likely due to the reduced whole-body O2 cost
of constant-work-rate exercise following dietary NO3- supplementation [15, 51, 76]. Bailey et al. [9] reported
that the decrease in O2 cost correlates with a reduced ATP cost of muscle force production, creating a
reduction in the phosphocreatine degradation, as well as a reduced accumulation of adenosine diphosphate
and inorganic phosphate concentration during low and high-intensity exercise (knee extensions) after beetroot
juice supplementation when compared to a placebo. Moreover, beetroot juice supplementation significantly
reduced muscle ATP hydrolysis during both low and high-intensity exercise bouts. The authors speculated that
the possible mechanisms behind the in vivo decrease in O2 cost of exercise following NO3- supplementation is
predominantly a result of a reduction in total ATP cost of muscle force production, and not an increase in
mitochondrial phosphate/O2 ratio. Alternatively, Jones [60] suggests that the decreases in steady-state 𝑉
̇O2
and phosphocreatine after dietary NO3- supplementation could potentially be due to the simultaneous
improvement of mitochondrial efficiency and muscle oxygenation. These findings suggest a fatigue protocol
Page 28 of 38
such as TTE may be more suited for dietary NO3- studies looking at the physiological mechanisms affecting
performance and exercise capacity.
The ergogenic effect of dietary NO3- was more apparent when TTE tests were used as the main
outcome measure. Protocols involving exercising until exhaustion have been suggested to have a greater
variability than TT tests [61, 62]. In a study by Saris et al. [61], times to exhaustion across five trials resulted in
a high coefficient of variation (CV) of 26.6%, with an individual CV range from 17.4% to 39.5%. In the same
study, two time-trial protocols produced a CV of 3.5% and 3.4%, and the individual CV ranged from 1.7% to
5.8% and 0.8% to 5.8% respectively. Thus it initially appears surprising that the ergogenic effects were
significant only when the more variable TTE measures were used. However, Amann et al. [63] found similar
sensitivities between TT and TTE protocols suggesting TTE protocols are a valid option when determining the
effects of an intervention on endurance performance. Jeukendrup et al. [64] suggested the difference in the
variability between protocols could be attributed to differences in the influence of psychological factors such
as motivation and monotony on the outcome measure and that TTE protocols measure endurance capacity
rather than exercise performance, which is better measured by TT protocols. Clearly further research is
required to determine why the present analysis shows a greater ergogenic effect of dietary NO3- when TTE
protocols are used as the outcome measure rather than TT protocols. It is worth noting that a TT protocol has
been suggested to be the most appropriate and reliable choice for an intervention resembling “real-life”
endurance exercise performance (63) and therefore, these protocols may be the most ecologically valid option
when assessing the impact of dietary NO3- supplementation on performance [65, 66].
Despite not being statistically significant, the 0.8% improvement in TT performance following dietary
NO3- supplementation may be meaningful for athletes. To put this into perspective, the difference between
first and twelfth place in the 10000m men’s running final at the 2012 London Olympics was only 0.66% [67];
therefore, it is still prudent to recommend dietary NO3- supplementation to aid endurance exercise
performance, when small improvements in performance can be particularly meaningful. In addition, using
dietary NO3- supplementation to increase TTE during training may result in the completion of more intervals,
enhancing those physiological adaptations that improve TT performance.
Page 29 of 38
Moderator variables, including exercise type, exercise duration, dose duration, NO3- type, dose
amount, study quality, fitness level (𝑉
̇O2max), and percentage NO2- change, did not appear to have any
significant interactions on the effects of dietary NO3- supplementation on exercise performance.
A chronic dosage regime appears to show a trend towards a slightly better performance outcome
than acute on the TTE protocol (ES: 0.29; p = 0.11). Interestingly, there were two studies that directly
compared chronic and acute doses on performance. Vanhatalo et al. [10] found that chronic ingestion (15
days) of beetroot juice had a greater effect on peak power output, gas exchange threshold, and blood pressure
compared to an acute dosage (2.5 hours prior to testing). Boorsma et al. [22] also reported a slight
improvement in TT performance after a chronic dosing protocol (8 days), whereas participants consuming an
acute dose (2.5 hours prior to testing) of dietary NO3- did not improve exercise efficiency or performance.
