Nitrate and nitrite bioavailability in plasma and saliva: Their association with blood pressure - A systematic review and meta-analysis

In this study, we conducted a systematic review and meta-analysis to determine plasma and salivary nitrate (NO ₃ ⁻) and nitrite (NO ₂ ⁻) concentrations under resting and fasting conditions in different type of individuals and their association with blood pressure levels. A total of 77 studies, involving 1918 individuals aged 19–74 years (males = 906; females = 1012), which measured plasma and/or salivary NO ₃ ⁻ and NO ₂ ⁻ using the chemiluminescence technique, were included. Mean plasma NO ₃ ⁻ and NO ₂ ⁻ concentrations were 33.9 μmol/L and 158.3 nmol/L, respectively. Subgroup analyses revealed lower plasma NO ₃ ⁻ and NO ₂ ⁻ concentrations in individuals with cardiometabolic risk (NO ₃ ⁻: 21.2 μmol/L; 95 % CI, 13.4–29.0; NO ₂ ⁻: 122.8 nmol/L; 95 % CI, 75.3–138.9) compared to healthy (NO ₃ ⁻: 33.9 μmol/L; 95 % CI, 29.9–37.9; NO ₂ ⁻: 159.5 nmol/L; 95 % CI, 131.8–187.1; P < 0.01) and trained individuals (NO ₃ ⁻: 43.0 μmol/L; 95 % CI, 13.2–72.9; NO ₂ ⁻: 199.3 nmol/L; 95 % CI, 117.6–281; P < 0.01). Mean salivary NO ₃ ⁻ and NO ₂ ⁻ concentrations were 546.2 μmol/L and 197.8 μmol/L, respectively. Salivary NO ₃ ⁻, but no NO ₂ ⁻, concentrations were higher in individuals with cardiometabolic risk (680.0 μmol/L; 95 % CI, 510.2–849.8; P = 0.001) compared to healthy individuals (535.9 μmol/L; 95 % CI, 384.2–687.6). A significant positive association (coefficient, 15.4 [95 % CI, 0.255 to 30.5], P = 0.046) was observed between salivary NO ₃ ⁻ and diastolic blood pressure (DBP). These findings suggest that the health status is positively associated with plasma NO ₃ ⁻ and NO ₂ ⁻ concentrations, but the circulatory levels of these anions are not associated with blood pressure. Only salivary NO ₃ ⁻ showed a significant positive association with DBP.