Article

# Hydration Status in Active Youth

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## Abstract

Fluid balance is crucial for maintaining health. It is well documented that dehydration increases physiologic strain and decreases athletic performance, especially in hot environments. Although there are numerous studies evaluating hydration status in adults, limited data concerning hydration levels in athletic youth exist. Nevertheless, most of these studies clearly indicate that (a) dehydration is a major and common problem within children exercising in the heat; and (b) children do not have the capacity to translate hydration awareness to successful hydration strategies. Further research is needed, and constant efforts must be made toward the development of more efficient hydration strategies in order to educate young people about the benefits of optimal hydration status.

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... Previous studies have shown that children demonstrate a similar pattern of insufficient drinking and dehydration as adults when they participate in physical activities or exercise in a hot environment (5,6). Moreover, children may be in a state of chronic hypohydration when fluids are not replenished (3,14,33). Therefore, it has been suggested recently that children cannot translate hydration awareness into successful hydration strategies (14). ...
... Moreover, children may be in a state of chronic hypohydration when fluids are not replenished (3,14,33). Therefore, it has been suggested recently that children cannot translate hydration awareness into successful hydration strategies (14). ...
... To our knowledge, the current study is the first to investigate the effect of flavors on the ad libitum fluid intake in Chinese children when they exercise in a hot and humid environment. As children usually do not have successful hydration strategies (14), it is very important to encourage children to drink enough fluids when exercising in a hot environment. The perceived taste of the beverage is important for fluid replacement during exercise (35). ...
Article
The purpose of the current study was to examine the effect of flavor on voluntary drinking and thermoregulatory responses in Chinese boys and girls exercising intermittently in a hot environment. Fourteen boys and girls (9 to 11 years old) performed four 3-hour intermittent exercise sessions (20-min walking sessions at 50% VO2peak followed by a 25-minute rest period) in a hot and humid environment (~30 °C ambient temperature and ~70% relative humidity). The participants consumed 1 of 4 beverages ad libitum in a randomized sequence by using a Latin-square principle: unflavored water (W), orange-flavored water (OF), lemon-flavored water (LF), and grape-flavored water (GF). No differences were observed in the total fluid intake (W vs. OF vs. LF vs. GF: Boys, 441 ± 114 vs. 493 ± 106 vs. 387 ± 83 vs. 568 ± 146 ml; Girls, 613 ± 131 vs. 923 ± 204 vs. 825 ± 157 vs. 790 ± 166 ml), urine and sweat output, and physiological perceptual variables among trials and between sexes. The results suggested that Chinese children can maintain body fluid balance while exercising moderately in a hot and humid environment by ad libitum drinking. The flavor of the beverages had no impact on the voluntary drinking and the state of hydration in the current study.
Conference Paper
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Article
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Article
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We aimed to evaluate whether an intervention program emphasizing in increased fluid intake can improve exercise performance in children exercising in the heat. Ninety-two young athletes participated in the study (age: 13.8 ± 0.4 years, weight: 54.9 ± 1.5 kg). Thirty-one (boys: 13, girls: 18) children served as the control group (CON) and 61 (boys: 30, girls: 31) as the intervention (INT). Volunteers had free access to fluids. Hydration was assessed on the basis of first morning urine. A series of field tests were used to evaluate exercise performance. All tests occurred outdoors in the morning (mean ambient temperature=28 °C). After baseline testing, INT attended a lecture on hydration, and urine color charts were mounted in all bathrooms. Additionally, water accessibility was facilitated in training, dining and resting areas. Hydration status was improved significantly in the INT [USG: pre=1.031 ± 0.09, post=1.023 ± 0.012, P<0.05; urine osmolality (mOsm/kg water): pre=941 ± 30, post=782 ± 34, P<0.05], while no statistically significant changes were found in the CON [USG: pre=1.033 ± 0.011, post=1.032 ± 0.013, P>0.05; urine osmolality (mOsm/kg water) 970 ± 38 vs 961 ± 38, P>0.05]. Performance in an endurance run was improved significantly only in INT (time for 600 m: pre=189 ± 5 s, post=167 ± 4 s, P<0.05). Improving hydration status by ad libitum consumption of water can enhance performance in young children exercising in the heat.
