No effect of beetroot juice supplementation on exercise economy and performance in recreationally active females despite increased torque production

Abstract

This study investigated the effects of acute and chronic beetroot juice (BRJ) supplementation on submaximal exercise oxygen uptake (VO₂ ), time trial (TT) performance, and contractile properties of the plantar flexors in females. Study 1: Using a double blind, randomized, crossover design, 12 recreationally active females using hormonal contraceptives supplemented acutely (2.5 h) and chronically (8 days) with 280 mL BRJ/d (~26 mmoles nitrate [ ${\text{NO}}_3^{-}$ ]) or a ${\text{NO}}_3^{-}$ -free placebo (PLA). On days 1 and 8, participants cycled for 10 min at 50% and 70% VO_2peak and completed a 4 kJ/kg body mass TT. Plasma [ ${\text{NO}}_3^{-}$ ] and nitrite ([NO₂^- ]) increased significantly following BRJ supplementation versus PLA. There was no effect of BRJ supplementation on VO₂ at 50% or 70% VO_2peak , or TT performance. Study 2: 12 recreationally active females (n = 7 from Study 1) using hormonal contraceptives participated in a baseline visit and were supplemented acutely (2.5 h) and chronically (8 days) with 280 mL BRJ/d. Maximum voluntary strength (MVC) of the plantar flexors was assessed and a torque-frequency curve performed. BRJ had no effect on MVC, voluntary activation, peak twitch torque, time to peak torque, or half relaxation time. Following both acute (46.6 ± 4.9% of 100 Hz torque) and chronic (47.2 ± 4.4%) supplementation, 10 Hz torque was significantly greater compared to baseline (32.9 ± 2.6%). In summary, BRJ may not be an effective ergogenic aid in recreationally active females as it did not reduce submaximal exercise VO₂ or improve aerobic TT performance despite increasing low frequency torque production.

Keywords: Beetroot juice; contractile properties; females; oxygen uptake; performance; torque-frequency.

Figure 1

(A) Plasma nitrate (${\text{NO}}_3^{-}$) at baseline (B) and two hours post‐ingestion (2 h) of 140 mL beetroot juice (BRJ) or nitrate‐free placebo (PLA) both acutely and chronically. (B) Plasma nitrite (${\text{NO}}_2^{-}$) at B and 2 h of 140 mL BRJ or PLA both acutely and chronically.Values are mean ± SE, n = 12. *Significantly different than all other conditions. ^†Significantly greater than conditions with no symbol.

Figure 2

(A) Mean oxygen uptake (VO ₂) at 50% peak oxygen uptake (VO _2peak) following acute and chronic supplementation with beetroot juice (BRJ) or a nitrate‐free placebo (PLA). (B) Mean VO ₂ at 70% VO2peak following acute and chronic supplementation with BRJ or PLA. Data reported as mean ± SE, n = 12.

Figure 3

Mean time trial completion following acute and chronic supplementation with beetroot juice (BRJ) or a nitrate‐free placebo (PLA). Values reported as mean ± SE, n = 12.

Figure 4

Time trial performance parameters following acute and chronic supplementation with beetroot juice (BRJ) or a nitrate‐free placebo (PLA). (A) Mean time to complete each 20% split (B) Mean heart rate (HR) at each 20% split. (C) Mean rating of perceived exertion (RPE) at each 20% split. (D) Mean power output at each 20% split. Values reported as mean ± SE, n = 12. *Significantly greater than all other time points; a, significantly greater than 20%; b, significantly greater than 20%, 40% and 60%; c, significantly greater than 20, 40, 60 and 80%.

Figure 5

Absolute torque produced with 100 Hz stimulation during the torque‐frequency curve of the plantar flexors following stimulation of the tibial nerve at baseline and following acute and chronic BRJ supplementation. Values reported as mean ± SE, n = 12.

Figure 6

Torque‐frequency curve of the plantar flexors following stimulation of the tibial nerve at baseline and following acute and chronic BRJ supplementation. Data normalized to a percentage of torque produced at 100 Hz. Values reported as mean ± SE, n = 12. *Significantly greater compared to baseline.