Taking into account the results from the meta-analysis and also these studies, it would appear that chronic
dosing may be more likely to produce a benefit; however, further research is needed to understand what
length of dietary NO3- supplementation period elicits the best outcome.
Interestingly, level of fitness did not influence the ergogenic effect of dietary NO3- supplementation
according to the continuous variable meta-regression, however, the subjects involved in the trials had a similar
fitness level; therefore, we were unable to determine confidently the effect training status has on the
response to dietary NO3- supplementation. The only study to directly compare individual aerobic fitness levels
with dietary NO3- supplementation observed positive improvements in sedentary and moderately trained
individuals, but not highly trained subjects [14]. Further research should be specifically targeted towards the
level of fitness variable before definitive conclusions can be made regarding its effect on the dietary NO3-
supplementation response.
A potential limitation of this meta-analysis is the possible effect of publication bias with the
suggestion of small study bias. However, these types of studies typically employ small sample sizes. Thus, there
may be other sources of funnel plot asymmetry, e.g. true heterogeneity and chance [30]. Although studies
included in the meta-analysis showed no statistical heterogeneity, they still varied considerably in study
design. Differences in exercise mode, dose duration and amount, mode of NO3- delivery, test duration, and
NO3- type along with a lack of repetition when measuring these variables made it difficult to draw conclusions
and make interpretations from the results. Additionally, univariate meta-regression does have limitations that
Page 30 of 38
can diminish its ability to make valid conclusions. The main limitation of the uni-variate approach is that
potential moderators cannot be assessed in isolation in trials with large numbers of characteristics. The
findings of this meta-analysis demonstrate that there is enough evidence to suggest dietary NO3-
supplementation can improve endurance exercise performance; however, more experimental trials need to be
conducted with a research focus on potential moderator variables to provide definitive conclusions and
recommendations for dietary NO3- supplementation and its effect on endurance exercise performance.
With respect to moderator variables, future research might also be designed to isolate the ergogenic
effect of nitrate ingestion for individuals possessing different muscle fibre type proportions. For example,
research conducted by Hernandez et al. [71] on the effect of dietary NO3- ingestion observed an enhanced
contractile force in fast-twitch muscles in the NO3- supplemented mice. The results translate to an activation of
fast-twitch muscle fibres at a lower frequency but still achieving the same force after dietary NO3-
supplementation, therefore, a reduced effort required to perform a given task. Dietary NO3- supplementation
appears to be particularly effective at improving physiological responses in type II muscle [11, 37, 71, 72] and
can lead to increased force production at higher contraction velocities [73], and improved performance during
short-duration high-intensity intermittent exercise [74, 75] when type II muscle fibre recruitment is high. This
provides an interesting avenue for future research investigating the effects of dietary NO3- supplementation on
performance during intermittent and power exercise tests.
5. Conclusion
In summary, the findings of this systematic review and meta-analysis provide convincing evidence
that dietary NO3- supplementation is likely to elicit a positive outcome when testing endurance exercise
capacity, but is less likely to be effective for TT performance. The design of the test protocol selection may
influence the conclusion regarding the ergogenic effect of dietary NO3- supplementation. Further work is
needed to understand the optimal dosing strategies, which population is most likely to benefit, and under
which conditions dietary nitrates are likely to be most effective for enhancing performance.
Page 31 of 38
Compliance with Ethical Standards
Funding
No sources of funding were used to assist in the preparation of this article.
Conflicts of interest
Nicholas McMahon, Michael Leveritt and Toby Pavey declare they have no conflicts of interest relevant to the
content of this review.
Acknowledgements
The authors would like to express their gratitude to Julie Hansen and Scott Macintyre for their assistance in
developing a search strategy, and to several authors cited herein for providing access to data.