Article
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The purpose of this study was to examine the extent to which lighter runners might be more advantaged than larger, heavier runners during prolonged running in warm humid conditions. Sixteen highly trained runners with a range of body masses (55-90 kg) ran on a motorised treadmill on three separate occasions at 15, 25 or 35C, 60% relative humidity and 15 kmh-1 wind speed. The protocol consisted of a 30-min run at 70% peak treadmill running speed (sub-max) followed by a self-paced 8-km performance run. At the end of the sub-max and 8-km run, rectal temperature was higher at 35C (39.5&#450.4C, P<0.05) compared with 15C (38.6&#450.4C) and 25C (39.1&#450.4C) conditions. Time to complete the 8-km run at 35C was 30.4&#452.9 min (P<0.05) compared with 27.0&#451.5 min at 15C and 27.4&#451.5 min at 25C. Heat storage determined from rectal and mean skin temperatures was positively correlated with body mass (r=0.74, P<0.0008) at 35C but only moderately correlated at 25C (r=0.50, P<0.04), whereas no correlation was evident at 15C. Potential evaporation estimated from sweat rates was positively associated with body mass (r=0.71, P<0.002) at 35C. In addition, the decreased rate of heat production and mean running speed during the 8-km performance run were significantly correlated with body mass (r=-0.61, P<0.02 and r=-0.77, P<0.0004, respectively). It is concluded that, compared to heavier runners, those with a lower body mass have a distinct thermal advantage when running in conditions in which heat-dissipation mechanisms are at their limit. Lighter runners produce and store less heat at the same running speed; hence they can run faster or further before reaching a limiting rectal temperature.
Article
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Authors of most field studies have not observed decrements in physiologic function and performance with increases in dehydration, although authors of well-controlled laboratory studies have consistently reported this relationship. Investigators in these field studies did not control exercise intensity, a known modulator of body core temperature. To directly examine the effect of moderate water deficit on the physiologic responses to various exercise intensities in a warm outdoor setting. Semirandomized, crossover design. Field setting. Patients or Other Seventeen distance runners (9 men, 8 women; age = 27 +/- 7 years, height = 171 +/- 9 cm, mass = 64.2 +/- 9.0 kg, body fat = 14.6% +/- 5.5%). Participants completed four 12-km runs (consisting of three 4-km loops) in the heat (average wet bulb globe temperature = 26.5 degrees C): (1) a hydrated, race trial (HYR), (2) a dehydrated, race trial (DYR), (3) a hydrated, submaximal trial (HYS), and (4) a dehydrated, submaximal trial (DYS). Main Outcome Measure(s): For DYR and DYS trials, dehydration was measured by body mass loss. In the submaximal trials, participants ran at a moderate pace that was matched by having them speed up or slow down based on pace feedback provided by researchers. Intestinal temperature was recorded using ingestible thermistors, and participants wore heart rate monitors to measure heart rate. Body mass loss in relation to a 3-day baseline was greater for the DYR (-4.30% +/- 1.25%) and DYS trials (-4.59% +/- 1.32%) than for the HYR (-2.05% +/- 1.09%) and HYS (-2.0% +/- 1.24%) trials postrun (P < .001). Participants ran faster for the HYR (53.15 +/- 6.05 minutes) than for the DYR (55.7 +/- 7.45 minutes; P < .01), but speed was similar for HYS (59.57 +/- 5.31 minutes) and DYS (59.44 +/- 5.44 minutes; P > .05). Intestinal temperature immediately postrun was greater for DYR than for HYR (P < .05), the only significant difference. Intestinal temperature was greater for DYS than for HYS postloop 2, postrun, and at 10 and 20 minutes postrun (all: P < .001). Intestinal temperature and heart rate were 0.22 degrees C and 6 beats/min higher, respectively, for every additional 1% body mass loss during the DYS trial compared with the HYS trial. A small decrement in hydration status impaired physiologic function and performance while trail running in the heat.
