Abstract
BackgroundIncreasing nitric oxide bioavailability may induce physiological effects that enhance endurance exercise performance. This review sought to evaluate the performance effects of consuming foods containing compounds that may promote nitric oxide bioavailability.MethodsScopus, Web of Science, Ovid Medline, EMBASE and SportDiscus were searched, with included studies assessing endurance performance following consumption of foods containing nitrate, L-arginine, L-citrulline or polyphenols. Random effects meta-analysis was conducted, with subgroup analyses performed based on food sources, sex, fitness, performance test type and supplementation protocol (e.g. duration).ResultsOne hundred and eighteen studies were included in the meta-analysis, which encompassed 59 polyphenol studies, 56 nitrate studies and three L-citrulline studies. No effect on exercise performance following consumption of foods rich in L-citrulline was identified (SMD=-0.03, p=0.24). Trivial but significant benefits were demonstrated for consumption of nitrate and polyphenol-rich foods (SMD=0.15 and 0.17, respectively, p<0.001), including performance in time-trial, time-to-exhaustion and intermittent-type tests, and following both acute and multiple-day supplementation, but no effect of nitrate or polyphenol consumption was found in females. Among nitrate-rich foods, beneficial effects were seen for beetroot, but not red spinach or Swiss chard and rhubarb. For polyphenol-rich foods, benefits were found for grape, (nitrate-depleted) beetroot, French maritime pine, Montmorency cherry and pomegranate, while no significant effects were evident for New Zealand blackcurrant, cocoa, ginseng, green tea or raisins. Considerable heterogeneity between polyphenol studies may reflect food-specific effects or differences in study designs and subject characteristics. Well-trained males (V̇O2max ≥65 ml.kg.min-1) exhibited small, significant benefits following polyphenol, but not nitrate consumption.ConclusionFoods rich in polyphenols and nitrate provide trivial benefits for endurance exercise performance, although these effects may be food dependent. Highly trained endurance athletes do not appear to benefit from consuming nitrate-rich foods but may benefit from polyphenol consumption. Further research into food sources, dosage and supplementation duration to optimise the ergogenic response to polyphenol consumption is warranted. Further studies should evaluate whether differential sex-based responses to nitrate and polyphenol consumption are attributable to physiological differences or sample size limitations.OtherThe review protocol was registered on the Open Science Framework (https://osf.io/u7nsj) and no funding was provided.
Highlights
Increasing nitric oxide bioavailability may induce physiological effects that enhance endurance exercise performance
Of the 123 studies that were eligible for inclusion, 103 used a crossover design and 21 used a parallel design. 101 were reported as doubleblind, 13 as single-blind, and 10 did not specify blinding. 62 studies were classified as polyphenol studies, 57 were NO3--focused and four studies tested L-citrulline effects, with 40 different food sources represented
While no included studies were focused on L-arginine, it is an additional nutritional component found in watermelon and almonds, and studies that used these foods were classified as L-citrulline and polyphenol studies, respectively
Summary
Increasing nitric oxide bioavailability may induce physiological effects that enhance endurance exercise performance. Nitric oxide (NO) is a signalling molecule that is involved in numerous physiological processes including skeletal muscle contraction [1], endothelial function [2], mitochondrial biogenesis and respiration [3], muscle repair [4] and antioxidant defences [5–7] Given these functions are important during exercise, aerobic exercise, there has been considerable interest in enhancing NO production to improve endurance exercise performance. Increased synthesis of NO is proposed to reduce the oxygen cost of ATP resynthesis, lower the ATP cost of cross-bridge formation and promote vasodilation, thereby enhancing skeletal muscle blood flow and oxygen perfusion which may speed oxygen uptake kinetics [8] These effects may translate to increased endurance exercise performance by improving exercise efficiency, decreasing the oxygen deficit at exercise onset and reducing the VO2 slow component [9]. Such changes in NO bioavailability are typically evaluated by plasma or urine NO3- and NO2-, as these are end products of endogenous NO production, but can be inferred via measures of vascular function such as flow-mediated dilation (FMD)
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