The use of dietary nitrate (NO3—) supplementation, primarily in the form of concentrated beetroot juice, has become increasing popular amongst endurance athletes in an attempt to improve exercise performance. The objective of this research paper is to determine if there is sufficient scientific evidence to support its use as an ergogenic aid, determine whether outcomes are affected by moderator variables, and following, recommend a suitable supplementation protocol.
Research suggests that the performance impact of NO3— supplementation is dependent on a number of factors including type, duration and intensity of the exercise performed (Hoon et al. 2013; Maughan et al. 2018; McMahon, Leveritt & Pavey 2016; Reynolds et al. 2016), the fitness of the participant (Jones et al. 2018), and the dose and duration of supplementation (Jones 2014; Jones et al. 2018; Hoon et al. 2013 & 2014; Peeling et al. 2015; Wylie et al. 2013 & 2016).
Type, Duration & Intensity
Recent meta-analysis investigating the effect of dietary NO3— supplementation on endurance exercise performance found that performance outcomes varied depending on the exercise protocol tested (McMahon, Leveritt & Pavey 2016). In time to exhaustion (TTE) trials, the results of the meta-analysis showed that dietary NO3— was associated with a statistically significant effect in favour of supplementation with improvements of 4% to 25% reported. However, in the same study, only small, non-significant effects of 1% to 3% in favour of dietary NO3— supplementation were observed in time trials (TT) and graded-exercise tests (GXT) protocols. The researchers suggest that the improvements in exercise performance observed in TTE protocols is likely due to the reduced whole-body O2 cost of constant work-rate exercise following NO3— supplementation.
The findings presented above are similar to that of a previous meta-analysis conducted by Hoon et al. (2013). Specifically, the researchers found that dietary NO3— supplementation had a statistically significant moderate benefit on performance in TTE protocols but only a small, non-significant performance benefit in TT and GXT trials. In combination, the meta-analyses by McMahon, Leveritt & Pavey (2016) and Hoon et al. (2013) provide clear evidence that dietary NO3— supplementation can boost aerobic exercise capacity, particularly when measured by TTE trials.
In addition, there is some evidence to suggest that the effect of dietary NO3— supplementation may be impacted by exercise duration and intensity. In a review paper published by the International Olympic Committee (IOC), the researchers concluded that in trials of high-intensity, intermittent, team-sport exercise of 12-40 min in duration, performance improvements have been reported (Maughan et al. 2018). However, for exercise bouts lasting >40 min in duration, the researchers found that there was limited evidential support for dietary NO3— supplementation. In terms of exercise bouts of short duration (<12 min) the research is equivocal, with some studies showing potential benefits in 2000m rowing performance (Hoon et al. 2014) and others showing no effect of acute supplementation on repeated sprint performance (Reynolds et al. 2016).
Fitness of the Individual
The efficacy of dietary NO3— supplementation may be vary depending on the training status of the individual. In a wide-ranging review paper, Jones et al. (2018) found that whilst improvements in moderately trained endurance athletes (VO2max <60mL/kg/min) are well documented, the evidence on high trained athletes (VO2max >60mL/kg/min) was less equivocal with published research reporting conflicting outcomes. Jones et al. (2018) suggests that the reason for the apparent lack of effect may be due to physiological adaptations present in highly trained athletes includes higher endogenous nitric oxide (NO) synthesis as well as greater skeletal muscle capillarisation and Ca2+ handling capabilities that blunt the effects of dietary NO3— supplementation.
Dose and Duration of Supplementation
Dietary NO3— is a natural component of many foods, most notably leafy green and root vegetables (Jones 2014). However, in terms of its use as an ergogenic aid, the most common source of dietary NO3— that has been evaluated for its efficacy is concentrated beetroot juice (Hoon et al. 2013; McMahon, Leveritt & Pavey 2016).
Research has shown that performance benefits are generally seen with 2-3 hours of ingestion following a NO3— bolus of 310-560 mg (Hoon et al. 2014; Peeling et al. 2015). In addition, Jones et al. (2018) concluded that acute NO3—consumption studies suggest that a NO3— dose of 310-527 mg is likely to improve exercise economy but in order to improve both exercise economy and performance a NO3— dose of >527 mg may be necessary. That said, there does appear to be an upper limit to the benefits that can derived from dietary NO3— supplementation. In a study investigating the dose-response relationships of dietary NO3— supplementation, Wylie et al. (2013) found that there was no additional improvement in exercise performance after ingesting 1042 mg of dietary nitrate as compared to 521 mg.
In terms of prolonged NO3— consumption, research has shown that performance benefits have been maintained for up to 28 days if supplementation is continued with NO3— doses of 372 mg per day (Wylie et al. 2016), and moreover, that there may be a cumulative influence of repeated NO3— intake (McMahon, Leveritt & Pavey 2016). Interestingly, Jones et al. (2018) found that there was some evidence to suggest that prolonged consumption (> 6 days) of dietary NO3— may have a positive impact on exercise performance in highly trained athletes and therefore maybe a particularly useful strategy for this cohort where performance gains appear harder to obtain.
