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

Purpose: This study evaluated the change (Δ) in plasma volume (PV), nitrate [NO], and nitrite [NO] concentration following changes in posture in the presence and absence of elevated plasma [NO] and [NO] METHODS: Fourteen healthy participants completed two trials that were preceded by either supplementation with NO-rich beetroot juice (BR; total of ∼31 mmol NO) 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 [NO] and [NO] were made by gas-phase chemiluminescence and ΔPV was estimated using the Dill and Costill method.

Results: Plasma [NO] 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 [NO] remained elevated following exercise only in CON (125 ± 61 nM P = 0.02). Plasma [NO] 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 [NO]. Researchers should therefore carefully consider the effect of posture when measuring this variable.