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To assess the hydration status and level of hydration knowledge of youths at summer sports camps. Sixty-seven active youths, 57 males (mean +/- SD, 12 +/- 2 y, 136 +/- 16 cm, 50.6 +/- 21.1 kg) and 10 females (13 +/- 2 y, 153 +/- 8 cm, 45.2 +/- 9.0 kg) participated in 4 d of sports camp. Hydration status was assessed before the first practice (AM) and after the second practice (PM). Participants completed surveys assessing hydration knowledge (HAQ) and hydration habits on day 3 and a self-assessment (EQ#1). Mean AM urine specific gravity (USG) and urine osmolality (Uosm) scores ranged from minimal to significant dehydration across 4 d, even when temperatures were mild. Correlations between hydration indices and EQ#1, ranging from 0.11 to -0.51, were statistically significant (P < .05), indicating that subjects recognized when they were doing a good or bad job hydrating. HAQ did not correlate strongly with hydration indices suggesting other impediments to hydration. Thirst correlated negatively with EQ#1 (from -0.29 to -0.60). Hydration at summer sports camp is a concern and special efforts need to be made to help youths develop hydration strategies.
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This study was performed to determine whether a) children voluntarily dehydrate while exercising in hot climate; b) such dehydration affects their well-being and thermoregulation. Eleven 10p to 12-yr old, partially acclimatized boys underwent two work-in-the-heat protocols (cycle rides, 45% aerobic capacity at 39 degrees C, 45% rh). During one session they drank only voluntarily when thirsty (VD). In the other, drinking was forced (FD) to replenish fluid losses. VD induced a progressively increasing fluid loss (0.3% of body wt.h-1) due to insufficient drinking (72% of intake in FD). URinary output was lower (55.7 vs. 81.6 ml.h-1) and its osmolality higher (880 vs. 523 meq.1-1) than during FD. Sweat rate, rectal (Tre) and mean skin (T-sk) temperatures, heart rate, rate of perceived exertion, sweat gland counts, blood hemoglobin, hematocrit (Hct), serum electrolytes, and total proteins did not differ between sessions. However, the rise of Tre, Hct, and proteins positively correlated with hypohydration level. It is concluded that exercising children progressively dehydrate when not forced to drink. At equal levels of % weight loss they have greater Tre rise than do lean adults.
Article
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Twelve 9- to 12-year-old children performed four exercise-in-the-heat trials (35 degrees C, 45% RH), which differed in the fluids consumed. In each trial, subjects were kept euhydrated while cycling one 20-min and two 15-min bouts at 50% peak VO2 followed by a 90% peak VO2 bout until exhaustion. Thereafter, they could drink ab libitum while resting. One drink was water, and the other three drinks had 6% CHO with different Na+: 0, 8.8, and 18.5 mEq.L-1. All drinks had the same grape flavor. The perceived thirst was similar among trials and it did not increase while subjects were exercising. On average, subjects felt their stomach "somewhat full" with no difference among drinks. Thermal sensations, RPE, and overall comfort were similar among trials. During a 30-min recovery, volume intake was similar among drinks (201 +/- 27 ml). In conclusion, the drink composition did not affect perceptual responses to drinking while euhydrated children exercised in the heat, nor did it affect drinking behavior during recovery.
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Dehydration alters cardiovascular, thermoregulatory, central nervous system, and metabolic functions. One or more of these alterations will degrade endurance exercise performance when dehydration exceeds 2% of body weight. These performance decrements are accentuated by heat stress. To minimize the adverse consequences of body water deficits on endurance exercise performance, it is recommended that fluid intake be sufficient to minimize dehydration to less than 2% of body weight loss. This can usually be achieved with fluid intakes of under 1 L x h(-1).
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This study investigated the relationship between runners' perceptions of fluid needs and drinking behavior under conditions of compensable heat stress (ambient temperature = 20.5 +/- 0.7 degrees C, 68.9 degrees F; relative humidity = 76.6%). Eighteen experienced runners (15 men, 40.5 +/- 2.5 y, and 3 women, 42 +/- 2.3 y) were given ad libitum access to a sports drink (6% carbohydrate-electrolyte solution) at Miles 2, 4, 6, and 8. After the run (75.5 +/- 8.0 min), subjects completed questionnaires that required them to estimate their individual fluid intake and sweat loss. Dehydration averaged 1.9% +/- 0.8% of initial body weight (a mean sweat loss of 21.6 +/- 5.1 mL.kg-1.h-1). Subjects replaced only 30.5% +/- 18.1% of sweat loss and underestimated their sweat loss by 42.5% +/- 36.6% (P <or= 0.001). Subjects' self-estimations of fluid intake (5.2 +/- 3.2 mL.kg-1.h-1) were not significantly different from actual fluid intake (6.1 +/- 3.4 mL.kg-1.h-1) and were significantly correlated (r = 0.63, P = 0.005). The data indicate that even under favorable conditions, experienced runners voluntarily dehydrate (P <or= 0.001), possibly because they are unable to accurately estimate sweat loss and consequently cannot subjectively judge how much fluid to ingest to prevent dehydration. This conclusion suggests that runners should not depend on self-assessment to maintain adequate hydration, underscores the need for runners to enhance their ability to self-assess sweat losses, and suggests that a predetermined regimen of fluid ingestion might be necessary if they wish to maintain more optimal hydration.
