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 randomised, crossover study, ten healthy volunteers consumed 12.8 mmol ¹⁵N-labelled potassium nitrate (K¹⁵NO₃; NIT) or potassium chloride placebo (PLA). Muscle biopsies were taken at baseline, at 1-h 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 analysed using chemiluminescence to determine absolute [NO₃^-] and [NO₂^-], and by mass spectrometry to determine the proportion of NO₃^- and NO₂^- that was ¹⁵N-labelled.

Results

Neither muscle [NO₃^-] nor [NO₂^-] were altered by PLA. Following NIT, muscle [NO₃^-] (but not [NO₂^-]) was elevated at 1-h (from ~35 to 147 nmol.g^-1, P<0.001) and 3-h, with almost all of the increase being ¹⁵N-labelled. There was a significant reduction in ¹⁵N-labelled muscle [NO₃^-] from pre- to post-exercise. Relative to PLA, mean muscle torque production was ~7% greater during the first 18 contractions following NIT. This improvement in torque was correlated with the pre-exercise ¹⁵N-labelled muscle [NO₃^-] and the magnitude of decline in ¹⁵N-labelled 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.