Stephen A. Mears

Mild hypohydration increases the frequency of driver errors during a prolonged, monotonous driving task.

UNLABELLED The aim of the present study was to examine the effect of mild hypohydration on performance during a prolonged, monotonous driving task. METHODS Eleven healthy males (age 22±4y) were instructed to consume a volume of fluid in line with published guidelines (HYD trial) or 25% of this intake (FR trial) in a crossover manner. Participants came to the laboratory the following morning after an overnight fast. One hour following a standard breakfast, a 120min driving simulation task began. Driver errors, including instances of lane drifting or late breaking, EEG and heart rate were recorded throughout the driving task RESULTS Pre-trial body mass (P=0.692), urine osmolality (P=0.838) and serum osmolality (P=0.574) were the same on both trials. FR resulted in a 1.1±0.7% reduction in body mass, compared to -0.1±0.6% in the HYD trial (P=0.002). Urine and serum osmolality were both increased following FR (P<0.05). There was a progressive increase in the total number of driver errors observed during both the HYD and FR trials, but significantly more incidents were recorded throughout the FR trial (HYD 47±44, FR 101±84; ES=0.81; P=0.006) CONCLUSIONS: The results of the present study suggest that mild hypohydration, produced a significant increase in minor driving errors during a prolonged, monotonous drive, compared to that observed while performing the same task in a hydrated condition. The magnitude of decrement reported, was similar to that observed following the ingestion of an alcoholic beverage resulting in a blood alcohol content of approximately 0.08% (the current UK legal driving limit), or while sleep deprived.

Chronic ingestion of a low dose of caffeine induces tolerance to the performance benefits of caffeine

ABSTRACT This study examined effects of 4 weeks of caffeine supplementation on endurance performance. Eighteen low-habitual caffeine consumers (<75 mg · day−1) were randomly assigned to ingest caffeine (1.5–3.0 mg · kg−1day−1; titrated) or placebo for 28 days. Groups were matched for age, body mass, V̇O2peak and Wmax (P > 0.05). Before supplementation, all participants completed one V̇O2peak test, one practice trial and 2 experimental trials (acute 3 mg · kg−1 caffeine [precaf] and placebo [testpla]). During the supplementation period a second V̇O2peak test was completed on day 21 before a final, acute 3 mg · kg−1 caffeine trial (postcaf) on day 29. Trials consisted of 60 min cycle exercise at 60% V̇O2peak followed by a 30 min performance task. All participants produced more external work during the precaf trial than testpla, with increases in the caffeine (383.3 ± 75 kJ vs. 344.9 ± 80.3 kJ; Cohen’s d effect size [ES] = 0.49; P = 0.001) and placebo (354.5 ± 55.2 kJ vs. 333.1 ± 56.4 kJ; ES = 0.38; P = 0.004) supplementation group, respectively. This performance benefit was no longer apparent after 4 weeks of caffeine supplementation (precaf: 383.3 ± 75.0 kJ vs. postcaf: 358.0 ± 89.8 kJ; ES = 0.31; P = 0.025), but was retained in the placebo group (precaf: 354.5 ± 55.2 kJ vs. postcaf: 351.8 ± 49.4 kJ; ES = 0.05; P > 0.05). Circulating caffeine, hormonal concentrations and substrate oxidation did not differ between groups (all P > 0.05). Chronic ingestion of a low dose of caffeine develops tolerance in low-caffeine consumers. Therefore, individuals with low-habitual intakes should refrain from chronic caffeine supplementation to maximise performance benefits from acute caffeine ingestion.

Hypohydration impairs endurance performance: a blinded study

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.

Assessing Hydration Status and Reported Beverage Intake in the Workplace

The aim was to examine the hydration status of adults working in different jobs at the beginning and end of a shift and their reported water intake. One hundred and fifty-six subjects (89 males, 67 females) were recruited from workplaces within the local area (students, teachers, security, office, firefighters, catering). A urine sample was obtained at the start and end of the shift and was analyzed for osmolality (Uosm), specific gravity (USG), and sodium and potassium concentrations. Euhydration was considered Uosm <700 mOsmol/kg or USG <1.020. At the end of the shift, subjects were asked to report all water intake from beverages during the shift. Females had lower Uosm than males at the start (656 [range, 85-970] vs 738 [range, 164-1090] mOsmol/kg) and end (461 [range, 105-1014] vs 642 [range, 130-1056] mOsmol/kg; P < .05) of their working day. Fifty-two percent of individuals who appeared hypohydrated at the start of the shift were also hypohydrated at the end. Reported water intake from beverages was greater in males compared with females (1.2 [range, 0.0-3.3] vs 0.7 [range, 0.0-2.0] L, respectively; P < .0001). In conclusion, a large proportion of subjects exhibited urine values indicating hypohydration, and many remained in a state of hypohydration at the end of the shift.

