Goulet, E. D., Lamboley, C. R., Rousseau, S., & Dionne, I. J. (2006). Effect of hyperhydration on performance during prolonged cycling in a temperate environment when fluid is consumed during exercise. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 1513.

"Taken as a whole, research shows that, compared with pre-exercise euhydration (PEE), pre-exercise hyperhydration (PEH) improves endurance performance (EP) during moderate-duration exercise (18-105 min) conducted in hot ambient temperatures and this, independently of whether or not fluids are ingested during exercise. In a temperate climate, it has been shown that PEH enhances EP compared with PEE during an ~ 2 h long cycling period in which no fluids are ingested. It still unclear whether PEH may improve EP compared with PEE during a moderate-duration exercise conducted in a temperate climate when fluid is consumed during exercise. It is possible that the weak thermal and cardiovascular stress imposed on the body due to the nature of the environment, combined with the ingestion of fluid during exercise, which delays the attainment of the critical level of dehydration deleterious for EP, may render the surplus of fluid provided by PEH useless". This study compared the effect of hyperhydration with pre-exercise euhydration during a 2-hour cycling exercise conducted at 26.5 ºC (55% RH), when fluid is ingested during exercise. Trained Ss (N = 8) underwent either hyperhydration (26 ml/kg bodyweight [BW] of water with 1.2 g glycerol/kg BW [to maximize fluid retention]) or waiting a period of 80 minutes, immediately followed by 2 hours of cycling at 65% VO2max, which was interspersed by five two-min intervals at 80% VO2max. Following the cycling period, Ss underwent an incremental cycling test to exhaustion. During exercise, Ss drank 2.5 ml of sports drink/kg BW every 20 minutes, up to the 100th minute. Heart rate, rectal temperature, perceived exertion, perceived heat-stress and perceived thirst were measured every 20 minutes during the two hours of cycling. During the cycling test, heart rate and rectal temperature were measured every three minutes.

Ss started the exercises either hyperhydrated or euhydrated, depending upon the manipulation. During hyperhydration and euhydration respectively, fluid ingestion during exercise replaced 36.0 + 3.6% and 39.2 + 5.3% of the sweat losses. Hyperhydration decreased perceived thirst during the two-hour cycling exercise, but had no effect on heart rate, rectal temperature, sweat rate, perceived exertion or perceived heat-stress. During the cycling test, hyperhydration had no effect on heart rate or rectal temperature, nor on the time to exhaustion.

Implication. There is no benefit from hyperhydrating before a moderate-duration (<2 hours), moderately intense (<65% VO2max) exercise in a temperate environment (<26.5 ºC) when fluid is replaced at a rate >40% of sweat losses during exercise.

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