The effect of exercise-induced hypohydration upon treadmill running performance

Introduction:
Whilst it is often cited that hypohydration (HYP) levels (>2% body mass (BM) losses) result in impaired endurance performance, it has been suggested that endurance runners that drink ad libitum (AL) during exercise and avoid becoming thirsty can negate this impairment (Sawka & Noakes 2007; Noakes 2012). Speculation also exists in the literature that HYP during exercise can potentially benefit the runner by reducing BM leading to improved running economy (RE) during exercise. However little research exists investigating the links between BM lost during exercise-induced HYP, alterations to RE, and performance (Saunders et al. 2004). Therefore the aim of this study was to investigate the impact of variations in exercise-induced HYP on 5km time-trial (TT) performance in trained runners.

Method:
Forty-eight trained male runners were familiarised to procedures and reported to the laboratory on five occasions. An initial visit established VO2peak which preceded four 90 minute treadmill running trials performed at a running speed equivalent to 65% VO2peak. During the 90 minute trials hydration status was manipulated through fluid ingestion with participants completing trials at 0%HYP, ~1%HYP, No Fluid, and AL in a random order. Expired gases were analysed at 30, 60 and 90 minutes of the trials. On completion of each 90 minute trial participants undertook a self-paced, blinded maximal 5km performance TT on the treadmill. Results The AL trials resulted in a 1.7 ±0.4% reduction in BM and a 5km TT of 1404 ±177s. AL trials did not result in the fastest performance time; where individual fastest trials were analysed, a significantly faster mean time of 1361 ±171s (p<0.05) was recorded at a significantly higher BM (only 1.2 ±0.7% BM loss) when compared to AL (p<0.05). RE was significantly lower during the 90 minute run prior to fastest 5km TT performance compared to the AL trial (p<0.05).

Discussion:
This study demonstrates that an AL drinking strategy did not result in fastest 5km treadmill TT performance following a 90 minute preload run. Faster times were recorded by participants with increased fluid ingestion during the preload where significantly better RE was also apparent despite a higher mean BM when compared to AL trials. A drinking strategy with water ingestion 0.5% BM above AL in a 90 minute period when exercising at this intensity would appear to be more appropriate in attempting to maximise performance using this protocol.

Noakes, T.D. (2012) “Thirst as a signal for fluid intake” in Waterlogged; Human Kinetics; Champaign; IL. Pp.57–58 Saunders, P.U, Pyne, D.B, Telford, R.D. & Hawley, J.A. (2004) “Factors affecting running economy in trained distance runners” in Sports Med; Vol. 34;(7); Pp. 465-485 Sawka, M.N. & Noakes, T.D. (2007) “Does dehydration impair exercise performance?” in Med & Sci in Sports & Exercise; Vol. 39;(8); Pp. 1209-1217