Voluntary water intake during and following moderate exercise in the cold.

Exercising in cold environments results in water losses, yet examination of resultant voluntary water intake has focused on warm conditions. The purpose of the study was to assess voluntary water intake during and following exercise in a cold compared with a warm environment. Ten healthy males (22 ± 2 years, 67.8 ± 7.0 kg, 1.77 ± 0.06 m, VO₂peak 60.5 ± 8.9 ml·kg⁻¹·min⁻¹) completed two trials (7-8 days). In each trial subjects sat for 30 min before cycling at 70% VO₂peak (162 ± 27W) for 60 min in 25.0 ± 0.1 °C, 50.8 ± 1.5% relative humidity (RH; warm) or 0.4 ± 1.0 °C, 68.8 ± 7.5% RH (cold). Subjects then sat for 120 min at 22.2 ± 1.2 °C, 50.5 ± 8.0% RH. Ad libitum drinking was allowed during the exercise and recovery periods. Urine volume, body mass, serum osmolality, and sensations of thirst were measured at baseline, postexercise and after 60 and 120 min of the recovery period. Sweat loss was greater in the warm trial (0.96 ± 0.18 l v 0.48 ± 0.15 l; p < .0001) but body mass losses over the trials were similar (1.15 ± 0.34% (cold) v 1.03 ± 0.26% (warm)). More water was consumed throughout the duration of the warm trial (0.81 ± 0.42 l v 0.50 ± 0.49 l; p = .001). Cumulative urine output was greater in the cold trial (0.81 ± 0.46 v 0.54 ± 0.31 l; p = .036). Postexercise serum osmolality was higher compared with baseline in the cold (292 ± 2 v 287 ± 3 mOsm.kg⁻¹, p < .0001) and warm trials (288 ± 5 v 285 ± 4 mOsm·kg⁻¹; p = .048). Thirst sensations were similar between trials (p > .05). Ad libitum water intake adjusted so that similar body mass losses occurred in both trials. In the cold there appeared to a blunted thirst response.

Ecologically Valid Carbohydrate Intake during Soccer-Specific Exercise Does Not Affect Running Performance in a Fed State

This study assessed the effect of carbohydrate intake on self-selected soccer-specific running performance. Sixteen male soccer players (age 23 ± 4 years; body mass 76.9 ± 7.2 kg; predicted VO2max = 54.2 ± 2.9 mL∙kg−1∙min−1; soccer experience 13 ± 4 years) completed a progressive multistage fitness test, familiarisation trial and two experimental trials, involving a modified version of the Loughborough Intermittent Shuttle Test (LIST) to simulate a soccer match in a fed state. Subjects completed six 15 min blocks (two halves of 45 min) of intermittent shuttle running, with a 15-min half-time. Blocks 3 and 6, allowed self-selection of running speeds and sprint times, were assessed throughout. Subjects consumed 250 mL of either a 12% carbohydrate solution (CHO) or a non-caloric taste matched placebo (PLA) before and at half-time of the LIST. Sprint times were not different between trials (CHO 2.71 ± 0.15 s, PLA 2.70 ± 0.14 s; p = 0.202). Total distance covered in self-selected blocks (block 3: CHO 2.07 ± 0.06 km; PLA 2.09 ± 0.08 km; block 6: CHO 2.04 ± 0.09 km; PLA 2.06 ± 0.08 km; p = 0.122) was not different between trials. There was no difference between trials for distance covered (p ≥ 0.297) or mean speed (p ≥ 0.172) for jogging or cruising. Blood glucose concentration was greater (p < 0.001) at the end of half-time during the CHO trial. In conclusion, consumption of 250 mL of 12% CHO solution before and at half-time of a simulated soccer match does not affect self-selected running or sprint performance in a fed state.

