Abstract
Objectives
Dietary inorganic nitrate from foods such as red beetroot juice (RBJ) can contribute to nitric oxide (NO) bioavailability through the enterosalivary nitrate-nitrite-NO pathway. A critical step in this pathway is the reduction of nitrate to nitrite by oral bacteria. We investigated the effects of inorganic nitrate supplementation, as RBJ or placebo + potassium nitrate (PBO+NIT), on the oral microbiota, and its relationship with saliva and plasma NO metabolites and vascular endothelial function.
Methods
In a randomized, double-blind, placebo-controlled trial, we measured the abundance of oral nitrate-reducing bacteria in saliva samples from 15 middle-aged/older adults with overweight and obesity using 16 rRNA sequencing. We also assessed the relationship of oral nitrate-reducing bacteria with the physiological responses to acute (4 hours) and chronic (4 weeks) RBJ, PBO+NIT, nitrate-free RBJ, and placebo supplementation via measurement of saliva and plasma nitrate/nitrite (NOx), plasma nitrite levels, and reactive hyperemia index (RHI).
Results
A significant decrease in the alpha diversity metric, Pileou's Evenness, was detected after chronic consumption of PBO+NIT (0.69 ± 0.05 at week 0 vs. 0.65 ± 0.05 at week 4; P < 0.05), while there was a trend for a decline following RBJ consumption (0.69 ± 0.05 at week 0 vs. 0.65 ± 0.05 at week 4; P = 0.08). No significant differences in abundance of nitrate-reducing bacteria were observed after chronic supplementation, although abundance of the species Neisseria subflava was trending toward an increase in the RBJ group (10.8% at week 0 vs. 12.2% at week 4; P = 0.07). Plasma and saliva NOx increased from baseline and remained elevated for the 4-hour testing period after acute and chronic RBJ and PBO+NIT supplementation (all P < 0.05), while plasma nitrite only peaked at 2 hours in the RBJ group after acute supplementation and was significantly higher than PBO+NIT group (P < 0.01). RHI change from baseline to 4 hours was positively correlated with total abundance of nitrate-reducing species after chronic RBJ supplementation (r = 0.5; P = 0.05).
Conclusions
Acute and chronic RBJ and PBO+NIT supplementation increases NO metabolites and may alter the oral microbiota to favorably affect vascular endothelial function in middle-aged/older adults with overweight and obesity.
Funding Sources
NIFA, USDA.