Page 32 of 38
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Page 37 of 38
Electronic Supplementary Material Appendix S1. PEDro scale criteria and operational definitions
PEDro scale
1.
eligibility criteria were specified
no yes where:
2.
subjects were randomly allocated to groups (in a crossover study, subjects
were randomly allocated an order in which treatments were received)
no yes where:
3.
allocation was concealed
no yes where:
4.
the groups were similar at baseline regarding the most important prognostic
indicators
no yes where:
5.
there was blinding of all subjects
no yes where:
6.
there was blinding of all therapists who administered the therapy
no yes where:
7.
there was blinding of all assessors who measured at least one key outcome
no yes where:
8.
measures of at least one key outcome were obtained from more than 85%
of the subjects initially allocated to groups
no yes where:
9.
all subjects for whom outcome measures were available received the
treatment or control condition as allocated or, where this was not the case,
data for at least one key outcome was analysed by “intention to treat”
no yes where:
10.
the results of between-group statistical comparisons are reported for at least
one key outcome
no yes where:
11.
the study provides both point measures and measures of variability for at
least one key outcome
no yes where:
The PEDro scale is based on the Delphi list developed by Verhagen and colleagues at the Department of
Epidemiology, University of Maastricht [77].
Page 38 of 38
Criterion
Operational Definition
All criteria
Points are awarded only when a criterion is clearly satisfied. If on a literal reading of the trial report, it is
possible that a criterion was not satisfied, a point should not be awarded for that criterion.
Criterion 1
This criterion is satisfied if the report describes the source of subjects and a list of criteria used to determine who
was eligible to participate in the study.
Criterion 2
A study is considered to have used random allocation if the report states that allocation was random. The precise
method of randomization need not be specified. Procedures such as coin tossing and dice rolling should be
considered random. Quasi-randomization allocation procedures such as allocation by hospital record number
or birth date, or alternation, do not satisfy this criterion.
Criterion 3
Concealed allocation means that the person who determined if a subject was eligible for inclusion in the trial
was unaware, when this decision was made, of which group the subject would be allocated to. A point is
awarded for this criterion, even if it is not stated that allocation was concealed, when the report states that
allocation was by sealed opaque envelopes or that allocation involved contacting the holder of the allocation
schedule who was “off-site.”
Criterion 4
At a minimum, in studies of therapeutic interventions, the report must describe at least one measure of the severity
of the condition being treated and at least one (different) key outcome measure at baseline. The rater must be
satisfied that the groups’ outcomes would not be expected to differ, on the basis of baseline differences in
prognostic variables alone, by a clinically significant amount. This criterion is satisfied even if only baseline
data of subjects completing the study are presented.
Criteria 4, 711
Key outcomes are those outcomes that provide the primary measure of the effectiveness (or lack of effectiveness)
of the therapy. In most studies, more than one variable is used as an outcome measure.
Criteria 57
Blinding means the person in question (subject, therapist, or assessor) did not know which group the subject had
been allocated to. In addition, subjects and therapists are only considered to be “blind” if it could be expected
that they would have been unable to distinguish between the treatments applied to different groups. In trials in
which key outcomes are self-reported (e.g., visual analog scale, pain diary), the assessor is considered to be
blind if the subject was blind.
Criterion 8
This criterion is satisfied only if the report explicitly states both the number of subjects initially allocated to groups
and the number of subjects from whom key outcome measurements were obtained. In trials in which outcomes
are measured at several points in time, a key outcome must have been measured in more than 85% of subjects
at one of those points in time.
Criterion 9
An intention-to-treat analysis means that, where subjects did not receive treatment (or the control condition) as
allocated and where measures of outcomes were available, the analysis was performed as if subjects received
the treatment (or control condition) they were allocated to. This criterion is satisfied, even if there is no mention
of analysis by intention to treat, if the report explicitly states that all subjects received treatment or control
conditions as allocated.
