Background: Recent randomized controlled trials have suggested that dietary nitrate (NO3-), found in beetroot and other vegetables, and inorganic NO3- salts decrease metabolic rate under resting and exercise conditions. Objective: Our aim was therefore to determine from a systematic review and meta-analysis whether dietary NO3- supplementation significantly reduces metabolic rate, expressed as oxygen uptake (VO2), under resting and exercise conditions in healthy humans and those with cardiorespiratory diseases. Design: A systematic article search was performed on electronic databases (PubMed, Scopus and Web of Science) from February to March 2015. The inclusion criteria included 1) randomized controlled trials; 2) studies reporting the effect of NO3- on VO2 under resting and/or exercise conditions; 3) comparison between dietary NO3- supplementation and placebo. Random-effects models were used to calculate the pooled effect size. Results: Twenty nine randomized placebo-controlled trials were included in the systematic review, and 26 of which were included in the meta-analysis. Dietary NO3- supplementation significantly decreases VO2 during submaximal intensity exercise [-0.26 (95% IC: -0.38, -0.15), p < 0.01], but not in the sub-analysis of subjects with chronic diseases [-0.09 (95% IC: -0.50, 0.32), p = 0.67]. When data were separately analyzed by submaximal intensity domains, NO3- supplementation reduces VO2 during moderate [-0.29 (95% IC: -0.48,-0.10), p < 0.01] and heavy [-0.33 (95% IC: -0.54,-0.12), p < 0.01] intensity exercise. When the studies with the largest effects were excluded from the meta-analysis, there is a trend for a VO2 decrease under resting condition in dietary NO3- supplementation [-0.28 (95% IC: -0.62, 0.05), p = 0.10]. Conclusion: Dietary NO3- supplementation decreases VO2 during exercise performed in the moderate and heavy intensity domains in healthy subjects. The present meta-analysis did not show any significant effect of dietary NO3- supplementation on metabolic rate in subjects with chronic diseases, despite enhanced exercise tolerance.
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