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

Abstract

Purpose: Dietary nitrate (NO₃^-) supplementation can lower systolic blood pressure (SBP) and improve exercise performance. Salivary flow rate (SFR) and pH are key determinants of oral NO₃^- reduction and purported to peak in the afternoon. We tested the hypotheses that NO₃^--rich beetroot juice (BR) would increase plasma [nitrite] ([NO₂^-]), 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. NO₃^--depleted beetroot juice (PL) or BR (~ 13 mmol NO₃^-) were ingested in the MORN, AFT and EVE. SFR and pH, salivary and plasma [NO₃^-] and [NO₂^-], 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 [NO₂^-] 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 [NO₂^-] and lowering central SBP across the day, which may have implications for informing NO₃^- supplementation strategies.

Keywords: Beetroot juice; Cardiovascular health; Circadian rhythms; Exercise performance; Nitric oxide.

Fig. 1

Mean salivary flow rate (SFR; upper panel) and salivary pH (lower panel) from 1 to 2.5 h following ingestion of nitrate-depleted and nitrate-rich beetroot juice in the morning (PL-MORN and BR-MORN), afternoon (PL-AFT and BR-AFT) and evening (PL-EVE and BR-EVE). The bars represent the group mean ± SEM responses with the filled circles representing individual participants. No differences observed between conditions (P > 0.050)

Fig. 2

Salivary nitrate concentration ([NO₃⁻], upper panel) and salivary nitrite concentration ([NO₂⁻], lower panel) at baseline, 1 h and 2.5 h following ingestion of nitrate-depleted and nitrate-rich beetroot juice in the morning (PL-MORN and BR-MORN), afternoon (PL-AFT and BR-AFT) and evening (PL-EVE and BR-EVE). Data presented as the group mean ± SEM responses with the filled circles representing individual participants. *Denotes higher than PL-MORN, PL-AFT and PL-EVE in BR-MORN, BR-AFT and BR-EVE (P < 0.050). #denotes salivary [NO₃⁻] lower than baseline at 2.5 h in PL-AFT and PL-EVE (P < 0.050)

Fig. 3

Plasma nitrate concentration ([NO₃⁻], upper panel) and plasma nitrite concentration ([NO₂⁻], lower panel) 2.5 h following ingestion of nitrate-depleted and nitrate-rich beetroot juice in the morning (PL-MORN and BR-MORN), afternoon (PL-AFT and BR-AFT) and evening (PL-EVE and BR-EVE). The bars represent the group mean ± SD responses with the filled circles representing individual participants. *Denotes higher than PL-MORN, PL-AFT and PL-EVE in BR-MORN, BR-AFT and BR-EVE (P < 0.050)

Fig. 4

Time to exhaustion (TTE) during severe-intensity cycling exercise following ingestion of nitrate-depleted or nitrate-rich beetroot juice in the morning (PL-MORN and BR-MORN), afternoon (PL-AFT and BR-AFT) and evening (PL-EVE and BR-EVE) (upper panel). The bars represent the group mean ± SEM responses with the filled circles representing individual participants. No differences observed between conditions (P > 0.050)