Influence of acute dietary nitrate supplementation timing on nitrate metabolism, central and peripheral blood pressure and exercise tolerance in young men.

Purpose: Dietary nitrate (NO) supplementation can lower systolic blood pressure (SBP) and improve exercise performance. Salivary flow rate (SFR) and pH are key determinants of oral NO reduction and purported to peak in the afternoon. We tested the hypotheses that NO-rich beetroot juice (BR) would increase plasma [nitrite] ([NO]), lower SBP and improve exercise performance to a greater extent in the afternoon (AFT) compared to the morning (MORN) and evening (EVE).

Method: Twelve males completed six experimental visits in a repeated-measures, crossover design. NO-depleted beetroot juice (PL) or BR (~ 13 mmol NO) were ingested in the MORN, AFT and EVE. SFR and pH, salivary and plasma [NO] and [NO], brachial SBP and central SBP were measured pre and post supplementation. A severe-intensity exercise tolerance test was completed to determine cycling time to exhaustion (TTE).

Results: There were no between-condition differences in mean SFR or salivary pH. The elevation in plasma [NO] after BR ingestion was not different between BR-MORN, BR-AFT and BR-EVE. Brachial SBP was unchanged following BR supplementation in all conditions. Central SBP was reduced in BR-MORN (- 3 ± 4 mmHg), BR-AFT (- 4 ± 3 mmHg), and BR-EVE (- 2 ± 3 mmHg), with no differences between timepoints. TTE was not different between BR and PL at any timepoint.

Conclusion: Acute BR supplementation was ineffective at improving TTE and brachial SBP and similarly effective at increasing plasma [NO] and lowering central SBP across the day, which may have implications for informing NO supplementation strategies.