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The intent of this study was to assess the influence of drink flavor and composition on voluntary drinking and hydration status in girls exercising intermittently in the heat (35 ± 1°C, 45–50% relative humidity). Twelve physically active, nonacclimatized girls (9–12 years) performed three 3 h identical sessions, each consisting of four 20 min cycling bouts at 50% $$(\ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{{{\text{2}}\max }} ),$$ separated by 25 min of rest. One of the three beverages (chilled to 8–10°C) was assigned to each session: unflavored water (W), grape-flavored water (FW) and grape-flavored water plus 6% carbohydrate and 18 mmol l−1 NaCl (CNa). Drinking was ad libitum. Body weight (BW), drink intake (DI), heart rate, rectal and skin temperatures, and perceptions of thirst and stomach fullness were monitored periodically. Total DI was 759, 940, 1,045 g in W, FW and CNa, respectively (P < 0.05 for CNa–W and FW–W). BW changes were −0.15, 0.16, and 0.45% in W, FW and CNa, respectively, but only the difference between CNa and W was significant (P < 0.05). Other physiological and perceptual variables were not different between trials. In conclusion, beverage flavoring regardless if its combination with carbohydrate and NaCl, mildly enhanced voluntary drinking in young non-acclimatized girls. In contrast to previous research in young boys, euhydration was maintained in the girls by an adequate intake of unflavored water.
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This study examined the effects of beverage composition on the voluntary drinking pattern, body fluid balance and body temperature responses of heat-acclimatized trained girls exercising intermittently in outdoor conditions (WBGT = 30.9 +/- 0.2 degrees C). Twelve trained, heat-acclimatized girls (age = 10.6 +/- 0.2 years) performed three 3-h sessions, each consisting of four 20-min cycling bouts at 60% VO2max, alternating with 25-min rest. One of three beverages was assigned: unflavored water (W), flavored water (FW) or flavored water plus 6% carbohydrate and 18 mmol/l NaCl (CNa). Drinking was ad libitum. Total intake was similar among conditions (W = 953.3 +/- 107.8 ; FW = 1026.5 +/- 138.1; CNa = 906.4 +/- 107.5 g). A mild hypohydration occurred during the three conditions (W = -1.12%; FW = -0.95%; CNa = -0.74% BW, P > 0.05). Sweat loss, higher than previously reported for sedentary girls, was not different among conditions (W = 1,051.5 +/- 90.8; FW = 979.9 +/- 72.8; CNa = 1,052.7 +/- 52.6 g). The average amount of urine produced (W = 269.8 +/- 85.9; FW = 320.8 +/- 87.2; CNa = 85.6 +/- 9.3 g) was 73 and 68% lower [corrected] during CNa compared to [corrected] FW and W, respectively, [corrected] (CNa vs. FW, P < 0.05), CNa vs W, P = 0.06) [corrected] The increase in rectal temperature, heart rate and all perceptual variables did not differ among conditions. In conclusion, flavoring of the water and addition of 6% carbohydrate plus 18 mmol/l NaCl do not prevent mild hypohydration in trained, heat-acclimatized girls with high sweating rates. However, there is a tendency towards a greater fluid retention with the CNa beverage.
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Results of new research indicate that, contrary to previous thinking, youth do not have less effective thermoregulatory ability, insufficient cardiovascular capacity, or lower physical exertion tolerance compared with adults during exercise in the heat when adequate hydration is maintained. Accordingly, besides poor hydration status, the primary determinants of reduced performance and exertional heat-illness risk in youth during sports and other physical activities in a hot environment include undue physical exertion, insufficient recovery between repeated exercise bouts or closely scheduled same-day training sessions or rounds of sports competition, and inappropriately wearing clothing, uniforms, and protective equipment that play a role in excessive heat retention. Because these known contributing risk factors are modifiable, exertional heat illness is usually preventable. With appropriate preparation, modifications, and monitoring, most healthy children and adolescents can safely participate in outdoor sports and other physical activities through a wide range of challenging warm to hot climatic conditions.
