Changes in body posture alter plasma nitrite but not nitrate concentration in humans

Purpose: This study evaluated the change (Delta) in plasma volume (PV), nitrate [NO3-], and nitrite [NO2-] concentration following changes in posture in the presence and absence of elevated plasma [NO3-] and [NO2-] Methods: Fourteen healthy participants completed two trials that were preceded by either supplementation with NO3- -rich beetroot juice (BR; total of similar to 31 mmol NO3-) or no supplementation (CON). Both trials comprised 30 min of lying supine followed by 2 min of standing, 2 min of sitting and 5 min of sub-maximal cycling. Measurements of plasma [NO3- ] and [NO2-] were made by gas-phase chemiluminescence and APV was estimated using the Dill and Costill method. Results: Plasma [NO2-] decreased from baseline (CON: 120 +/- 49 nM, BR: 357 +/- 129 nM) after lying supine for 30 min (CON 77 +/- 30 nM; BR 231 +/- 92 nM, both P < 0.01) before increasing during standing (CON 109 +/- 42 nM; BR 297 +/- 105 nM, both P < 0.01) and sitting (CON 131 +/- 43 nM; BR 385 +/- 125 nM, both P < 0.01). Plasma [NO2-] remained elevated following exercise only in CON (125 +/- 61 nM P = 0.02). Plasma [NO3-] was not different between measurement points in either condition (P > 0.05). PV increased from baseline during the supine phase before decreasing upon standing, sitting, and exercise in both trials (all P< 0.05). Conclusions: Changing body posture causes rapid and consistent alterations in plasma [NO2-]. Researchers should therefore carefully consider the effect of posture when measuring this variable.