Acute nitrate supplementation and respiratory muscle economy during respiratory muscle metaboreflex activation

BACKGROUND: The respiratory muscle metaboreflex (RMMR) increases sympathetic outflow during fatiguing respiratory muscle (RM) work, elevating blood pressure, heart rate while constricting limb vasculature. Dietary nitrate (NO3-) supplementation enhances nitric oxide (NO) bioavailability, and may improve respiratory muscle contractile efficiency, potentially lowering RM metabolic cost and oxygen demand, during inspiratory loading. Whether increased NO availability improves RM economy during RMMR activation in humans remains unknown. We hypothesized that acute ingestion of nitrate-rich beetroot juice (BRJ) would attenuate respiratory and limb muscle deoxygenation, reduce dyspnea, and improve neuromuscular efficiency (NMER) during a resistive breathing challenge. METHODS: Seventeen healthy adults (9 males, 8 females) completed three randomized, double-masked, crossover visits: baseline (BASE), placebo (PLA), and BRJ (200 µmol·kg - ¹). In a randomized, double-blinded, following supplementation, participants performed a resistive breathing challenge at 60% of maximal inspiratory pressure until task failure to activate the RMMR. Plasma NO3-/NO2-, limb (LM) and RM oxy- and deoxygenation ([O2Hb], [HHb]), diaphragm electromyography, and dyspnea intensity and unpleasantness were assessed. Data were analyzed with one- and two-way repeated-measures ANOVA with Tukey’s post-hoc testing. RESULTS: BRJ increased plasma NO3- (Δ = 676 ± 27.2μM; p < 0.001) and NO2- (Δ = 0.503 ± 0.0815μM; p < 0.001) ~2-2.5hr post-ingestion, with concentrations remaining significantly higher than BASE and PLA through the end of testing (p < 0.001). Inspiratory muscle strength showed a significant treatment effect (p = 0.042), decreasing from BASE (-77.0 ± 19.3 cmH2O) to PLA (-83.9 ± 21.3 cmH2O) and BRJ (-85.1 ± 17.0 cmH2O). However, no significant differences from BASE vs PLA (p = 0.151), BRJ (p = 0.123), or PLA vs BRJ (p > 0.999) were observed. LM Δ[O2Hb] or Δ[HHb] remained unchanged, with no condition or interaction effects (all p > 0.05), although Δ[HHb] LM rose progressively with inspiratory load (p = 0.008). RM Δ[O2Hb] demonstrated a significant effect of condition (p = 0.047), although no comparisons emerged ( p > 0.05). Both Δ[O2Hb] RM and Δ[HHb] RM increased with task duration (p < 0.001), yet neither variable demonstrated treatment effects. NMER was unaffected by supplementation (p = 0.838), indicating no improvement in diaphragm contractile economy. Dyspnea intensity (p < 0.001) and unpleasantness (p < 0.001) increased markedly from 20-100% of task duration, with no condition or interaction effects (all p > 0.43). CONCLUSION: Despite substantially elevating circulating NO 3 - /NO 2 - , acute dietary nitrate supplementation did not attenuate respiratory or limb muscle deoxygenation, improve respiratory muscle neuromuscular efficiency, or reduce dyspnea during high-intensity inspiratory loading. These findings indicate that enhanced NO bioavailability alone is insufficient to modify respiratory muscle economy or alter the physiological consequences of the RMMR. Whether chronic supplementation, higher dosing strategies, or integration with whole-body exercise yields different outcomes warrants further investigation. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.