Influence of Dietary Nitrate Supplementation on High-Intensity Intermittent Running Performance at Different Doses of Normobaric Hypoxia in Endurance-Trained Males George P. Robinson, Sophie C. Killer, Zdravko Stoyanov, Harri Stephens, Luke Read, Lewis J. James, and Stephen J. Bailey Loughborough University This study investigated whether supplementation with nitrate-rich beetroot juice (BR) can improve high-intensity intermittent running performance in trained males in normoxia and different doses of normobaric hypoxia. Eight endurance-trained males ( ˙ VO 2peak , 62 ± 6 ml·kg -1 ·min -1 ) completed repeated 90 s intervals at 110% of peak treadmill velocity, from an initial step incremental test, interspersed by 60 s of passive recovery until exhaustion (T lim ). Participants completed the first three experimental trials during days 3, 5, and 7 of BR or nitrate-depleted beetroot juice (PLA) supplementation and completed the remaining experimental visits on the alternative supplement following at least 7 days of washout. The fraction of inspired oxygen during visits 1–3 was either 0.209, 0.182, or 0.157, equivalent to an altitude of 0, 1,200, and 2,400 m, respectively, and this order was replicated on visits 4–6. Arterial oxygen saturation declined dose dependently as fraction of inspired oxygen was lowered (p < .05). Plasma nitrite concentration was higher pre- and postexercise after BR compared with PLA supplementation (p < .05). There was no difference in T lim between PLA and BR at 0 m (445 [324, 508] and 410 [368, 548] s); 1,200 m (341 [270, 390] and 332 [314, 356] s); or 2,400 m (233 [177, 373] and 251 [221, 323] s) (median and [interquartile range]; p > .05). The findings from this study suggest that short-term BR supplementation does not improve high-intensity intermittent running performance in endurance-trained males in normoxia or at doses of normobaric hypoxia that correspond to altitudes at which athletes typically train while on altitude training camps. Keywords: beetroot juice, exercise performance, nitric oxide Dietary supplementation with inorganic nitrate (NO 3 - ), typi- cally in the form of NO 3 - -rich beetroot juice (BR), has emerged as a nutritional intervention to enhance exercise performance (Jones et al., 2018). The ergogenic effects of dietary NO 3 - supplementa- tion are attributed to the stepwise reduction of NO 3 - to nitrite (NO 2 - ) and then nitric oxide (NO), which can lead to improve- ments in skeletal muscle perfusion, metabolism, and contractile function (Jones et al., 2018). Initial studies indicated that dietary NO 3 - supplementation could improve continuous endurance per- formance in recreationally active or moderately trained individuals ( ˙ VO 2peak < 60 ml·kg -1 ·min -1 ; Bailey et al., 2009; Cermak et al., 2012; Porcelli et al., 2015; Wylie et al., 2013a), with subsequent studies revealing equivocal effects in well-trained endurance ath- letes ( ˙ VO 2peak > 60 ml·kg -1 ·min -1 ; Besco ´ s et al., 2012; Boorsma et al., 2014; Bourdillon et al., 2015; Christensen et al., 2013; Porcelli et al., 2015; Rokkedal-Lausch et al., 2019; Shannon et al., 2017). More recently, NO 3 - supplementation has been reported to augment physiological processes, such as perfusion, calcium han- dling and contractility, to a greater extent in type II (fast-twitch), compared with type I (slow-twitch), skeletal muscle (Jones et al., 2016) and to improve high-intensity intermittent performance in moderately trained individuals (Aucouturier et al., 2015; Thompson et al., 2016; Wylie et al., 2013b, 2016) and trained team sport athletes (Nyakayiru et al., 2017). However, the effect of NO 3 - supplementation on high-intensity intermittent performance in endurance-trained athletes is equivocal (Bond et al., 2012; Christensen et al., 2013; Pawlak-Chaouch et al., 2019). Since the reduction of NO 2 - to NO is enhanced as PO 2 declines (Jones et al., 2016), there has been great interest in the ergogenic potential of NO 3 - supplementation in hypoxia. In re- creationally active or moderately trained individuals, NO 3 - sup- plementation has been reported to improve continuous endurance performance in normobaric hypoxia (Cocksedge et al., 2020; Kelly et al., 2014; Masschelein et al., 2012; Muggeridge et al., 2014), with greater improvements in hypoxia compared with normoxia (Cocksedge et al., 2020; Kelly et al., 2014). Although the effects of NO 3 - supplementation on continuous endurance performance in normobaric hypoxia in trained individuals are less clear (Arnold et al., 2015; Bourdillon et al., 2015; MacLeod et al., 2015; Nybäck et al., 2017; Rokkedal-Lausch et al., 2019), there is evidence to suggest that, in contrast to normoxia (Porcelli et al., 2015), improved performance with NO 3 - supplementation in hypoxia is not linked to aerobic fitness (Shannon et al., 2016). To date, we are only aware of one study to have assessed the effect of NO 3 - supplementation on repeated sprint cycling performance (four sets of 9 × 4 s) in normobaric hypoxia, with this study reporting no effect on peak or mean power output throughout the test, but a reduction in work decrement with BR supplementation in the first set (Kent et al., 2019). While this suggests that NO 3 - supplemen- tation could confer some ergogenic effect for team sports athletes in hypoxia, the effect of NO 3 - supplementation on high-intensity The authors are with the School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, Leicestershire, United Kingdom. Bailey (S.Bailey2@lboro.ac.uk) is corresponding author. 1 International Journal of Sport Nutrition and Exercise Metabolism, 2021, 31, 1-8 https://doi.org/10.1123/ijsnem.2020-0198 © 2021 Human Kinetics, Inc. ORIGINAL RESEARCH Unauthenticated | Downloaded 08/17/21 02:32 AM UTC