EXERCISE ASSOCIATED HYPONATREMIA (EAH) AND FLUID INTAKE DURING THE 2016 LONDON MARATHON

Background Exercise-Associated Hyponatremia (EAH) is a potentially fatal condition during endurance exercise (Petzold et al, 2007). Excessive fluid intake over and above normal fluid losses is the most common risk factor in the development of EAH. A “drink to thirst” fluid regime is considered best practice (Hew-Butler et al, 2015). Objective To investigate fluid intake and changes in serum sodium concentrations ([Na]+) in runners during a marathon Design Prospective observational study. Setting 2016 London Marathon. Participants 28 amateur athletes were recruited at the marathon registration. Independent Variables Online and printed medical advice was available to all participants. A drink to thirst fluid regime was recommended to all athletes by the event organisers. Main Outcome Measures Serum sodium samples were collected and body mass measured at the marathon start and finish lines. Questionnaires regarding estimated fluid intake data were collected at the finish. Results The average volume of fluid intake during the marathon was 1.4 litres (range 0–5.5 litres). Change in serum sodium concentration was inversely correlated with change in body mass (r=−0.383, p=0.044) and with estimated fluid intake (r=−0.406, p=0.032). A significant correlation between fluid intake and weight change was not found. The average pre-race and post-race [Na]+ were 141.7 mmol/L and 141.5 mmol/L, respectively (p=0.8673). One runner (1/28, 3.6%) was found to have asymptomatic hyponatraemia ([Na]++=130 mmol/L). This runner had consumed an estimated 5l of fluid over the course of the marathon. A trend in correlation was seen between fluid intake and race time (r=0.349, p=0.069). Conclusions Runners consumed potentially dangerous volumes of fluid during a marathon despite the written guidance recommending against such practices. The consequence of this behaviour is overhydration and an increased risk of developing potentially fatal EAH. The optimal medium for runner education on safe drinking practices is yet to be determined.

Thirst responses following high intensity intermittent exercise when access to ad libitum water intake was permitted, not permitted or delayed

UNLABELLED An increase in subjective feelings of thirst and ad libitum drinking caused by an increase in serum osmolality have been observed following high intensity intermittent exercise (HIIE) compared to continuous exercise. The increase in serum osmolality is closely linked to the rise in blood lactate and serum sodium concentrations. However, during an ensuing recovery period after HIIE when serum osmolality will decrease, the resultant effect on sensations of thirst and subsequent water intake is unclear. Therefore the aim of the study was to assess the sensations of thirst and subsequent effect on ad libitum water consumption when water intake was immediately allowed, delayed or prevented following a period of HIIE. METHODS Twelve males (26±4 years, 80.1±9.3 kg, 1.81±0.05 m, V̇O2peak 60.1±8.9 ml·kg(-1)·min(-1)) participated in three randomised trials undertaken 7-14 days apart. Participants rested for 30 min then completed a 60 min HIIE exercise period (20×1 min at 100% V̇O2peak with 2 min rest) followed by 60 min of recovery, during which ad libitum water intake was provided immediately (W), delayed until the final 30 min (W30) or not permitted (NW). Body mass was measured at the start and end of the trial. Blood lactate and serum sodium concentrations serum osmolality and sensation of thirst were measured at baseline, immediately post-exercise and during the recovery. RESULTS Body mass loss was different between all trials (W: 0.25±0.45, W30: 0.49±0.37, NW: 1.29±0.37%; p<0.05). Sensations of thirst peaked post-exercise and decreased in W and W30 following water ingestion (p<0.05). Total voluntary water intake was greater in W trial (0.846±0.417 vs. 0.630±0.277l; p<0.05) but was similar during the first 30 min period of allowed drinking (0.618±0.297 vs. 0.630±0.277l; p>0.05). Serum osmolality (299±6 vs. 298±5 vs. 298±3 mOsmol·kg(-1)), blood lactate (7.1±1.1 vs. 7.2±1.1 vs. 7.1±1.2 mmol·l(-1)) and serum sodium concentrations (142±2 vs. 145±2 vs. 145±2 mmol·l(-1)) peaked post-exercise (W vs. W30 vs. NW; p<0.05) but were not different between trials (p>0.05). CONCLUSIONS Sensations of thirst were increased following HIIE and remained until satiated by water intake. This was despite the likely primary stimulus, serum osmolality, decreasing during the recovery period following a post-exercise peak. A combined effect of reduction in blood lactate and serum sodium concentrations, restoration of plasma volume and water intake contributed to the similar decrease in serum osmolality observed throughout the trials.