Criterion 10
A between-group statistical comparison involves statistical comparison of one group with another. Depending on
the design of the study, this may involve comparison of 2 or more treatments or comparison of treatment with a
control condition. The analysis may be a simple comparison of outcomes measured after the treatment was
administered or a comparison of the change in one group with the change in another (when a factorial
analysis of variance has been used to analyse the data, the latter is often reported as a group time
interaction). The comparison may be in the form hypothesis testing (which provides a P value, describing the
probability that the groups differed only by chance) or in the form of an estimate (eg, the mean or median
difference, a difference in proportions, number needed to treat, a relative risk or hazard ratio) and its
confidence interval.
Criterion 11
A point measure is a measure of the size of the treatment effect. The treatment effect may be described as a
difference in group outcomes or as the outcome in (each of) all groups. Measures of variability include
standard deviations, standard errors, confidence intervals, interquartile ranges (or other quartile ranges), and
ranges. Point measures and/or measures of variability may be provided graphically (e.g., standard deviations
may be given as error bars in a figure) as long as it is clear what is being graphed (e.g., as long as it is clear
whether error bars represent standard deviations or standard errors). Where outcomes are categorical, this
criterion is considered to have been met if the number of subjects in each category is given for each group.
... Meta-analyses by Hoon et al. (2013) and McMahon et al. (2017) surveyed the effects of nitrate supplementation on exercise tolerance and endurance performance. In these studies, graded exercise tests and time-to-exhaustion tests assessed exercise tolerance, while time trials measured endurance exercise performance (Van De Walle and Vukovich, 2018). ...
... The current thought suggests that nitrate supplementation affords the greatest advantage in untrained individuals, but it is an effect that is diminished as the individual becomes more trained (Van De Walle and Vukovich, 2018). Furthermore, researchers suspect not only a unique effect but also a specific mechanism of action when nitrate supplementation is utilized in different modes of training (endurance vs. resistance) (Jones, 2014a;McMahon et al., 2017;Tan, Pennell, et al., 2023). Other avenues of moderation include primary fiber type recruited during exercise (Jones et al., 2016), nitrate supplementation dosage (Coggan et al., 2018;Van De Walle and Vukovich, 2018), duration of supplementation (Van De Walle and Vukovich, 2018), and the age of the individuals (Stanaway et al., 2017). ...
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Nitric oxide (NO) is a ubiquitous signaling molecule known to modulate various physiological processes, with specific implications in skeletal muscle and broader applications in exercise performance. This review focuses on the modulation of skeletal muscle function, mitochondrial adaptation and function, redox state by NO, and the effect of nitrate supplementation on exercise performance. In skeletal muscle function, NO is believed to increase the maximal shortening velocity and peak power output of muscle fibers. However, its effect on submaximal contraction is still undetermined. In mitochondria, NO may stimulate biogenesis and affect respiratory efficiency. NO also plays a role in the redox state within the skeletal muscle, partially through its interaction with respiratory chain enzymes and transcriptional regulators of antioxidant production. Nitrate supplementation leads to an increased bioavailability of NO in skeletal muscle. Thus, nitrate supplementation has been investigated for its ability to impact performance outcomes in endurance and resistance exercise. The effect of nitrate supplementation on endurance exercise is currently indecisive, although evidence indicates that it may extend the time to exhaustion in endurance exercise. Alternatively, the effect of nitrate supplementation on resistance exercise performance has been less studied. Limited research indicates that nitrate supplementation may improve repetitions to failure. Further research is needed to investigate the influence of training status, age, sex, and duration of supplementation to further elucidate the impact of nitrate supplementation on exercise performance.
... NO regulates mitochondrial respiration and appears to have a role in optimising contractile velocity, power and endurance [11][12][13][14][15]. In younger individuals, dietary inorganic nitrate supplementation has been shown to improve multiple aspects of muscle function [16][17][18][19][20][21][22][23][24]. For example, exercise capacity has been shown to increase following NO supplementation, with a reduction in oxygen cost of exercise, in multiple studies in young active people, possibly explained by more efficient oxidative phosphorylation and reduced energy cost. ...
... Most previous reviews have included studies focussed on younger participants, often as an adjunct to exercise training or to improve performance in those undertaking sporting activity, and hence, many studies to date have included individuals with high baseline physical performance. However, their findings are broadly similar to the present review of studies of older adults [20][21][22][23][24][57][58][59]. ...