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Thermoregulatory responses to exercise differ in prepubertal athletes compared with their adult counterparts. It is important, therefore, to consider fluid requirements specific to this age group to prevent risks of dehydration and diminished sports performance. Relative to their body size, children demonstrate lower sweat water losses during exercise than adults. Nonetheless, percentage levels of incurred dehydration are similar in pre- and postpubertal athletes. Moreover, voluntary (ad libitum) drinking volumes in children in respect to their body size are comparable or greater than those of adults. Given an adequate opportunity to drink during exercise, volume intake driven by thirst should be expected to prevent significant levels of dehydration in child athletes. The amount can be calculated conservatively as an hourly fluid intake of 13 mL/kg (6 mL/lb) bodyweight. Equally important is post-exercise fluid replenishment (approximately 4 mL/kg [2 mL/lb] for each hour of exercise) to avoid initiating subsequent exercise bouts in a dehydrated state. Choice of fluid should be dictated by taste preference, since volume of intake, rather than fluid content, is the most critical issue in child athletes. Since children may lack motivation for proper fluid intake behaviours, the responsibility falls to coaches and parents to assure that young athletes receive appropriate hydration during and after exercise bouts.
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The purpose of this study was to determine the influence of hydration status on pacing of trail runners in the heat (wet bulb globe temperature = 26.2 +/- 1.8 degrees C). A randomized, crossover design was used and the participation occurred within a 2-week period. Seventeen competitive, well-trained distance runners (9 men, 8 women, age 27 +/- 7 years, height 171 +/- 9 cm, weight 64.2 +/- 9.0 kg, body fat 14.6 +/- 5.5%) completed the study. Subjects started maximum effort trials that were either hydrated (HYR) and dehydrated (DHR). Each trial subjects ran three 4-km loops with a 4-minute rest between loops. Significance was set at p < or = 0.05. The DHR had a significantly greater body mass loss at the pre- and posttrial time points (-2.05 +/- 1.25 and -4.3 +/- 1.25%, respectively) vs. HYR (-0.79 +/- 0.95 and -2.05 +/- 1.09%, respectively). Subjects ran the 12 km faster (p < 0.001) in HYR (3,191 +/- 366 seconds) vs. DHR (3,339 +/- 450 seconds). Differences between fastest and slowest loops during HYR (54 +/- 40 seconds) were significantly smaller than DHR (111 +/- 93 seconds; p = 0.041). Additionally, loop times were slower for loop 1 (HYR 1,039 +/- 116 seconds vs. DHR 1,071 +/- 123 seconds; p = 0.028), loop 2 (HYR 1,066 +/- 123 seconds vs. DHR 1,105 +/- 148 seconds; p = 0.01) and loop 3 (HYR 1,081 +/- 132 seconds vs. DHR 1,168 +/- 189 seconds; p = 0.003) when dehydrated. Percent of the race completed by loop as calculated by finishing time was significantly different at loop 2 between HYR (33.6 +/- 0.36%) and DHR (33.1 +/- 0.35%, p = 0.002) and loop 3 (33.8 +/- 0.75% vs. 34.9 +/- 1.35%, respectively, p = 0.01). Total variation from the mean pace for the duration of the HYR compared to the DHR approached significance (p = 0.064). Average percent of variance approached significance between trials (p = 0.057). Differences between the fastest and slowest loops between trials demonstrated an increased ability for hydrated individuals to evenly pace themselves. While total variation from the mean pace was not significantly different, it could have practical applicability. These findings reveal that dehydration is associated with decreases in a runners' ability to evenly pace themselves during a competitive situation.
