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Body water losses of >2 % of body mass are defined as hypohydration and can occur from sweat loss and/or diuresis from both cold and altitude exposure. Hypohydration elicits intracellular and extracellular water loss proportionate to water and solute deficits. Iso-osmotic hypovolemia (from cold and high-altitude exposure) results in greater plasma loss for a given water deficit than hypertonic hypovolemia from sweat loss. Hypohydration does not impair submaximal intensity aerobic performance in cold–cool environments, sometimes impairs aerobic performance in temperate environments, and usually impairs aerobic performance in warm–hot environments. Hypohydration begins to impair aerobic performance when skin temperatures exceed 27 °C, and with each additional 1 °C elevation in skin temperature there is a further 1.5 % impairment. Hypohydration has an additive effect on impairing aerobic performance in warm–hot high-altitude environments. A commonality of absolute hypovolemia (from plasma volume loss) combined with relative hypovolemia (from tissue vasodilation) is present when aerobic performance is impaired. The decrement in aerobic exercise performance due to hypohydration is likely due to multiple physiological mechanisms, including cardiovascular strain acting as the ‘lynchpin’, elevated tissue temperatures, and metabolic changes which are all integrated through the CNS to reduce motor drive to skeletal muscles.

To present evidence-based recommendations that promote optimized fluid-maintenance practices for physically active individuals.   Both a lack of adequate fluid replacement (hypohydration) and excessive intake (hyperhydration) can compromise athletic performance and increase health risks. Athletes need access to water to prevent hypohydration during physical activity but must be aware of the risks of overdrinking and hyponatremia. Drinking behavior can be modified by education, accessibility, experience, and palatability. This statement updates practical recommendations regarding fluid-replacement strategies for physically active individuals.   Educate physically active people regarding the benefits of fluid replacement to promote performance and safety and the potential risks of both hypohydration and hyperhydration on health and physical performance. Quantify sweat rates for physically active individuals during exercise in various environments. Work with individuals to develop fluid-replacement practices that promote sufficient but not excessive hydration before, during, and after physical activity.

Employing a methodology reported in a recent theoretical study on male astronauts, this study estimated the effects of body size and aerobic countermeasure (CM) exercise in a four-person, all-female crew composed of individuals drawn from a stature range (1.50- to 1.90-m) representative of current space agency requirements (which exist for stature, but not for body mass) upon total energy expenditure (TEE), oxygen (O 2 ) consumption, carbon dioxide (CO 2 ) and metabolic heat (H prod ) production, and water requirements for hydration, during space exploration missions. Assuming geometric similarity across the stature range, estimates were derived using available female astronaut data (mean age: 40-years; BMI: 22.7-kg·m −2 ; resting VO 2 and VO 2max : 3.3- and 40.5-mL·kg −1 ·min −1 ) on 30- and 1080-day missions, without and with, ISS-like countermeasure exercise (modelled as 2 × 30-min aerobic exercise at 75% VO 2max , 6-day·week −1 ). Where spaceflight-specific data/equations were not available, terrestrial equivalents were used. Body size alone increased 24-h TEE (+ 30%), O 2 consumption (+ 60%), CO 2 (+ 60%) and H prod (+ 60%) production, and water requirements (+ 17%). With CM exercise, the increases were + 25–31%, + 29%, + 32%, + 38% and + 17–25% across the stature range. Compared to the previous study of theoretical male astronauts, the effect of body size on TEE was markedly less in females, and, at equivalent statures, all parameter estimates were lower for females, with relative differences ranging from -5% to -29%. When compared at the 50th percentile for stature for US females and males, these differences increased to − 11% to − 41% and translated to larger reductions in TEE, O 2 and water requirements, and less CO 2 and H prod during 1080-day missions using CM exercise. Differences between female and male theoretical astronauts result from lower resting and exercising O 2 requirements (based on available astronaut data) of female astronauts, who are lighter than male astronauts at equivalent statures and have lower relative VO 2max values. These data, combined with the current move towards smaller diameter space habitat modules, point to a number of potential advantages of all-female crews during future human space exploration missions.