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Background Sarcopenia, the loss of muscle strength and mass with age, is a major cause of morbidity for older people. Dietary nitrate supplementation has been proposed as an intervention to improve skeletal muscle function via action as nitric oxide (NO) donors. However, the effect of nitrate supplementation on physical performance and muscle strength in older people is unclear. We aimed to systematically review evidence on whether dietary nitrate supplementation improves markers of muscle strength, muscle mass and physical performance in older people. Methods We conducted a systematic review of randomised controlled trials according to a prespecified protocol by two reviewers. We included interventional studies using dietary nitrate supplementation, mean participant age of >60 years, with or without muscle weakness. Outcomes of interest were physical performance, muscle strength and muscle mass. Risk of bias was assessed using a modified version of the Cochrane Risk of Bias tool. Results were grouped by intervention and outcome measures and were described by narrative synthesis. Results Twenty‐eight studies were included, with a size range of 8–72 participants. Intervention duration ranged from a single dose to 12 weeks. Seven studies were in healthy older people. Most studies had a high or unclear risk of bias; three had a low risk of bias. One‐hundred‐two outcomes were reported; 67 were related to physical performance, and 35 were related to muscle strength. No included study measured muscle mass. Thirty‐three outcomes showed significant improvement, two showed significant worsening and 67 showed no statistically significant difference. Meta‐analysis was not possible due to data heterogeneity. Subgroup analyses for different doses of nitrate (above or below 10 mmol nitrate per day), duration of treatment or specific commonly measured outcomes did not indicate any subgroup more likely to show positive results. The proportion of positive outcomes was similar in studies using beetroot extract, nitrate alone or exercise as a co‐intervention. Conclusions Current evidence is insufficient to decide if dietary nitrate supplementation improves skeletal muscle function in older people. Future studies should be longer, larger and target older people with sarcopenia or frailty.
... Nitrates are bioactive compounds that produce nitric oxide (NO) in the body. Dietary nitrates are found in beetroot juice, pomegranate extract, and green leafy vegetables [67][68][69][70][71] Nitrates were first studied for their role in cardiovascular health and recognized as a signaling molecule [72]. NO's vasodilation properties led to studying its effects on exercise performance. ...
... A limitation of these studies is the small sample size. However, numerous meta-analyses and systematic analyses support these findings [67][68][69][70][71]. Beneficial effects on sports performance are reported in doses ranging from 5 to 16.8 mmol (300-1041 mg) consumed 2-3 h before exercise [81]. ...
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Background/Objectives: Sports supplements have become popular among fitness enthusiasts for enhancing the adaptive response to exercise. This review analyzes five of the most effective ergogenic aids: creatine, beta-alanine, nitrates, caffeine, and protein. Methods: We conducted a narrative review of the literature with a focus on the sport supplements with the most robust evidence for efficacy and safety. Results: Creatine, one of the most studied ergogenic aids, increases phosphocreatine stores in skeletal muscles, improving ATP production during high-intensity exercises like sprinting and weightlifting. Studies show creatine supplementation enhances skeletal muscle mass, strength/power, and muscular endurance. The typical dosage is 3–5 g per day and is safe for long-term use. Beta-alanine, when combined with the amino acid histidine, elevates intramuscular carnosine, which acts as a buffer in skeletal muscles and delays fatigue during high-intensity exercise by neutralizing hydrogen ions. Individuals usually take 2–6 g daily in divided doses to minimize paresthesia. Research shows significant performance improvements in activities lasting 1–4 min. Nitrates, found in beetroot juice, enhance aerobic performance by increasing oxygen delivery to muscles, enhancing endurance, and reducing oxygen cost during exercise. The recommended dosage is approximately 500 milligrams taken 2–3 h before exercise. Caffeine, a central nervous system stimulant, reduces perceived pain while enhancing focus and alertness. Effective doses range from 3 to 6 milligrams per kilogram of body weight, typically consumed an hour before exercise. Protein supplementation supports muscle repair, growth, and recovery, especially after resistance training. The recommended intake for exercise-trained men and women varies depending on their specific goals. Concluions: In summary, creatine, beta-alanine, nitrates, caffeine, and protein are the best ergogenic aids, with strong evidence supporting their efficacy and safety.
