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.