The Effect of Dietary Nitrate on the Contractile Properties of Human Skeletal Muscle: A Systematic Review and Meta-Analysis.

The propose of this study was to systematically review the current literature and meta-analyse the effects of dietary nitrate (NO3 - ) supplementation on the contractile properties of skeletal muscle. A literature search of three databases was conducted in June 2021, with 19 studies meeting the inclusion criteria. Studies were included if a placebo versus dietary NO3 - -only supplementation protocol was used in healthy human, assessed muscle contraction or activities that was < 3 minutes in duration and focused on the lower-body. For the meta-analysis, a pooled standardised mean difference (SMD) was determined for maximum voluntary contraction (MVC) ( n  = 11), cycling, running and inertial load squad peak power output (PPO) ( n =  8), mean power output (MPO) ( n  = 6) and time to PPO ( n  = 4). NO3 - supplementation demonstrated a small improvement in PPO (SMD = 0.25, P = 0.030) and MPO (SMD = 0.28, P = 0.030) when compared to the placebo. NO3 - also resulted in an enhanced time to PPO (SMD = -0.78, P < 0.001). There was no clear effect of NO3 - on isometric MVC (SMD = 0.03, P = 0.758). This review reports that NO3 - supplementation may have potential to enhance PPO, MPO and time to PPO during dynamic exercise, which may transfer to brief explosive actions commonly observed in sporting activities. Due to the variability in studies, we encourage researchers to use this work to explore areas where evidence in lacking and standardize the study design and procedures.Key teaching pointsFindings from this meta-analysis highlight the potential positive ergogenic effect of dietary NO3-supplementation on PPO, MPO and time to PPO during short duration (<10 s) dynamic exercise.NO3- supplementation might be considered as an ergogenic aid when executing power-based actions (e.g., 100 m sprinter or weightlifter).This review highlights that further research is required to address some of the contrasting findings presented here using a standardised procedure to allow for improved synthesis.