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Sweat evaporation can be a key thermoregulatory mechanism and it causes a loss of water from all compartments of the body. Hypohydration can also develop with restricted fluid intake or with intake of diuretics. Hypohydration can affect physical and/or mental performance and/or have implications for dietary recommendations. A variety of different types and modes of exercise performance can be influenced by hydration state. Reviews of the published literature are currently most conclusive for endurance exercise. Dehydration equivalent to 2% body mass loss during exercise in a hot environment (31-32 degrees C) impairs endurance performance, but when the exercise is performed in a temperate environment (20-21 degrees C) a 2% body mass loss appears to have a lesser and inconsequential effect. In cold environments a body mass loss >2% may be tolerable for endurance exercise. There is a less conclusive picture as to the effects of hypohydration on other types of physical performance, including strength and power activities, team sports and the skills component of many sports, and for mental performance. A number of physiological mechanisms are responsible for the effects observed. Fluid consumption can be used to attenuate the development of a water deficit or to correct it. The composition and temperature of a drink and the volume and rate of its consumption can all influence the physiological responses to ingestion and can impact on exercise performance.
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Humans may lose large amounts of water and electrolytes from sweat during prolonged exercise in a hot climate. Gender and maturational differences for the total sweat electrolyte losses have not been reported. The purpose of this study was to compare sweat electrolyte losses of prepubescent (PP), pubescent (P) and young adult (YA) males and females, under the same environmental conditions and relative exercise intensities. Twenty-five females (8 PP, 9 P, 8 YA) and 26 males (10 PP, 8 P, 8 YA) cycled for two 20-min bouts at 50% of their peak VO2 in a climatic chamber (42 degrees C, 18% relative humidity). Sweat was collected from a plastic bag attached to the lower back. Total body sweat loss was calculated from the differences in nude body weight corrected for fluid intake, urine, and respiratory water loss. Sweat [Na+] and [Cl-] tended to increase with maturation while sweat [K+] was lower in YA compared with that of PP. Children had a lower sweating rate than YA, even when corrected for body surface area. As a result, total Na+ and Cl- losses per kg body weight from sweat (mEq.kg-1.h-1) were higher in YA compared with those of PP and P; however, no maturational difference was found in K+ losses. Within the same maturational group, there were no gender differences in any of the electrolyte losses. These results may be useful in recommending "optimal" fluid-electrolyte drinks for children exercising in the heat.
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Sweating rate (SR) of boys is lower than that of men. To assess the association between the response of individual sweat glands and physical growth and maturation, three groups of circumpubertal boys cycled at 50% VO2max in a climatic chamber (42 degrees C, 20% relative humidity). Based on Tanner staging (pubic hair), 16 were classified as prepubertal (PP, Tanner 1), 15 as midpubertal (MP, Tanner 2-4), and five as late-pubertal (LP, Tanner 5). Population density (PD) of the heat-activated sweat glands, the mean area of sweat drops (DA), and the proportion of skin covered by sweat (%A) were measured by skin photography and computer-assisted imaging analysis. Other measurements included rectal and skin temperatures (Tre, Tsk, respectively), heart rate (HR), and total body SR. The rise in HR, Tre and Tsk did not differ among groups. Whole body SR was significantly higher in the LP group compared with PP (PP = 4.95 +/- 0.23, MP = 5.79 +/- 0.20, LP = 6.70 +/- 0.42 ml.min-1.m-2) (mean +/- SEM). PD was significantly higher in the PP group (PP = 128 +/- 8, MP = 97 +/- 9, LP = 74 +/- 9 glands.cm-2), while DA was higher in the LP group (PP = 5.47 +/- 0.59, MP = 6.92 +/- 0.47, LP = 12.83 +/- 1.41 microns2.10(4)). %A did not differ among groups. The calculated SR per gland was higher among the LP groups compared with the less mature ones (PP = 4.6 +/- 0.3, MP = 7.2 +/- 0.8, LP = 9.6 +/- 1.0 nl.min-1).(ABSTRACT TRUNCATED AT 250 WORDS)
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During exercise in a hot climate, children have been reported to have a less effective temperature regulation capability, compared with adults. It is likely that the transition from a child-like to an adult-like response occurs during puberty. To assess the association between the thermoregulatory response and physical maturation, three groups of circum-pubertal boys cycled at 50% VO2max (three 20-min bouts with 10-min rests), in a climatic chamber (42 degrees C, 20% relative humidity). Based on Tanner staging (pubic hair), 10 were classified as prepubertal (PP), 13 as midpubertal (MP), and eight as late pubertal (LP). Measurements included rectal and skin temperatures (Tre, Tsk), heart rate (HR), sweating rate (SR), oxygen consumption (VO2), and forearm blood flow (FBF). Tre, Tsk, and HR increased with time, with no significant difference among groups. Relative VO2 (ml O2.kg-1) was similar among groups. FBF was consistently higher in PP compared with LP. In spite of the higher SR (PP = 4.9 +/- 0.2, MP = 5.7 +/- 0.3, LP = 6.6 +/- 0.4 ml.min-1.m-2) (mean +/- SEM) among LP compared with PP, the rate of heat storage (PP = 5.5 +/- 0.4, MP = 5.3 +/- 0.4, LP = 6.8 +/- 0.3, kJ.h-1.kg-1) was also significantly higher among those in the LP group. Three of eight LP did not complete the session due to high Tre, while two of the 10 PP were unable to complete the session even though the physiologic heat strain was not high. The results suggest that the transition from a child-like to an adult-like thermoregulatory effectiveness in a hot, dry climate may occur at a somewhat later stage, but not during puberty.
