Oral Temperature and pH Influence Dietary Nitrate Metabolism in Healthy Adults

This study tested the hypothesis that the increases in salivary and plasma [NO₂^-] after dietary NO₃^- supplementation would be greater when oral temperature and pH were independently elevated, and increased further when oral temperature and pH were elevated concurrently. Seven healthy males (mean ± SD, age 23 ± 4 years) ingested 70 mL of beetroot juice concentrate (BR, which provided ~6.2 mmol NO₃^-) during six separate laboratory visits. In a randomised crossover experimental design, salivary and plasma [NO₃^-] and [NO₂^-] were assessed at a neutral oral pH with a low (T_Lo-pH_Norm), intermediate (T_Mid-pH_Norm), and high (T_Hi-pH_Norm) oral temperature, and when the oral pH was increased at a low (T_Lo-pH_Hi), intermediate (T_Mid-pH_Hi), and high (T_Hi-pH_Hi) oral temperature. Compared with the T_Mid-pH_Norm condition (976 ± 388 µM), the mean salivary [NO₂^-] 1-3 h post BR ingestion was higher in the T_Mid-pH_Hi (1855 ± 423 µM), T_Hi-pH_Norm (1371 ± 653 µM), T_Hi-pH_Hi (1792 ± 741 µM), T_Lo-pH_Norm (1495 ± 502 µM), and T_Lo-pH_Hi (2013 ± 662 µM) conditions, with salivary [NO₂^-] also higher at a given oral temperature when the oral pH was increased (p < 0.05). Plasma [NO₂^-] was higher 3 h post BR ingestion in the T_Mid-pH_Hi, T_Hi-pH_Hi, and T_Lo-pH_Hi conditions, but not the T_Lo-pH_Norm and T_Hi-pH_Norm conditions, compared with T_Mid-pH_Norm (p < 0.05). Therefore, despite ingesting the same NO₃^- dose, the increases in salivary [NO₂^-] varied depending on the temperature and pH of the oral cavity, while the plasma [NO₂^-] increased independently of oral temperature, but to a greater extent at a higher oral pH.