Impact of Dietary Nitrate Supplementation on Executive Function During Hypoxic Exercise

Introduction

Appropriate and effective executive function seems to be essential for successful performance in various sports including mountain climbing and skiing at high altitude (Williams and Ericsson, 2005). However, in a recent meta-analysis review, McMorris et al. (2017) concluded that hypoxia has a negative effect on executive function. A previous study demonstrated that 2 days of high-dose nitrate ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} ) supplementation increases cerebral blood flow to the dorsolateral prefrontal cortex or the anterior cingulate cortex in normoxia compared with same days of low-dose \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} supplementation (Presley et al., 2011). Dietary \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} supplementation enhances bioavailability of nitric oxide (NO) (Jones, 2014). NO is known to play a pivotal role in cerebral vasodilation, cerebral blood flow, and the neurovascular coupling of local neural activity (Attwell et al., 2010), which may improve executive function. Indeed, a previous study showed that a single dose of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} supplementation can modulate the cerebral hemodynamics assessed by near-infrared spectroscopy (NIRS), and potentially improve executive performance in normoxia (Wightman et al., 2015). However, others have not found an improvement of executive performance with a single dose of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} supplementation during exercise in normoxia (Thompson et al., 2014), and at rest and during exercise in hypoxia (Lefferts et al., 2016; Shannon et al., 2017). In addition to these previous studies that have investigated effects of a single dose of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} supplementation on executive function, only in normoxia, consecutive days of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} supplementation improved executive performance during exercise under normoxia (Thompson et al., 2015). As it is observed that plasma nitrite, which can covert to NO, was increased dose dependently with \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} supplementation (Wylie et al., 2013), consecutive days of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} supplementation may improve executive function in hypoxia because of altered cerebral hemodynamics.

In this study, we hypothesized that a short-term dietary beetroot (BR) juice supplementation might improve executive function both at rest and during exercise under hypoxia. To test this, we used NIRS to assess tissue oxygenation at the frontal lobe, which has been used in previous studies (Ando et al., 2013; Komiyama et al., 2015; Wightman et al., 2015).

Materials and Methods

Treatments

This study adopted a randomized, double-blind, and crossover design. The study protocol is given in Figure 1. In brief, two exercise trials were performed with at least 10 days of washout period in a random order. The trials consisted of hypoxic exercise (fraction inspired oxygen, fraction of inspired oxygen [FiO₂] = 0.1395) with placebo (PL) or BR juice. The subjects consumed \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} -rich (8.4 mmol) BR (Beet it; James White Drinks Ltd., Ipswich, United Kingdom) or PL (140 mL/d, for 4 days), wherein \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} was depleted through the manufacturing process, for 4 days, including the main test day.

OPEN IN VIEWER

FIG. 1.

Overview of experimental procedure. Supplementation was taken at the same time each day (days 1–3) and 2.5 hours before the exercise trial (day 4). Executive function using CWST was assessed 30 minutes after the start of rest (Rest) and 15 minutes after the start of the submaximal leg cycling exercise (Ex) in hypoxia. Baseline values of the NIRS metrics were measured for 30 seconds before hypoxic exposure. CWST, color–word Stroop task; deoxy-Hb, deoxygenated hemoglobin; FiO₂, fraction of inspired oxygen; NIRS, near-infrared spectroscopy; oxy-Hb, oxygenated hemoglobin; P_ETCO₂, partial pressure of end-tidal carbon dioxide; SpO₂, arterial oxygen saturation; total-Hb, total hemoglobin.

Results

Dietary BR ingestion significantly increased plasma \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} levels compared with PL ingestion (211.4 ± 26.6 vs. 39.0 ± 8.2 mmol/L, t = −5.77, p < 0.001). Table 1 shows the physiological responses during the CWST at rest and during exercise. BR supplementation did not affect all physiological variables (all p > 0.05). Hypoxic exercise slightly increased oxy-Hb (p > 0.05) and significantly increased deoxy-Hb and total-Hb compared with rest (p < 0.01). In addition, SpO₂ during exercise was significantly decreased than rest in both trials (Rest vs. Ex; PL, 88.0% ± 0.8% vs. 75.6% ± 5.6%; BR, 87.0% ± 0.9% vs. 74.6% ± 1.7%; both p ≤ 0.05). Figure 2 presents the CRT and response accuracy in the CWST during hypoxic exposure. The CRT was shortened during Ex than at rest with no differences between PL and BR (p < 0.01; Fig. 2A). There was a worsening trend for response accuracy in BR during Ex compared with that in PL (p = 0.066; Fig. 2B).