Article
The phenomenon of involuntary dehydration, the delay in full restoration of a body water deficit by drinking, has been described extensively but relatively little is known about its physiological mechanism. It occurs primarily in humans when they are exposed to various stresses including exercise, environmental heat and cold, altitude, water immersion, dehydration, and perhaps microgravity, singly and in various combinations. The level of involuntary dehydration is approximately proportional to the degree of total stress imposed on the body. Involuntary dehydration appears to be controlled by more than one factor including social customs that influence what is consumed, the capacity and rate of fluid absorption from the gastrointestinal system, the level of cellular hydration involving the osmotic-vasopressin interaction with sensitive cells or structures in the central nervous system, and, to a lesser extent, hypovolemic-angiotensin II stimuli. Since humans drink when there is no apparent physiological stimulus, the psychological component should always be considered when investigating the total mechanisms for drinking.
Article
Fourteen postheatstroke (HS) subjects and nine normal control subjects (C) with similar (V̇O2max) underwent a 180-min heat-tolerance test (ambient temperature 40°C, relative humidity 40%; 12 steps.min⁻¹ on a 30-cm high bench). In group HS nine subjects (NHS) thermoregulated similar to group C, while five subjects were identified as heat intolerant (HI), characterized by significantly higher heart rate (HR), rectal temperature (Tre), and heat storage (ΔS) than groups NHS and C (at the end of the 2nd h, HR 159 ± 9, 124 ± 4, and 118 ± 3 beats.min⁻¹, Tre 38.9 ± 0.1, 37.9 ± 0.1, and 37.9 ± 0.1 °C, ΔS 24 ± 5, 10 ± 2, and 12 ± 4 W.m⁻² in groups HI, NHS, and C, respectively). The work efficiency (η) was significantly lower in groups HI and NHS when compared with C (10.2 ± 0.7, 10.4 ± 0.5, and 11.8 ± 0.6%, respectively). Skin surface area-to-body mass ratio (AD/W) in group HI was significantly lower than in group NHS or C (247 ± 7,271 ± 5, and 272 ± 4 cm².kg⁻¹, respectively). Multiple linear regression analysis between each one of the dependent variables, HR, Tre, and ΔS, mean weighted skin temperature, and sweat rate, against the two independent variables, η and AD/W, yielded significant correlations (r = 0.74, 0.73, 0.58, 0.59, and 0.44, respectively). The results suggest that a reduced η and/or a reduced AD/W play a role in heat intolerance, causing increased heat production and a concomitantly less effective heat dissipation.
Article
This study was intended to assess the influence of drink flavor and composition on voluntary drinking and hydration status in children exercising intermittently at 35 +/- 1 degrees C and 45-50% relative humidity. Twelve boys (9-12 yr) performed three 3-h identical sessions (four 20-min cycling bouts at 50% maximal O2 uptake followed by 25-min rest). One of three beverages (chilled to 8-10 degrees C) was assigned to each session in a Latin-square sequence: unflavored water (W), grape-flavored water (FW), and grape-flavored water plus 6% carbohydrate and 18 mmol/l NaCl (CNa). Drinking was ad libitum. Body weight, heart rate, rectal and skin temperatures, and thirst and stomach fullness perceptions were monitored periodically. Total intake was 610, 882, and 1,157 g in W, FW, and CNa, respectively (CNa-W and CNa-FW; P < 0.05). Hypohydration was observed with W (-0.65% body wt) and FW (-0.32% body wt), but drinking CNa resulted in slight overhydration (+0.47% body wt, CNa-W, CNa-FW; P < 0.05). Other physiological and all perceptual variables were insignificantly different among trails. In conclusion, while flavoring of water reduces children's voluntary dehydration, further addition of 6% carbohydrates and 18 mmol/l NaCl prevents it altogether.
