Plain language summary
The ability to increase endurance to improve their physical fitness is of great interest to athletes. Several dietary components have been shown to increase endurance especially those which contain nitrates. It is thought that nitrates, which are found in beetroots, pomegranates, green leafy vegetables, collard greens, lettuce and spinach, aid endurance through their action on improving blood flow to the muscles, helping to improve contraction of the muscles, through increased energy production, through improved oxygen flow and through sugar and nutrient balance. However, studies in humans on the effects of nitrates on exercise endurance have been conflicting. This systematic review and meta-analysis aimed to summarise the research on the use of nitrates during exercise. The results showed that nitrate supplementation improved muscle power, time to exhaustion and distance travelled, although no difference was found to perceived exertion, time trial performance and work done. It was concluded that nitrate supplementation is of benefit to improve exercise endurance, based on very low to moderate quality evidence. This study could be used by health care professionals to recommend a nitrate rich diet and possibly a nitrate supplement to improve exercise performance.
Abstract
BACKGROUND Nitrate supplementation is thought to improve performance in endurance sports. OBJECTIVE To meta-analyze studies evaluating the effect of nitrate supplementation on endurance sports performance among adults. DATA SOURCES We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE, Web of Science and CINAHL without language restrictions. METHODS We included studies that: 1) compared nitrate supplementation with placebo; 2) enrolled adults engaging in an endurance-based activity; and 3) reported a performance measure or surrogate physiologic outcome. We evaluated risk of bias using the Cochrane Collaboration tool and pooled data with a random-effects model. We used the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach to evaluate confidence in estimates. RESULTS We included 73 studies (n = 1061). Nitrate supplementation improved power output (MD 4.6 watts, P < 0.0001), time to exhaustion (MD 25.3 s, P < 0.00001), and distance travelled (MD 163.7 m, P = 0.03). We found no significant difference on perceived exertion, time trial performance and work done. Nitrate supplementation decreased VO2 (MD - 0.04 L/min, P < 0.00001) but had no significant effect on VO2max or blood lactate levels. CONCLUSION The available evidence suggests that dietary nitrate supplementation benefits performance-related outcomes for endurance sports.
Methodological quality
Jadad score
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Not applicable
Allocation concealment
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Not applicable