... To date, various meta-analyses have been conducted on the effects of dietary nitrate supplementation and exercise performance [20,[52][53][54][55]. The overarching result from these meta-analyses is that dietary nitrate has a small-to-moderate effect size on time-toexhaustion tests, while time trials were less likely to have any benefits [20,55]. ...
... To date, various meta-analyses have been conducted on the effects of dietary nitrate supplementation and exercise performance [20,[52][53][54][55]. The overarching result from these meta-analyses is that dietary nitrate has a small-to-moderate effect size on time-toexhaustion tests, while time trials were less likely to have any benefits [20,55]. Notably, the heterogeneity in methodology results in meta-analyses typically incorporating a myriad of exercise modalities (e.g., cycling, running, rowing, and kayaking [20,29] and/or exercise protocols (e.g., time trials, steady-state exercise, time-to-exhaustion trials, and sprints [53]). ...
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This systematic review and meta-analysis investigated the influence of dietary nitrate supplementation on performance metrics during cycling sprint exercise according to the PRISMA guidelines. Searches were conducted on MEDLINE, PubMed, ScienceDirect, Scopus, and SPORTDiscus databases up to September 2023. Inclusion criteria were healthy recreationally active men and women who consumed nitrate-rich and nitrate-deficient beetroot juice to assess performance outcomes of mean power, peak power, time-to-peak power, and minimum power during 30-s cycling sprints. Risk of bias was assessed using the Cochrane Risk of Bias 2 and TESTEX tools and funnel plots. A random effects model was performed on six studies and showed that dietary nitrate had significant effects on time-to-peak power (SMD: −0.66, 95% CI: −1.127 to −0.192, p = 0.006) but not on mean power, peak power, or minimum power. Subgroup analysis revealed that an acute low nitrate dose improved time-to-peak power (SMD: −0.977, 95% CI: −1.524 to −0.430, p < 0.001) but not after a multiday moderate nitrate dose (SMD: −0.177, 95% CI: −0.619 to −0.264, p = 0.431). These data suggest that acute nitrate supplementation can benefit time-to-peak power during 30-s cycling sprints, but due to the limited availability of data and heterogeneity in methodology, these results should be interpreted with caution. There was insufficient data on women to analyze sex-based differences. Future studies are required to provide insight on how supplementation regimen and population impact the effects of dietary nitrate for enhancing cycling sprint performance.
... Ergogenic aids are promoted but are not necessarily proven to support performance enhancement and/or recovery. Examples of ergogenic aids that bear substantial evidence include caffeine, creatine and dietary nitrate (McMahon et al. 2017;Wu et al. 2020;Wu et al. 2024). According to the United States Anti-Doping Agency (USADA), manufacturers may designate energy or sports drinks as either a food or dietary supplement (Energy Drinks vs. 2022). ...
... It has also been reported that red spinach extract increases the time to exhaustion during high-intensity exercise and causes an increase in exhaled NO [28]. However, recent meta-analyses support the suggestion that dietary nitrate, normally taken by athletes in the form of beetroot juice, has the potential to can potentially improve performance in various sports and exercise activities [29,30]. Previous studies reported that maximum sprint cycling performance [31,32] and 180 m sprint running performance [33] improved after NO3 supplementation, and Thompson, Vanhatalo [32] reported that NO3 supplementation could improve performance during 5, 10, and 20 m sprint runs. ...
