¹⁵N-labeled dietary nitrate supplementation increases human skeletal muscle nitrate concentration and improves muscle torque production

Aim: Dietary nitrate (NO3-) supplementation increases nitric oxide bioavailability and can enhance exercise performance. We investigated the distribution and metabolic fate of ingested NO3- at rest and during exercise with a focus on skeletal muscle. Methods: In a randomized, crossover study, 10 healthy volunteers consumed 12.8 mmol N-15-labeled potassium nitrate ((KNO3)-N-15; NIT) or potassium chloride placebo (PLA). Muscle biopsies were taken at baseline, at 1- and 3-h post-supplement ingestion, and immediately following the completion of 60 maximal intermittent contractions of the knee extensors. Muscle, plasma, saliva, and urine samples were analyzed using chemiluminescence to determine absolute [NO3-] and [NO2-], and by mass spectrometry to determine the proportion of NO3- and NO2- that was N-15-labeled. Results: Neither muscle [NO3-] nor [NO2-] were altered by PLA. Following NIT, muscle [NO3-] (but not [NO2-]) was elevated at 1-h (from similar to 35 to 147 nmol/g, p < 0.001) and 3-h, with almost all of the increase being N-15-labeled. There was a significant reduction in N-15-labeled muscle [NO3-] from pre- to post-exercise. Relative to PLA, mean muscle torque production was similar to 7% greater during the first 18 contractions following NIT. This improvement in torque was correlated with the pre-exercise N-15-labeled muscle [NO3-] and the magnitude of decline in N-15-labeled muscle [NO3-] during exercise (r = 0.66 and r = 0.62, respectively; p < 0.01). Conclusion: This study shows, for the first time, that skeletal muscle rapidly takes up dietary NO3-, the elevated muscle [NO3-] following NO3- ingestion declines during exercise, and muscle NO3- dynamics are associated with enhanced torque production during maximal intermittent muscle contractions.