Effect of ingestion pattern on rehydration and exercise performance subsequent to passive dehydration

Accelerated muscle contractility and decreased muscle steadiness following sauna recovery do not induce greater neuromuscular fatigability during sustained submaximal contractions

Acute whole-body hyperthermia (WBH) increases blood markers concentration of stress, impairs motor drive to exercising muscles, and decreases resistance to neuromuscular fatigability. The functional natural residual consequences of WBH on neuromuscular functions remain unclear. We aimed to investigate the effects of residual WBH on voluntary and electrically induced ankle plantar flexor contractility properties, motor drive transmission (reflexes), muscle torque steadiness, resistance to neuromuscular fatigability, and markers of stress as the body temperature recovers naturally to normothermia. WBH was induced by Finnish sauna bathing in 16 apparently healthy young (24 ± 4 years) adult men. Motor performance was monitored before and 2 h after the sauna, and immediately after submaximal exercise (120 s at 50% of maximal voluntary contraction). Markers of stress were monitored before and 2 h after the sauna. Finnish sauna exposure induced moderate to severe WBH (rectal temperature, 38.5-39.6 °C). At 2 h after the sauna, rectal temperature had recovered to the preheating level (preheating 37.11 ± 0.33 °C versus postheating 37.00 ± 0.29 °C, p > .05). Post-sauna recovery was accompanied by slowed salivary free cortisol diurnal kinetics, whereas noradrenaline, dopamine, and serotonin did not persist into the 2 h recovery after the sauna. Although recovery to normothermia after a sauna led to a greater acceleration of muscle contractility properties and decreased muscle steadiness, sustained isometric submaximal contraction did not provoke greater neuromuscular fatigability.

The Effect of Fluid Intake Following Dehydration on Subsequent Athletic and Cognitive Performance: a Systematic Review and Meta-analysis

Background

The deleterious effects of dehydration on athletic and cognitive performance have been well documented. As such, dehydrated individuals are advised to consume fluid in volumes equivalent to 1.25 to 1.5 L kg⁻¹ body mass (BM) lost to restore body water content. However, individuals undertaking subsequent activity may have limited time to consume fluid. Within this context, the impact of fluid intake practices is unclear. This systematic review investigated the effect of fluid consumption following a period of dehydration on subsequent athletic and cognitive performance.

Methods

PubMed (MEDLINE), Web of Science (via Thomas Reuters) and Scopus databases were searched for articles reporting on athletic (categorized as: continuous, intermittent, resistance, sport-specific and balance exercise) or cognitive performance following dehydration of participants under control (no fluid) and intervention (fluid intake) conditions. Meta-analytic procedures determined intervention efficacy for continuous exercise performance.

Results

Sixty-four trials (n = 643 participants) derived from 42 publications were reviewed. Dehydration decreased BM by 1.3–4.2%, and fluid intake was equivalent to 0.4–1.55 L kg⁻¹ BM lost. Fluid intake significantly improved continuous exercise performance (22 trials), Hedges’ g = 0.46, 95% CI 0.32, 0.61. Improvement was greatest when exercise was performed in hotter environments and over longer durations. The volume or timing of fluid consumption did not influence the magnitude of this effect. Evidence indicating a benefit of fluid intake on intermittent (10 trials), resistance (9 trials), sport-specific (6 trials) and balance (2 trials) exercise and on cognitive performance (15 trials) was less apparent and requires further elucidation.

Conclusions

Fluid consumption following dehydration may improve continuous exercise performance under heat stress conditions, even when the body water deficit is modest and fluid intake is inadequate for complete rehydration.

Electronic supplementary material

The online version of this article (doi:10.1186/s40798-017-0079-y) contains supplementary material, which is available to authorized users.

