Influence of iodide ingestion on nitrate metabolism and blood pressure following short-term dietary nitrate supplementation in healthy normotensive adults

Abstract

Uptake of inorganic nitrate (NO₃^-) into the salivary circulation is a rate-limiting step for dietary NO₃^- metabolism in mammals. It has been suggested that salivary NO₃^- uptake occurs in competition with inorganic iodide (I^-). Therefore, this study tested the hypothesis that I^- supplementation would interfere with NO₃^- metabolism and blunt blood pressure reductions after dietary NO₃^- supplementation. Nine healthy adults (4 male, mean ± SD, age 20 ± 1 yr) reported to the laboratory for initial baseline assessment (control) and following six day supplementation periods with 140 mL·day^-1 NO₃^--rich beetroot juice (8.4 mmol NO₃^-·day^-1) and 198 mg potassium gluconate·day^-1 (nitrate), and 140 mL·day^-1 NO₃^--rich beetroot juice and 450 μg potassium iodide·day^-1 (nitrate + iodide) in a randomized, cross-over experiment. Salivary [I^-] was higher in the nitrate + iodide compared to the control and NIT trials (P < 0.05). Salivary and plasma [NO₃^-] and [NO₂^-] were higher in the nitrate and nitrate + iodide trials compared to the control trial (P < 0.05). Plasma [NO₃^-] was higher (474 ± 127 vs. 438 ± 117 μM) and the salivary-plasma [NO₃^-] ratio was lower (14 ± 6 vs. 20 ± 6 μM), indicative of a lower salivary NO₃^- uptake, in the nitrate + iodide trial compared to the nitrate trial (P < 0.05). Plasma and salivary [NO₂^-] were not different between the nitrate and nitrate + iodide trials (P > 0.05). Systolic blood pressure was lower than control (112 ± 13 mmHg) in the nitrate (106 ± 13 mmHg) and nitrate + iodide (106 ± 11 mmHg) trials (P < 0.05), with no differences between the nitrate and nitrate + iodide trials (P > 0.05). In conclusion, co-ingesting NO₃^- and I^- perturbed salivary NO₃^- uptake, but the increase in salivary and plasma [NO₂^-] and the lowering of blood pressure were similar compared to NO₃^- ingestion alone. Therefore, increased dietary I^- intake, which is recommended in several countries worldwide as an initiative to offset hypothyroidism, does not appear to compromise the blood pressure reduction afforded by increased dietary NO₃^- intake.

Keywords: Entero-salivary circulation; Nitric oxide; Nitrite; Nutrition; Vascular health.