OPEN IN VIEWER

FIG. 2.

Summarized results of the CWST. Mean values of the correct response time (A) and response accuracy (B) in CWST are given. Plots linked with dashed line indicate an individual data, and bars indicate average data. The plots which revealed same values of response accuracy among participants were piled up at the same position.

Discussion

In this study, 4 days of BR supplementation increased plasma \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} levels before the exercise trial on day 4. However, the BR supplementation did not affect CRT (Fig. 2A) and all physiological variables (Table 1) at rest and during exercise, under hypoxic condition. These results were in accordance with previous studies, showing no differences in executive performances (Lefferts et al., 2016; Shannon et al., 2017), cardiorespiratory variables (Bourdillon et al., 2015), and cerebral oxygenation (Masschelein et al., 2012) between the PL and BR at rest and during exercise in hypoxia. From these literatures, CRT, cardiorespiratory, and cerebrovascular responses under hypoxic exercise may not be affected regardless of single- or multidose BR supplementation. On the contrary, hypoxic exercise significantly improved the CRT irrespective of PL or BR (Fig. 2A). As some previous studies also demonstrated improvement in reaction time during dynamic exercise in hypoxia (Ando et al., 2013; Komiyama et al., 2015), the improvement in CRT may be attributed to the beneficial effects of exercise per se, not to BR supplementation effects. However, the reasons why BR did not affect these variables in hypoxia and why acute hypoxic exercise improved CRT are unclear. A recent review reported that the response time of executive task during exercise was improved by cerebral metabolism rather than cerebral blood flow and oxygenation (Ogoh, 2017). Therefore, future studies to elucidate the underlying mechanisms with measurement of these cerebral outcomes are needed.

Unexpectedly, the response accuracy with BR during hypoxic exercise was not significantly, but marginally impaired compared with PL (p = 0.066; Fig. 2B). Although we can only speculate, one possible explanation might involve the trigeminovascular system by direct stimulation of NO. Because it has been suggested that direct stimulus of NO in the brain may be responsible for headache pain sensation (Ashina et al., 1999) and headache impaired executive function (Smith, 2016). It was reported that a single bout of moderate-intensity exercise increased vessel diameter in vertebral arteries (i.e., vasodilation) compared with resting condition (Sato and Sadamoto, 2010), suggesting that headache may increase during exercise rather than at rest. A previous study have also reported that BR supplementations rather than PL caused more severe headache under hypoxic condition (Rossetti et al., 2017). Moreover, Lawley et al. (2016) also demonstrated that only 10-minute exposure to severe hypoxia (FiO₂ = 0.11) could increase headache with intracranial pressure. Although we did not assess any headache symptoms of the participants, these previous findings raise the possibility that headache could be caused during hypoxic exercise with BR in this study. As it was reported that headache could also impair response accuracy (Smith, 2016), our results may be supported by the previous study. Nonetheless, the magnitude of the reduction in response accuracy was relatively small, ∼2%. Thus, a physiological significance of this reduction is uncertain, and also, we must acknowledge a possibility of type I error. However, under extreme environments (e.g., mountain climbing at high altitude), we consider that even a minor mistake or accident should not be ignored for people's safety.

In this study, we recruited only young men because age (Hamasaki et al., 2018) and sex (Esposito et al., 1996) could affect the cerebral blood flow during executive task. Therefore, it is unclear whether our results can be applied for other populations such as women, aged people, and diseased patients. Future studies should be expanded.

In summary, a short-term dietary BR supplementation did not affect CRT and physiological indices at rest and during exercise in hypoxia. In contrast, there was a trend for impairment of response accuracy during hypoxic exercise with BR compared with PL. These findings suggest that \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm{NO}}_3^ -$$ \end{document} supplementation may be against for an improvement of executive function, and thereby, may not be recommended during hypoxic exercise, at least, for the populations in this study.