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Background: Taekwondo is a complex martial art that requires speed, balance, agility, and endurance. This study aims to examine the effects of nitrate and L-arginine supplementation on acute aerobic and anaerobic performance, balance, agility, and recovery in elite taekwondo athletes. Method: This study was conducted as a double-blind, randomized, crossover study with the participation of 15 experienced taekwondo athletes aged 19.06 ± 0.96 years and 8.93 ± 1.27 years of training experience. Participants visited the laboratory a total of nine times, including a practice session and anthropometric measurements. These visits consisted of eight experimental sessions conducted at 72-hour intervals. The experimental sessions were conducted with nitrate, L-arginine, and a combination of both supplements (NIT*L-ARG) and placebo. Nitrate supplementation was provided by homogenizing fresh spinach (837.40 mg/kg), while L-ARG was given as a single dose of 6 g in powder form three hours before exercise. Results: NIT*L-ARG supplementation significantly improved the anaerobic performance of athletes in Wingate peak power and peak power (w/kg) compared to placebo and in mean power compared to NIT, L-ARG, and PLA. In addition, NIT*L-ARG supplementation significantly improved blood lactate levels and agility performance immediately after Wingate and Shuttle run tests. Conclusion: The combined intake of NIT*L-ARG was found to be effective in improving aerobic, anaerobic, and agility performances as well as fatigue levels of athletes. It was determined that taking NIT and L-ARG supplements alone contributed to the improvement of improving athletes' performance in Wingate mean power values and subsequent fatigue level compared to PLA.
... Questions where current evidence is inconclusive/for which no correct response is available were excluded from the Index. Data from recent reviews and an expert consensus statement on nitrate informed decision-making on correct/incorrect responses (4,10,25,34,35) . ...
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Objective Evaluate knowledge and beliefs about dietary nitrate among United Kingdom (UK)-based adults. Design An online questionnaire was administered to evaluate knowledge and beliefs about dietary nitrate. Overall knowledge of dietary nitrate was quantified using a 21-point Nitrate Knowledge Index. Responses were compared between sociodemographic groups. Setting UK. Participants A nationally representative sample of three hundred adults. Results Only 19% of participants had heard of dietary nitrate prior to completing the questionnaire. Most participants (∼70%) were unsure about the effects of dietary nitrate on health parameters (e.g., blood pressure, cognitive function, cancer risk) or exercise performance. Most participants were unsure of the average population intake (78%) and acceptable daily intake (ADI) (83%) of nitrate. Knowledge of dietary sources of nitrate was generally low, with only ∼30% of participants correctly identifying foods with higher/lower nitrate contents. Almost none of the participants had deliberately purchased, or avoided purchasing, a food based around its nitrate content. Nitrate Knowledge Index scores were generally low (median[IQR]: 5[8]), but were significantly higher in individuals who were currently employed vs. unemployed (median[IQR]: 5[7]vs.4[7]; p <0.001), in those with previous nutrition education vs. no nutrition education (median[IQR]: 6[7]vs.4[8]; p= 0.012), and in individuals who had heard of nitrate prior to completing the questionnaire vs. those who had not (median [IQR]: 9[8]vs.4[7]; p <0.001). Conclusions This study demonstrates low knowledge around dietary nitrate in UK-based adults. Greater education around dietary nitrate may be valuable to help individuals make more informed decisions about their consumption of this compound.
... The methodological rigor of the studies included in the analysis was assessed using the Physiotherapy Evidence Database (PEDro) scale [27]. The PEDro scale is commonly utilized in systematic review studies examining the effectiveness of supplements and nutritional ergogenic resources [28][29][30]. It provides a reliable and objective means of evaluating the internal validity of randomized controlled trials [31]. ...
