Acute nitrate supplementation and cardiovascular function during respiratory muscle metaboreflex activation

BACKGROUND: The respiratory muscle metaboreflex (RMMR) augments sympathetic outflow during fatiguing respiratory muscle (RM) work, elevating blood pressure, heart rate, and reducing limb blood flow. Dietary nitrate (NO 3 - ) supplementation increases circulating NO 3 - and nitrite (NO 2 - ), enhancing nitric oxide (NO) bioavailability, potentially improving diaphragm contractile efficiency, lowering RM metabolic cost and oxygen demand in the diaphragm, favoring reduced oxygen delivery during inspiratory loading. Whether increased NO availability can improve RM economy with increased inspiratory resistance in humans remain unknown. We hypothesized that acute beetroot juice (BRJ) supplementation would reduce RM and limb (LM) muscle deoxygenation, lower dyspnea, and improve respiratory neuromuscular efficiency (NMER) during a resistive breathing challenge. METHODS: In a randomized, double-masked, crossover design, 17 healthy adults (9 males, 8 females) completed three visits to the conditions: baseline (BASE), placebo (PLA), and BRJ (200µmol/kg body weight). Following supplementation, participants performed a resistive breathing challenge at 60% of maximal inspiratory pressure until task failure, a method previously used to activate the RMMR. Plasma NO 3 - /NO 2 - , LM and RM oxy- and deoxygenation ([O 2 Hb], [HHb]), diaphragm electromyography, and dyspnea intensity and unpleasantness were assessed. Data were analyzed using one- and two-way repeated-measures ANOVA (condition x percent stage of the breathing challenge/experimental sampling period) with Tukey’s post-hoc testing. RESULTS: BRJ increased plasma NO 3 - (Δ = 676 ± 27.2μM; p < 0.001) and NO 2 - (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 (all p < 0.001). Inspiratory strength showed a significant treatment effect (p = 0.042), decreasing from BASE (-77.0 ± 19.3 cmH 2 O) to PLA (-83.9 ± 21.3 cmH 2 O) and BRJ (-85.1 ± 17.0 cmH 2 O). However, no significant differences from BASE vs PLA (p = 0.151), BRJ (p = 0.123), or PLA vs BRJ (p > 0.999) were observed, indicating that supplementation did not meaningfully influence inspiratory muscle strength. LM Δ[O 2 Hb] 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), reflecting increased metabolic demand independent of treatment. RM Δ[O 2 Hb] demonstrated a significant effect of condition (p = 0.047), although no comparisons emerged (all adjusted p > 0.05). Both Δ[O 2 Hb] RM and Δ[HHb] RM increased with challenge stage (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: Acute NO 3 - supplementation substantially increased circulating NO 3 - /NO 2 - but did not reduce RM or LM oxygenation dynamics, dyspnea perception, or improve NMER during high-intensity inspiratory loading. These findings indicate that enhanced NO bioavailability alone is insufficient to modify respiratory muscle economy or downstream sympathetic-related consequences during fatiguing inspiratory work. Further research should determine whether chronic supplementation, higher dosing, or exercise-induced metaboreflex activation alters these responses. 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.