Effect of food sources of nitrate, polyphenols, L-arginine and L-citrulline on endurance exercise performance: a systematic review and meta-analysis of randomised controlled trials.
Level 1 - systematic review of randomized trials
Systematic review and meta-analysis of randomized controlled trials
PubMed 34965876 · doi:10.1186/s12970-021-00472-y
What was done
Authors conducted a systematic review and random-effects meta-analysis across Scopus, Web of Science, Medline, EMBASE, and SportDiscus to evaluate endurance exercise performance following the consumption of foods containing nitrate, polyphenols, L-arginine, or L-citrulline. Subgroup analyses evaluated specific food types, sex, training status, performance test protocols (time-trial, time-to-exhaustion, intermittent), and supplementation duration (acute vs. multi-day).
What was found
Across 118 included randomized controlled trials (59 polyphenol, 56 nitrate, 3 L-citrulline): - L-citrulline foods had no significant effect on performance (SMD = -0.03, p = 0.24). - Nitrate-rich foods yielded a trivial benefit overall (SMD = 0.15, p < 0.001). Benefits occurred with beetroot, but not red spinach, Swiss chard, or rhubarb. No benefit was observed in females or well-trained males (VO2max ≥ 65 ml·kg⁻¹·min⁻¹). - Polyphenol-rich foods yielded a trivial benefit overall (SMD = 0.17, p < 0.001). Benefits were observed for grape, nitrate-depleted beetroot, French maritime pine, Montmorency cherry, and pomegranate, but not New Zealand blackcurrant, cocoa, ginseng, green tea, or raisins. Polyphenols benefited well-trained males, but no benefit was found in females. - Both nitrate and polyphenols demonstrated benefits across acute and multi-day dosing, as well as across time-trial, time-to-exhaustion, and intermittent tests.
Why it matters
This review clarifies that whole-food sources of nitric oxide precursors provide only small, highly food-dependent endurance gains. It demonstrates that highly trained endurance athletes may benefit from polyphenols but not nitrate, and highlights the lack of evidence supporting ergogenic effects in female athletes.
Limits
There was considerable heterogeneity among polyphenol studies due to variable compounds, dosing protocols, and subject characteristics. Female participants were underrepresented, leaving it unclear whether the lack of effect in females reflects true biological differences or sample size limitations. L-citrulline was represented by only three studies, and L-arginine food studies were absent.
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