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Background Although recent studies have increasingly focused on examining the potential benefits of creatine supplementation to improve performance in swimming events, the impact of creatine supplementation on swimming performance remains a topic of debate and controversy. A comprehensive meta-analytical review was undertaken to evaluate the effects of creatine supplementation on the performance, physiological response, and body composition among swimmers. Methods The research methodology adhered strictly to the guidelines outlined by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). A comprehensive search was conducted across six databases (Cochrane Library, Web of Science, Scopus, Embase, PubMed, and SPORTDiscus) until March 23, 2024. Eligible studies that investigated the impact of creatine supplementation on swimming time, physiological parameters, and body composition in swimmers were included. For the meta-analysis, a random-effects model was employed to determine the collective effect and assess variations across distinct subgroups defined by swimming time, physiological metrics, and body composition. Meta-regression analysis was conducted on datasets comprising ten or more studies. Standardized mean differences (SMD) along with their corresponding 95% confidence intervals (CI) were calculated. To evaluate the methodological rigor of the included studies, the Physiotherapy Evidence Database (PEDro) scale was utilized. Results The systematic review included seventeen studies with a total of 361 subjects. No significant differences were observed in the overall effect during single sprint swimming (SMD: -0.05, 95% CI: -0.26, 0.15; p = 0.61), repeated interval swimming (SMD: -0.11; 95% CI: -0.46, 0.25; p = 0.56), physiological response (SMD: 0.04, 95% CI: -0.16, 0.23; p = 0.71), and body composition (SMD: 0.18; 95% CI: -0.05, 0.41; p = 0.12) between creatine and placebo groups. Conclusions Creatine supplementation exhibited ineffectiveness in enhancing the performance, physiological response, and body composition among swimmers.
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Systematic reviews should build on a protocol that describes the rationale, hypothesis, and planned methods of the review; few reviews report whether a protocol exists. Detailed, well-described protocols can facilitate the understanding and appraisal of the review methods, as well as the detection of modifications to methods and selective reporting in completed reviews. We describe the development of a reporting guideline, the Preferred Reporting Items for Systematic reviews and Meta-Analyses for Protocols 2015 (PRISMA-P 2015). PRISMA-P consists of a 17-item checklist intended to facilitate the preparation and reporting of a robust protocol for the systematic review. Funders and those commissioning reviews might consider mandating the use of the checklist to facilitate the submission of relevant protocol information in funding applications. Similarly, peer reviewers and editors can use the guidance to gauge the completeness and transparency of a systematic review protocol submitted for publication in a journal or other medium.
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Background and purpose: Assessment of the quality of randomized controlled trials (RCTs) is common practice in systematic reviews. However, the reliability of data obtained with most quality assessment scales has not been established. This report describes 2 studies designed to investigate the reliability of data obtained with the Physiotherapy Evidence Database (PEDro) scale developed to rate the quality of RCTs evaluating physical therapist interventions. Method: In the first study, 11 raters independently rated 25 RCTs randomly selected from the PEDro database. In the second study, 2 raters rated 120 RCTs randomly selected from the PEDro database, and disagreements were resolved by a third rater; this generated a set of individual rater and consensus ratings. The process was repeated by independent raters to create a second set of individual and consensus ratings. Reliability of ratings of PEDro scale items was calculated using multirater kappas, and reliability of the total (summed) score was calculated using intraclass correlation coefficients (ICC [1,1]). Results: The kappa value for each of the 11 items ranged from.36 to.80 for individual assessors and from.50 to.79 for consensus ratings generated by groups of 2 or 3 raters. The ICC for the total score was.56 (95% confidence interval=.47-.65) for ratings by individuals, and the ICC for consensus ratings was.68 (95% confidence interval=.57-.76). Discussion and conclusion: The reliability of ratings of PEDro scale items varied from "fair" to "substantial," and the reliability of the total PEDro score was "fair" to "good."
Article
Nitrate can impact a range of physiological functions through its conversion to nitric oxide (NO) and as such, it has received attention as a dietary supplement. Beetroot juice (BR) is a nitrate-rich dietary source which has been used in the athletic setting for evaluating its performance through time-to-exhaustion protocols, which test exercise capacity but not athletic performance using in most research cyclists as subjects. The aim of this study was to examine the effects of beetroot juice on muscle force production and repeated sprint performance through measurements on an isokinetic dynamometer through extension and flexion knee movements at 60 and 240 degrees and a non-motorized treadmill through a high intensity sprint (5×6 seconds) protocol after a single dose intake of BR (70ml with 0.4g nitrate), placebo (blackcurrant juice) or control (no juice) in young and healthy adults. The results did not show any differences between the three trials for the muscle strength measurements but a small difference in peak power production performing the sprint protocol, suggesting that either higher doses or longer intake period may be required prior to observation of an impact on either muscle strength or sprint performance.