Effects of lycium barbarum polysaccharides on neuropeptide Y and heat-shock protein 70 expression in rats exposed to heat

The purpose of the present study was to investigate the effects of high ambient temperature on the neuropeptide Y (NPY) mRNA level in the hypothalamus, the plasma concentration of corticotropin-releasing hormone (CRH), cortisol (Cor), heat-shock protein 70 (HSP70) and epinephrine (EPI), and the intervention of lycium barbarum polysaccharides (LBPs) in rats. Compared to the control (CN) group, the plasma levels of CRH, Cor, HSP70 and EPI were markedly increased, and the level of NPY mRNA was downregulated in the high ambient temperature (HT) group. By contrast, rats in the HT + LBP (HTL) group had: i) a significantly enhanced expression of HSP70 compared to the HT and CN groups; ii) clearly increased plasma levels of CRH, Cor and EPI compared to the CN group; and iii) a markedly upregulated expression of NPY mRNA compared to the HT group. Thus, the results showed that high-temperature environments may damage the body, and LBPs have a potentially protective function by increasing the expression of HSP70 and NPY.

The impact of passive hyperthermia on human attention networks: an fMRI study

An attention network test (ANT) provides a behavioral measure of the efficiency of the three attention networks (alerting, orienting and executive networks) within a single task. In the present study, we investigated the effect of passive hyperthermia on the attention network with event-related functional magnetic resonance imaging (fMRI). The behavioral results showed that passive hyperthermia of 50 °C and 40% relative humidity impaired the executive function, but showed no effect on the alerting and orienting networks. The fMRI results showed that: (i) passive hyperthermia enhanced the activity in the right superior frontal gyrus and depressed the activity in the right middle occipital gyrus, left inferior parietal lobule and left culmen in the alerting network, (ii) passive hyperthermia enhanced the activity in the temporal lobe and depressed the activity in the frontal lobe, parietal lobe and occipital lobe in the orienting network, and (iii) passive hyperthermia enhanced the activity in the dorsolateral prefrontal cortex but did not affect the activity in the anterior cingulate. We concluded that passive hyperthermia impaired executive function, especially the efficiency of resolving conflict and the negative effects of passive hyperthermia on alerting and orienting were overcome through variant regional brain activation.

Pre-shift fluid intake: effect on physiology, work and drinking during emergency wildfire fighting

Wildfire fighters are known to report to work in a hypohydrated state, which may compromise their work performance and health.

PURPOSE

To evaluate whether ingesting a bolus of fluid before the shift had any effect on firefighters' fluid consumption, core temperature, or the time they spent in high heart rate and work activity zones when fighting emergency wildfires.

METHODS

Thirty-two firefighters were divided into non-bolus (AD) and pre-shift drinking bolus (PS, 500 ml water) groups.

RESULTS

Firefighters began work hypohydrated as indicated by urine colour, specific gravity and plasma osmolality (P(osm)) results. Post-shift, firefighters were classified as euhydrated according to P(osm) and hypohydrated by urinary markers. No significant differences existed between the drinking groups in pre- or post-shift hydration status, total fluid intake, activity, heart rate or core temperature.

CONCLUSION

Consuming a bolus of fluid, pre-shift provided no benefit over non-consumption as both groups had consumed equivalent ad libitum volumes of fluid, 2.5 h into the shift. No benefits of bolus consumption were observed in firefighter activity, heart rate response or core temperature response across the shift in the mild weather conditions experienced. Ad libitum drinking was adequate to facilitate rehydration in firefighters upon completion of their emergency firefighting work shift.

Fluid provision and metabolic responses to soccer-specific exercise

The present study aimed to investigate the impact on metabolism of altering the timing and volume of ingested carbohydrate during soccer-specific exercise. Twelve soccer players performed a soccer-specific protocol on three occasions. On two, 7 ml kg(-1) carbohydrate-electrolyte or placebo were ingested at 0 and 45 min. On a third, the same total volume of carbohydrate-electrolyte was consumed but at 0, 15, 30, 45, 60 and 75 min. Carbohydrate-electrolyte ingestion increased blood glucose, insulin and carbohydrate oxidation, whilst suppressing NEFA, glycerol and fat oxidation (P < 0.05) although manipulating the schedule of carbohydrate ingestion elicited similar metabolic responses (P > 0.05). However, consuming fluid in small volumes reduced the sensation of gut fullness (P < 0.05). The results demonstrated that when the total volume of carbohydrate consumed is equal, manipulating the timing and volume of ingestion elicits similar metabolic responses. Furthermore, consuming a small volume of fluid at regular intervals reduces the sensation of gut fullness.

