Hypohydration impairs endurance performance: a blinded study

Abstract

The general scientific consensus is that starting exercise with hypohydration >2% body mass impairs endurance performance/capacity, but most previous studies might be confounded by a lack of subject blinding. This study examined the effect of hypohydration in a single blind manner using combined oral and intragastric rehydration to manipulate hydration status. After familiarization, seven active males (mean ± SD: age 25 ± 2 years, height 1.79 ± 0.07, body mass 78.6 ± 6.2, VO2peak 48 ± 7 mL·kg·min−1) completed two randomized trials at 34°C. Trials involved an intermittent exercise preload (8 × 15 min exercise/5 min rest), followed by a 15‐min all‐out performance test on a cycle ergometer. During the preload, water was ingested orally every 10 min (0.2 mL·kg body mass−1). Additional water was infused into the stomach via a gastric feeding tube to replace sweat loss (EU) or induce hypohydration of ~2.5% body mass (HYP). Blood samples were drawn and thirst sensation rated before, during, and after exercise. Body mass loss during the preload was greater (2.4 ± 0.2% vs. 0.1 ± 0.1%; P < 0.001), while work completed during the performance test was lower (152 ± 24 kJ vs. 165 ± 22 kJ; P < 0.05) during HYP. At the end of the preload, heart rate, RPE, serum osmolality, and thirst were greater and plasma volume lower during HYP (P < 0.05). These results provide novel data demonstrating that exercise performance in the heat is impaired by hypohydration, even when subjects are blinded to the intervention.

High Prevalence and Magnitude of Rapid Weight Loss in Mixed Martial Arts Athletes

“…Whilst acute dehydration (James et al, 2017) and energy restriction (James et al 2015) might impair performance, the impairments in thermoregulatory and cardiovascular function caused by hypovolaemia/hyperosmolality (Cheuvront and Kenefick, 2014) are of particular concern, and may exacerbate any negative health effects related to heat exposure used to facilitate weight loss (Casa et al, 2015). This is particularly relevant given that heat exposure has been implicated in a number of the deaths reported in those attempting to make weight (Centers for Disease Control, Prevention, 1998).…”

Mild hypohydration impairs cycle ergometry performance in the heat: A blinded study

“…Previous studies examining the role of fluid intake on exercise performance have not been able to match thirst between euhydrated and hypohydrated trials prior to performance tests. 16,17 In contrast, few investigations have observed performance differences even when thirst was matched by mouth rinsing or water ingestion. 3,15,18 In the present study, subjects during the HYP had lower power output and cycling speed in the second time trial, even when hypohydration was just 1% of bodyweight.…”

“…Studies that manipulate hydration status through fluid ingestion are limited by nature of drinking and its effects on subject bias. [12][13][14] Recently, few studies have attempted to blind subjects to their fluid replacement and overall hydration state via nasal/ oral-gastric tubes 15,16 or intravenous saline infusion. 17,18 The former studies administered fluids via gastric infusion and reported that dehydration of 2%-2.5% body mass resulted in impaired performance.…”

Fluid Intake Habits in Type 1 Diabetes Individuals during Typical Training Bouts

“…The latest National Athletic Trainers’ Association position statement suggests considering fluid replacement volume based on individual fluid losses to avoid a 2% or higher BM loss [18]; there is currently ongoing debate in the literature whether 2% BM loss inevitably leads to reduced performance, or whether people should follow the dictates of thirst [19]. However, recent results recommend preventing BM losses greater than the commonly accepted cut off of 2%, to optimize fluid replacement, as thirst sensation may be insufficient [20-22]. …”

“…Certainly, the optimal fluid replacement protocol to preserve euhydration during exercise is heavily debated. The most commonly referred strategies are as follows: (i) replacing fluid losses in order to avoid a 2% BM loss or (ii) following dictates of thirst [19, 31-33]; nevertheless, drinking to thirst has often been found insufficient to prevent dehydration [21, 34, 35], while fluid loss estimated through BM change was shown to impair endurance performance in a blinded study [22]. To date, the influence that T1DM may exert on thirst perception (i.e., the capacity of stimulating thirst depending on changes in blood osmolality), due to chronic fluctuations in blood glucose and osmolality, has not been fully defined, although high glucose levels increasing blood osmolality are likely to signal for increased thirst sensation [36, 37].…”