Background How hypohydration impacts non-bodyweight (BW)-dependent muscle performance and vertical jumping ability remains to be determined using meta-analytic procedures. Objectives Our objective was to determine the impact of hypohydration on muscle endurance, strength, anaerobic power and capacity and vertical jumping ability using a meta-analytic approach. Data sources Studies were located using database searches and cross-referencing. Synthesis methods Effect summaries were obtained using random-effects models; method of moments mixed-effects analysis-of-variance-like procedures were used to determine differences between groups; and restricted maximum likelihood random-effects meta-regressions were performed to determine relationships between variables, impact of confounders, and interaction effects. Results A total of 28 manuscripts met the inclusion criteria, producing six (upper body muscle endurance), ten (lower body muscle endurance), 14 (upper body muscle strength), 25 (lower body muscle strength), nine (muscle anaerobic power), nine (muscle anaerobic capacity), and 12 (vertical jumping ability) effect estimates. Hypohydration impaired overall muscle endurance by 8.3 ± 2.3 % ( P  < 0.05), with no significant difference between upper body (−8.4 ± 3.3 %) and lower body (−8.2 ± 3.2 %). As a whole, muscle strength fell by 5.5 ± 1.0 % ( P  < 0.05) with hypohydration; the difference between lower (−3.7 ± 1.8 %) and upper (−6.2 ± 1.1 %) body was non-significant. Anaerobic power (−5.8 ± 2.3 %) was significantly altered with hypohydration, but anaerobic capacity (−3.5 ± 2.3 %) and vertical jumping ability (0.9 ± 0.7 %) were not. No significant correlations were observed between the changes in any of the muscle performance variables or vertical jumping ability and the changes in hypohydration level. Using an active procedure to dehydrate participants decreased muscle performance by an additional 5.4 ± 1.9 % (2.76-fold) ( P  = 0.02) compared with using a passive dehydration procedure. Trained individuals demonstrated a 3.3 ± 1.7 % (1.76-fold) ( P  = 0.06) lesser decrease in muscle performance with hypohydration than did untrained individuals. Conclusion Hypohydration, or factors associated with dehydration, are likely to be associated with practically important decrements in muscle endurance, strength, and anaerobic power and capacity. However, their impact on non-BW-dependent muscle performance is substantially mitigated in trained individuals or when hypohydration is induced passively. Conversely, it is possible that body water loss (~3 % BW) may improve performance in BW-dependent tasks such as vertical jumping ability.

Precipitating factors that contribute to the severity of exertional heat stroke (EHS) are unclear. The purpose of this study was to determine the effect of prior illness (PI) on EHS severity. We performed a retrospective clinical record review of 179 documented cases of EHS at the Marine Corps Base in Quantico, Virginia. Approximately 30% of EHS cases had a medically documented PI. Anthropometrics (height, weight, body mass index) and commonly associated risk factors for EHS (age, number of days in training, wet bulb globe temperature, sleep patterns) did not differ between PI and no illness (NI) groups. PI patients presented with higher maximal rectal core temperatures (40.6 ± 1.0°C vs. 40.3 ± 1.2°C; P = 0.0419), and elevated pulse rates (118.1 ± 16.7 bpm vs. 110.5 ± 24.2 bpm; P = 0.0397). At the point of care, biomarker values were similar between PI and NI groups, with the exception of a trend toward elevated monocytes in those with PI (7.9 ± 2.9% vs 6.7± 2.7%; P = 0.0521). Rate and duration of cooling were similar between PI and NI patients. This study indicates that PI has a minimal effect on the patient presentation, severity and treatment outcome of EHS. The results of this study have important implications for military, civilian, and occupational populations who are at risk for EHS.

Background Neuroendocrine tumors, although relatively rare in incidence, are now the second most prevalent gastrointestinal neoplasm owing to indolent disease biology. A small but significant sub-group of neuroendocrine tumor patients suffer from diarrhea. This is usually secondary to carcinoid syndrome but can also be a result of short gut syndrome, bile acid excess or iatrogenic etiologies. Recently, an amino acid based oral rehydration solution (enterade® Advanced Oncology Formula) was found to have anti-diarrheal properties in preclinical models. Methods A retrospective chart review of all NET patients treated with enterade® AO was performed after IRB approval. Results Ninety-eight NET patients who had received enterade® AO at our clinic from May 2017 through June 2019 were included. Patients ( N  = 49 of 98) with follow up data on bowel movements (BMs) were included for final analysis. Eighty-four percent of patients (41/49) had fewer BMs after taking enterade® AO and 66% (27/41) reported more than 50% reduction in BM frequency. The mean number of daily BMs was 6.6 (range, 3–20) at baseline before initiation of therapy, while the mean number of BMs at 1 week time point post enterade® AO was 2.9 (range, 0–11). Conclusions Our retrospective observations are encouraging and support prospective validation with appropriate controls in NET patients. This is first published report of the potential anti-diarrheal activity of enterade® AO in NET patients.

