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.