Article - Effect of dietary nitrate on human muscle power: a systematic review and individual participant data meta-analysis.

Plain Language Summary

Previous reviews have concluded that dietary nitrate (NO3−) improves maximal neuromuscular power in humans, but these were based on a limited number of studies. This is the first systematic review and meta-analysis evaluating the effects of dietary NO3− supplementation on muscular power in humans. The study also aims to quantify the size of this beneficial effect. 19 studies with a total of 268 participants were included. Most of these used concentrated beetroot juice as the source of NO3− given as an acute dose (short term high level). A positive effect of dietary NO3− on muscle power was observed in all 19 studies. Analyses were done on sub groups - age, sex and the amount of muscle mass engaged in the activity. Dietary NO3− intake significantly increases maximal muscle power in humans. The magnitude of this effect has practical and clinical importance; not just for athletes but also for patient groups. This effect is independent of subject age, sex, or the amount of muscle mass engaged in the activity but may be greater with acute vs. repeated dosing. Further research is needed to determine factors such as the optimal supplementation regimen and target population.

Expert Review

Reviewer: Dr Marc Bubbs

20th Sep 2022

Conflict of interest

None

Take home message

This meta-analysis lends quantitative support to previous narrative reviews that nitrate supplementation can enhance maximal power output.
These findings are highly relevant to team and strength sport athletes, who may not otherwise be supplementing with nitrates.
These findings are also highly relevant for older populations, where risk of falls and fractures are high and can lead to significant adverse effects on health and quality of life.

Evidence category

A: Meta-analyses, position-stands, randomized-controlled trials (RCTs)

Summary review

In 2007, researchers uncovered the ingestion of dietary nitrates reduced the oxygen cost of submaximal exercise, and since, over 100 studies have examined the effects of nitrates on endurance performance.
With regards to the impact of nitrates on maximal force output, only trivial results had been previously found.
This review study found that while nitrates do not impact force development, they do demonstrate primary effect on the speed of muscle contraction (i.e. muscular power is the product of force x speed).
The reviews primary finding was that nitrate intake can significantly enhance muscular power, regardless of subject age or sex.

Clinical practice applications

These new findings highlight the ability of dietary nitrates to improve neuromuscular power production is highly relevant for team sport athletes, due to the explosive nature of these sports with constant accelerations and decelerations during training and competition.
In the general population, falls and fractures amongst older adults significantly reduces quality of life and costs the healthcare system hundreds of millions of pounds to treat.
Improved contractile properties of muscle, most notably speed of contraction, may offer protection to older adults as well as the benefit of additional nitric oxide (NO) to support vascular health as well.
The typical intake of dietary nitrates in the general population is about 31-185mg/day in Europe and 40-100mg/day in North America. Most studies use doses between 300-600mg of dietary nitrates. Increasing dietary or supplemental intake is key to achieving the neuromuscular effect.

Considerations for future research

The results of the present meta-analysis clearly demonstrate that dietary nitrates increases muscle power in humans, but the mechanism responsible for this effect is still unclear.
There are notable differences between rodent and human metabolism of dietary nitrates, therefore the biochemical mechanism by which nitrate intake improves human muscle power requires additional study.

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Abstract

BACKGROUND

Previous narrative reviews have concluded that dietary nitrate (NO) improves maximal neuromuscular power in humans. This conclusion, however, was based on a limited number of studies, and no attempt has been made to quantify the exact magnitude of this beneficial effect. Such information would help ensure adequate statistical power in future studies and could help place the effects of dietary NO on various aspects of exercise performance (i.e., endurance vs. strength vs. power) in better context. We therefore undertook a systematic review and individual participant data meta-analysis to quantify the effects of NO supplementation on human muscle power.

METHODS

The literature was searched using a strategy developed by a health sciences librarian. Data sources included Medline Ovid, Embase, SPORTDiscus, Scopus, Clinicaltrials.gov , and Google Scholar. Studies were included if they used a randomized, double-blind, placebo-controlled, crossover experimental design to measure the effects of dietary NO on maximal power during exercise in the non-fatigued state and the within-subject correlation could be determined from data in the published manuscript or obtained from the authors.

RESULTS

Nineteen studies of a total of 268 participants (218 men, 50 women) met the criteria for inclusion. The overall effect size (ES; Hedge's g) calculated using a fixed effects model was 0.42 (95% confidence interval (CI) 0.29, 0.56; p = 6.310 × 10). There was limited heterogeneity between studies (i.e., I = 22.79%, H = 1.30, p = 0.3460). The ES estimated using a random effects model was therefore similar (i.e., 0.45, 95% CI 0.30, 0.61; p = 1.064 × 10). Sub-group analyses revealed no significant differences due to subject age, sex, or test modality (i.e., small vs. large muscle mass exercise). However, the ES in studies using an acute dose (i.e., 0.54, 95% CI 0.37, 0.71; p = 6.774 × 10) was greater (p = 0.0211) than in studies using a multiple dose regimen (i.e., 0.22, 95% CI 0.01, 0.43; p = 0.003630).

CONCLUSIONS

Acute or chronic dietary NO intake significantly increases maximal muscle power in humans. The magnitude of this effect-on average, ~ 5%-is likely to be of considerable practical and clinical importance.