Heat stress, plasma concentrations of adrenaline, noradrenaline, 5-hydroxytryptamine and cortisol, mood state and cognitive performance

The primary aims of this paper were to examine the effect of heat stress on working memory, choice reaction time and mood state, and to investigate the relationship between heat induced changes in plasma concentrations of selected neurotransmitters and hormones, and cognition. Heat stress resulted in a deterioration of performance on a central executive task (random movement generation) but not on verbal and spatial recall, and choice reaction time tasks. Perceptions of vigour decreased and fatigue increased following exposure to heat stress. Plasma concentrations of cortisol and 5-hydroxytryptamine significantly increased following exposure to heat. Regression analyses showed that percent body mass loss and change from baseline (Delta) concentrations of cortisol, post-exposure to heat, were significant predictors of Delta random movement generation and Delta fatigue. A secondary purpose was to examine the effect of recovery on cognition and mood. Following recovery, the performance of the central executive task was poorer than pre-treatment. Mood states, catecholamines and 5-hydroxytryptamine concentrations returned to pre-treatment values, but cortisol fell to a level significantly lower. Regression correlations showed that Delta adrenaline and Delta scores, post-recovery, on the central executive task were significantly correlated. Delta noradrenaline correlated significantly with Delta fatigue. It was concluded that heat stress results in deterioration in the performance of central executive tasks and perceptions of mood state, and that this can be predicted by changes in body mass loss and plasma concentrations of the hormones cortisol and adrenaline.

Water: an essential but overlooked nutrient

Water is an essential nutrient required for life. To be well hydrated, the average sedentary adult man must consume at least 2,900 mL (12 c) fluid per day, and the average sedentary adult woman at least 2,200 mL (9 c) fluid per day, in the form of noncaffeinated, nonalcoholic beverages, soups, and foods. Solid foods contribute approximately 1,000 mL (4 c) water, with an additional 250 mL (1 c) coming from the water of oxidation. The Nationwide Food Consumption Surveys indicate that a portion of the population may be chronically mildly dehydrated. Several factors may increase the likelihood of chronic, mild dehydration, including a poor thirst mechanism, dissatisfaction with the taste of water, common consumption of the natural diuretics caffeine and alcohol, participation in exercise, and environmental conditions. Dehydration of as little as 2% loss of body weight results in impaired physiological and performance responses. New research indicates that fluid consumption in general and water consumption in particular can have an effect on the risk of urinary stone disease; cancers of the breast, colon, and urinary tract; childhood and adolescent obesity; mitral valve prolapse; salivary gland function; and overall health in the elderly. Dietitians should be encouraged to promote and monitor fluid and water intake among all of their clients and patients through education and to help them design a fluid intake plan. The influence of chronic mild dehydration on health and disease merits further research.

Intravenous vs. oral rehydration: effects on subsequent exercise-heat stress

This study compared the influence of intravenous vs. oral rehydration after exercise-induced dehydration during a subsequent 90-min exercise bout. It was hypothesized that cardiovascular, thermoregulatory, and hormonal variables would be the same between intravenous and oral rehydration because of similar restoration of plasma volume (PV) and osmolality (Osmo). Eight non-heat-acclimated men received three experimental treatments (counterbalanced design) immediately after exercise-induced dehydration (33 degrees C) to -4% body weight loss. Treatments were intravenous 0.45% NaCl (iv; 25 ml/kg), no fluid (NF), and oral saline (Oral; 25 ml/kg). After rehydration and rest (2 h total), subjects walked at 50% maximal O2 consumption for up to 90 min at 36 degrees C. The following observations were made: 1) heart rate was higher (P < 0.05) in Oral vs. iv at minutes 45, 60, and 75 of exercise; 2) rectal temperature, sweat rate, percent change in PV, and change in plasma Osmo were similar between iv and Oral; 3) change in plasma norepinephrine decreased less (P < 0.05) in Oral compared with iv at minute 45; 4) changes in plasma adrenocorticotropic hormone and cortisol were similar between iv and Oral after exercise was initiated; and 5) exercise time was similar between iv (77.4 +/- 5.4 min) and Oral (84.2 +/- 2.3 min). These data suggest that after exercise-induced dehydration, iv and Oral were equally effective as rehydration treatments. Thermoregulation, change in adrenocorticotropic hormone, and change in cortisol were not different between iv and Oral after exercise began; this is likely due to similar percent change in PV and change in Osmo.