Dietary nitrate supplementation increases nitrate and nitrite concentrations in human skin interstitial fluid

Acute dietary nitrate (NO3−) supplementation can increase [NO3−], but not nitrite ([NO2−]), in human skeletal muscle, though its effect on [NO3−] and [NO2−] in skin remains unknown. In an independent group design, 11 young adults ingested 140 mL of NO3−-rich beetroot juice (BR; 9.6 mmol NO3−), and 6 young adults ingested 140 mL of a NO3−-depleted placebo (PL). Skin dialysate, acquired through intradermal microdialysis, and venous blood samples were collected at baseline and every hour post-ingestion up to 4 h to assess dialysate and plasma [NO3−] and [NO2−]. The relative recovery rate of NO3− and NO2− through the microdialysis probe (73.1% and 62.8%), determined in a separate experiment, was used to estimate skin interstitial [NO3−] and [NO2−]. Baseline [NO3−] was lower, whereas baseline [NO2−] was higher in the skin interstitial fluid relative to plasma (both P < 0.001). Acute BR ingestion increased [NO3−] and [NO2−] in the skin interstitial fluid and plasma (all P < 0.001), with the magnitude being smaller in the skin interstitial fluid (e.g., 183 ± 54 vs. 491 ± 62 μM for △[NO3−] from baseline and 155 ± 190 vs. 217 ± 204 nM for △[NO2−] from baseline at 3 h post BR ingestion, both P ≤ 0.037). However, due to the aforementioned baseline differences, skin interstitial fluid [NO2−] post BR ingestion was higher, whereas [NO3−] was lower relative to plasma (all P < 0.001). These findings extend our understanding of NO3− and NO2− distribution at rest and indicate that acute BR supplementation increases [NO3−] and [NO2−] in human skin interstitial fluid.