Mild hypohydration increases the frequency of driver errors during a prolonged, monotonous driving task (Abstract, 2nd International and 4th Spanish Hydration Conference)

Purpose: Driver error is the largest cause of road traffic accidents, accounting for around 68 % of all vehicle crashes in the UK. During long and monotonous driving, most individuals display progressive signs of visual fatigue and a loss of vigilance. Since deficits in total body water (TBW) are associated with altered mood and decrements in aspects of cognitive function, it is possible that dehydrated drivers may be more susceptible to errors in judgement and car handling. With this in mind, the aim of the present study was to examine the effects of fluid restriction, on performance during a prolonged, monotonous driving task. Methods: Eleven healthy males (age 22 ± 4 y) completed a familiarisation trial, before two experimental trials were undertaken in a randomised manner. Each experimental trial took place over two days. On day 1 volunteers were instructed to consume a volume of fluid in line with published guidelines (HYD trial) or 25 % of this intake (FR trial). Participants came to the laboratory the following morning after an overnight fast (day 2). One hour following a standard breakfast, a 120 min driving simulation task began. During the HYD trial volunteers were provided with 200 mL of fluid every hour, and on the FR trial only 25 mL was made available each hour. Body mass, serum and urine osmolality, and subjective feelings were recorded during trials. Driver errors, including instances of lane drifting or late breaking, brain activity (EEG) and heart rate were recorded throughout the driving task. Results: Pre-trial body mass (P=0.692), urine osmolality (P=0.838) and serum osmolality (P=0.574) were the same on both trials. FR resulted in a 1.1±0.7 % reduction in body mass, compared to -0.1±0.6 % in the HYD trial (P = 0.002). Urine and serum osmolality were both increased following FR (P Conclusions: The results of the present study suggest that mild hypohydration, resulted in an increase in errors during a prolonged, monotonous drive, compared to that observed while performing the same task in a hydrated condition. The magnitude of decrement reported was similar to that observed following the ingestion of alcohol resulting in a blood alcohol content of approximately 0.08 % (the current UK legal driving limit), or while sleep deprived. There is no question that both drink-driving and driving while tired increases the risk of road traffic accidents, and many countries have instigated national campaigns to educate drivers of the associated risks. Given the present fin- dings, perhaps some attention should also be directed to encouraging appropriate hydration practices among drivers. Key words: Dehydration, Cognitive function, Road traffic accident Language: en

Voluntary fluid intake in the cold

When exercising in a cold environment, fluid losses can occur via sweat, cold induced diuresis and respiration but failure to replace lost fluid is common in the cold due to a blunted thirst response (Kenefick et al. Med Sci Sports Exerc 2004;36;1528–34). This study assessed voluntary fluid intake and measures of hydration status following moderate intensity exercise in the cold. Ten healthy males (age 22±2 years, mass 67.8±7.0 kg, height 1.77±0.06 m, VO2peak 60.5±8.9 ml.kg.min-1) completed two trials following familiarisation separated by 7–14 days. In each trial, subjects sat for 30 min before cycling at 70% VO2peak for 60 min in either 25.0±0.1°C, 50.8±1.5% RH (warm) or 0.4±1.0°C, 68.8±7.5% RH (cold). Subjects then sat for 120 min at 22.2±1.2°C, 50.5±8.0% RH. Ad libitum drinking was allowed during the exercise and recovery periods. Urine volume, body mass, serum osmolality and Na and K concentrations and sensations of thirst were measured at baseline, postexercise and after 60 and 120 min of the recovery period. Sweat loss was lower in the cold trial (0.48±0.15 l vs 0.96±0.18 l) (p<0.0001) but body mass losses over the trials were similar: 1.15±0.34% versus 1.03±0.26% for cold and warm trials respectively. More fluid was consumed in the warm trial (0.81±0.42 l) compared to the cold (0.50±0.49 l) (p=0.001) replacing 51±17 and 33±27% of the total fluid lost respectively (p=0.013). Cumulative urine output was greater in the cold trial (0.81±0.46 vs 0.54±0.31 l) (p=0.036). Serum osmolality, Na and K concentrations, thirst sensations and plasma volume changes were not different between trials (p>0.05). Postexercise serum osmolality was higher compared to baseline in the cold (292±2 vs 287±3 mOsm.kg-1, p<0.0001) and warm trials (288±5 vs 285±4 mOsm.kg-1; p=0.048). Voluntary fluid intake was less in the cold environment, however in both the warm and cold environment, ad libitum fluid intake, combined with fluid losses, resulted in similar changes in body mass.