Ingested inorganic nitrate (NO 3 ⁻) has multiple effects in the human body including vasodilation, inhibition of platelet aggregation, and improved skeletal muscle function. The functional effects of oral NO 3 ⁻ involve the in vivo reduction of NO 3 ⁻ to nitrite (NO 2 ⁻) and thence to nitric oxide (NO). However, the potential involvement of S-nitrosothiol (RSNO) formation is unclear. We hypothesised that the RSNO concentration ([RSNO]) in red blood cells (RBCs) and plasma is increased by NO 3 ⁻-rich beetroot juice ingestion. In healthy human volunteers, we tested the effect of dietary supplementation with NO 3 ⁻-rich beetroot juice (BR) or NO 3 ⁻-depleted beetroot juice (placebo; PL) on [RSNO], [NO 3 ⁻] and [NO 2 ⁻] in RBCs, whole blood and plasma, as measured by ozone-based chemiluminescence. The median basal [RSNO] in plasma samples (n = 22) was 10 (5–13) nM (interquartile range in brackets). In comparison, the median values for basal [RSNO] in the corresponding RBC preparations (n = 19) and whole blood samples (n = 19) were higher (p < 0.001) than in plasma, being 40 (30–60) nM and 35 (25–80) nM, respectively. The median RBC [RSNO] in a separate cohort of healthy subjects (n = 5) was increased to 110 (93–125) nM after ingesting BR (12.8 mmol NO 3 ⁻) compared to a corresponding baseline value of 25 (21–31) nM (Mann-Whitney test, p < 0.01). The median plasma [RSNO] in another cohort of healthy subjects (n = 14) was increased almost ten-fold to 104 (58–151) nM after BR supplementation (7 × 6.4 mmol of NO 3 ⁻ over two days, p < 0.01) compared to PL. In conclusion, RBC and plasma [RSNO] are increased by BR ingestion. In addition to NO 2 ⁻, RSNO may be involved in dietary NO 3 ⁻ metabolism/actions.

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