Article
Football players lose 3.5 to 5 kg of body weight during preseason practices because of heavy sweating. This fluid may be difficult to replace when practices occur 2 times per day on consecutive days. Football players are hypohydrated during twice-a-day preseason training in a hot, humid environment. Descriptive laboratory study. In 10 college football players, body weight was measured, and blood and urine samples were obtained before and after practices on days 2 through 8 of preseason training. Baseline samples were obtained when subjects were euhydrated. Blood samples were used to calculate plasma volume changes. Urine samples were analyzed for specific gravity, sodium, and potassium. Sweat rate was calculated. Core temperature was monitored during half- and full-padded practices. Mean wet bulb temperatures were 23.3 degrees C during morning practices and 23.7 degrees C during afternoon practices. Plasma volume was below baseline on day 2 and expanded by day 6. Urine specific gravity was higher than baseline for 12 of 20 measurements over the 8 days. It was 1.0175 +/- 0.006 at baseline but subsequently ranged from 1.0214 +/- 0.007 to 1.0321 +/- 0.004. Mean daily urine sodium dropped from baseline to day 2 (194 +/- 43 vs 43 +/- 38 mmol x L-1), remaining lower on days 3, 4, and 6 (40 +/- 39, 39 +/- 39, and 68 +/- 40 mmol x L-1, respectively). Urine potassium was lower on days 6 and 8 compared with baseline and day 3. Body weight was below baseline before and after both daily practices. Core temperature was higher in full pads; sweat rate and body weight loss were not different between half and full pads. Body weight, plasma volume, urine specific gravity, and urine sodium indicate that football players become dehydrated by day 2 of preseason training. Urine sodium increased to near baseline by day 8; urine specific gravity was elevated. Football players struggle to maintain euhydration during preseason twice-a-day sessions.
Article
The purpose of this study was to determine whether high-school football players showed risks of fluid deficits during two-a-day training (Part 1), and whether implementing a drinking strategy could acutely improve the markers of hydration (Part 2). In Part 1, pre-training urine specific gravity (USG) and pre- and post-training body weight were measured at the morning session for 5 consecutive days of two-a-day practices to monitor the hydration status of 13 varsity players. The mean pre-training body weight was consistently lower (mean decrease of 0.5 kg, p<0.05) following the first day of measurement. Pre-training USG values remained consistently high each day (range for daily means: 1.022+/-0.003 to 1.024+/-0.005). Part 2 consisted of assessing hydration status in 46 varsity and junior varsity players prior to morning training during two-a-day training before and following implementing a drinking strategy. In association with the strategy, mean body weight increased 0.5 kg (p<0.01) and mean USG decreased from 1.021 to 1.016 (p<0.01) following the drinking protocol. The slight decline in body weight and consistently high USG (Part 1) suggested that standard fluid replacement strategies were less than optimal for a majority of the players. Implementing a drinking strategy appeared to improve hydration status based on changes in body weight and USG (Part 2).
Article
Children possess certain physiological and anatomic characteristics that have traditionally been considered to impair thermoregulatory responses to exercise in the heat: low exercise economy, high ratio of body surface area to mass, diminished sweating capacity, and less cardiac output at the same work load compared with adults. Consequently, children have been regarded as an at-risk group for not only decrements of physical performance but also heat injury during physical activities performed in conditions of high ambient temperature. Recent investigations that have directly compared thermoregulatory responses to exercise in the heat in children and adults have challenged these traditional concepts. Such studies have failed to indicate group differences in heat dispersal when adult-child comparisons are appropriately considered in respect to relative exercise intensity. These findings imply that no maturational differences exist in thermal balance or endurance performance during exercise in the heat, nor that child athletes are more vulnerable to heat injury.