This study tested the accuracy of a novel, limited-availability web application (H2QTM) for predicting sweat rates in a variety of sports using estimates of energy expenditure and air temperature only. The application of predictions for group water planning was investigated for soccer match play. Fourteen open literature studies were identified where group sweat rates were reported (n = 20 group means comprising 230 individual observations from 179 athletes) with fidelity. Sports represented included: walking, cycling, swimming, and soccer match play. The accuracy of H2QTM sweat rates was tested by comparing to measured group sweat rates using the concordance correlation coefficient (CCC) with 95% confidence interval [CI]. The relative absolute error (RAE) with 95% [CI] was also assessed, whereby the mean absolute error was expressed relative to an acceptance limit of 0.250 L/h. The CCC was 0.98 [0.95, 0.99] and the RAE was 0.449 [0.279, 0.620], indicating that the prediction error was on average 0.112 L/h. The RAE was < 1.0 for 19/20 observations (95%). Drink volumes modeled as a proxy for sweat losses during soccer match play prevented dehydration (< 1% loss of body mass). The H2QTM web application demonstrated high group sweat prediction accuracy for the variety of sports activities tested. Water planning for soccer match play suggests the feasibility of easily and accurately predicting sweat rates to plan group water needs and promote optimal hydration in training and/or competition.

This study determined whether a torso-vest forced ambient air body ventilation system (BVS) reduced physiological strain during exercise-heat stress. Seven heat-acclimated volunteers attempted nine, 2-h treadmill walks at 200 W m −2 in three environments, −40°C, 20% rh (HD), 35°C, 75% rh (HW), and 30°C, 50% rh, (WW) wearing the Army Combat Uniform, interceptor body armor (IBA) and Kevlar helmet. Three trials in each environment were BVS turned on (BVS On ), BVS turned off (BVS Off ), and no BVS (IBA). In HD, BVS On significantly lowered core temperature ( T re ), heart rate (HR), mean skin temperature ( T sk ), mean torso skin temperature ( T torso ), thermal sensation (TS), heat storage ( S ), and physiological strain index (PSI), versus BVS Off and IBA ( P  < 0.05). For HW ( n  = 6), analyses were possible only through 60 min. Exercise tolerance time (min) during HW was significantly longer for BVS On (116 ± 10 min) versus BVS Off (95 ± 22 min) and IBA (96 ± 18 min) ( P  < 0.05). During HW, BVS On lowered HR at 60 min versus IBA, T sk from 30 to 60 min versus BVS Off and IBA, and PSI from 45 to 60 min versus BVS Off and at 60 min versus IBA ( P  < 0.05). BVS On changes in T re and HR were lower in HD and HW. During WW, BVS On significantly lowered HR, T sk , and T torso versus BVS Off and IBA ( P  < 0.05) during late exercise. Sweating rates were significantly lower for BVS On versus BVS Off and IBA in both HD and WW ( P  < 0.05), but not HW. These results indicate that BVS On reduces physiological strain in all three environments by a similar amount; however, in hot-dry conditions the BVS Off increases physiological strain.

It is well appreciated that a loss of body water (dehydration) can impair endurance performance and that the effect is magnified by environmental heat stress. A majority of professional sports medicine and nutrition organizations recommend drinking during exercise to replace sweat losses and prevent dehydration, while also avoiding frank over-hydration. Knowledge of sweating rate, which is highest in the heat for any given metabolic rate, is therefore considered key to developing a sound drinking strategy. Exercise duration and the provision of liquid fuel interacts with required drink volumes in important ways that are infrequently discussed but are of utmost practical concern. This review details some challenges related to the optimized coupling of fluid and fuel needs during prolonged exercise in the heat and the need for personalization.

The primary aim was to explore the impact of exertional-heat stress (EHS) promoted exercise-associated bacteraemia. A secondary aim was to examine if an amino acid beverage (AAB) intervention may mitigate exercise-associated bacteraemia. Counterbalanced randomised control trial. Twenty endurance trained male participants completed two randomised EHS trials. On one occasion, participants consumed a 237 mL AAB twice daily for 7 days prior, immediately before and every 20 min during EHS (2 h running at 60 % V̇O2max in 35 °C). On the other occasion, a water volume control (CON) equivalent was consumed. Whole blood samples were collected pre- and immediately post-EHS, and were analysed for plasma DNA concentration by fluorometer quantification after microbial extraction, and bacterial relative abundance by next generation 16s rRNA gene sequencing. Increased concentration of microbial DNA in plasma pre- to post-EHS was observed on CON (pre-EHS 0.014 ng/μL, post-EHS 0.039 ng/μL) (p < 0.001) and AAB (pre-EHS 0.015 ng/μL, post-EHS 0.031 ng/μL) (p < 0.001). The magnitude of change from pre- to post-exercise on AAB was 40 % lower, but no significant difference was observed versus CON (p = 0.455). Predominant bacterial groups identified included: phyla-Proteobacteria (88.0 %), family-Burkholderiaceae (59.1 %), and genus-Curvibacter (58.6 %). No significant variation in absolute and relative change in α-diversity and relative abundance for phyla, family, and genus bacterial groups was observed in AAB versus CON. The increased presence of microbial-bacterial DNA in systemic circulation in response to EHS appears positive in all participants. An amino acid beverage supplementation period prior to and consumption during EHS did not provide significant attenuation of EHS-associated bacteraemia.