The Effect of Dietary Nitrate Supplementation on Isokinetic Torque in Adults: A Systematic Review and Meta-Analysis.

Dietary nitrate (NO) supplementation, which can enhance performance in exercise settings involving repeated high-intensity efforts, has been linked to improved skeletal muscle contractile function. Although muscular strength is an important component of explosive movements and sport-specific skills, few studies have quantified indices of muscular strength following NO supplementation, particularly isokinetic assessments at different angular velocities. We performed a systematic review and meta-analysis to determine whether dietary NO supplementation improves peak torque, as assessed by the gold standard method of isokinetic dynamometry, and if this effect was linked to the angular velocity imposed during the assessment. Dialnet, Directory of Open Access Journals, MEDLINE, PubMed, SciELO, Scopus, and SPORTDiscus were searched for articles using the following search strategy: (nitrate OR beet*) AND (supplement* OR nutr* OR diet*) AND (isokinetic OR strength OR "resistance exercise" OR "resistance training" OR "muscular power"). The meta-analysis of data from 5 studies with 60 participants revealed an overall effect size of -0.01 for the effect of nitrate supplementation on isokinetic peak torque, whereas trivial effect sizes ranging from -0.11 to 0.16 were observed for independent velocity-specific (90°/s, 180°/s, 270°/s, and 360°/s) isokinetic peak torque. Four of the five studies indicated that dietary NO supplementation is not likely to influence voluntary knee extensor isokinetic torque across a variety of angular velocities. These results suggest that NO supplementation does not influence isokinetic peak torque, but further work is required to elucidate the potential of NO supplementation to influence other indices of muscular strength, given the dearth of experimental evidence on this topic.