Despite concentrated beetroot juice providing the main source of supplementation in the scientific literature, it is important to note that there are a wide range of unprocessed vegetables with a very high NO3— concentration (>250 mg/100g) that include spinach, rocket, cress, lettuce, radish, Swiss chard, chervil and red beetroot (McMahon, Leveritt & Pavey 2016).
Potential Side Effects
The IOC have reported that the evidence suggests that there appear to be few side effects associated with NO3—supplementation, however, the IOC warns that there does exist the potential for GI upset in susceptible individuals (Maughan et al. 2018).
2.0 Biochemical and Physiological Pathways
The ingestion of NO3— enhances NO bioavailability via the NO3—-nitrate-NO pathway, a reduction that is catalysed initially by anaerobic bacteria in the oral cavity leading to increased levels of NO in the blood and tissues (Duncan et al. 1995). NO is a free-radical gas, that is known to be involved in wide range of signalling and regulatory functions in the body, including skeletal muscle glucose uptake, neurotransmission, sarcoplasmic reticulum calcium handling, mitochondria respiration and skeletal muscle fatigue (Bailey et al. 2012).
In terms of the effects on exercise performance, research indicates that NO may reduce the O2 cost of exercise. One possible explanation appears to be a reduced Adenosine Triphosphate (ATP) cost of muscle force production. Bailey et al. (2010) was able to show that during both low-intensity and high-intensity exercise, that the rates of Adenosine Triphosphate (ATP) turnover during exercise from phosphocreatine (PCr) and oxidative phosphorylation were significantly reduced following NO3— supplementation, leading to a reduction in VO2 uptake.
Another possible mechanism identified through the research is an improvement in mitochondria efficiency. Specifically, Larsen et al. (2011), found that dietary NO3— supplementation improved the O2 cost of mitochondria ATP resynthesis. This improvement in mitochondria efficiency was mainly attributed by the authors to a reduction in the leakage of protons across the mitochondrial membrane.
Finally, NO bioavailability has also been shown to improve O2 delivery to the working muscles through effects on peripheral vasodilation which may contribute to improved performance during maximal exercise (Bailey et al. 2010).
3.0 Recommended Protocol
Based on the evidence, the following protocol for dietary NO3— supplementation should be observed.
Dietary NO3— supplementation should be achieved via the oral consumption of concentrated beetroot juice that has been certified through the Informed-Sport program. This will ensure that appropriate doses are meet and that the risk of cross-contamination from banned and/or dangerous substances is minimised. Beet It Sport Nitrate 400 (James White Drinks, Ipswich, UK) is a popular beetroot juice supplement that has been used by athletes for over 10 years and is Informed-Sport compliant.
In addition, athletes should avoid using antibacterial mouthwashes and chewing gum prior to ingestion to minimise any potential interference with the mouth nitrate-nitrate conversion.
Modality and Duration
Concentrated beetroot juice supplementation is recommended for use in endurance athletes where there are periods of moderate to high intensity physical activity lasting up to 40 min in duration. That would include, for example, most ball sport athletes, middle to long distance runners, road cyclists, kayakers and rowers.
Dose & Frequency
For moderately trained, recreational athletes, acute supplementation 2-3 hours before competition of 500-600 mg of dietary NO3— will be sufficient. However, in highly trained, competitive athletes, prolonged consumption of 400 mg of dietary NO3— per day for at least 6 days prior to exercise and/or competition is suggested.
For general health and potential training benefits, in periods not immediately leading up to competition (i.e. >1 week prior to competition), competitive athletes should maintain a dietary NO3— intake of >300 mg per day from the consumption of 100-150g of unprocessed vegetables such as spinach, rocket, cress, lettuce, radish, Swiss chard, chervil and red beetroot.
Concentrated beetroot juice has the potential to cause GI upset and therefore should be thoroughly trialed in training prior to competition.
4.0 AIS Categorisation
The AIS uses an ABCD classification system that ranks sports food and supplements into 4 groups based according to scientific evidence and other practical considerations that determine whether a product is safe, legal and effective in improving sports performance (AIS 2019).
Presently, beetroot juice and/or dietary NO3— per is a group A categorised supplement. Group A categorised supplements and sports foods are deemed by the AIS to have strong scientific evidence for use in specific situations in sport using evidence-based protocols (AIS 2019). Given the evidence presented above this categorisation would appear to be appropriate. Specifically, there are at least two meta-analyses and systematic reviews that have found dietary NO3— was associated with a statistically significant effect in favour of supplementation.
In addition, there is a reasonable body of evidence that allows for the formation of an evidence-based protocol for use in relation to the dosage amount and frequency, the training status of the athlete, as well as the modality, duration and intensity of the exercise for which dietary NO3— supplementation is beneficial.
Australian Institute of Sport, Supplements, Australian Institute of Sport, viewed 4 September 2019, <https://ais.gov.au/nutrition/supplements>
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