Rehydration with glycerol: endocrine, cardiovascular, and thermoregulatory responses during exercise in the heat.

The impact of rehydration with glycerol on cardiovascular and thermoregulatory responses during exercise in the heat was studied in eight highly trained male cyclists. Each subject completed three dehydration-rehydration experimental trials that differed only in the rehydration treatment, each separated by 7 days. Before each experimental day, subjects dehydrated to -4% of their body weight by exercise and water restriction. The experimental treatments were as follows: no fluid (NF), glycerol bolus (1 g/kg body wt) followed by water (G), and water alone (W). Rehydration (3% body weight) was given over an 80-min period. After rehydration, subjects cycled (74% peak O2 uptake) to exhaustion in a hot and wet (37 degrees C and 48% relative humidity) environment. For G, plasma volume was expanded (P < 0.05) during rehydration and remained higher than W (P < 0.05) during exercise. Exercise time to exhaustion during G (33 +/- 4 min) was longer (P < 0.05) compared with both W (27 +/- 3 min) and NF (19 +/- 3 min). Cutaneous vascular conductance was significantly elevated (P < 0.05) during G, but G provided no other thermoregulatory or cardiovascular benefits compared with W and NF. Fluid-regulating hormones (vasopressin, aldosterone, atriopeptin, and plasma renin activity) decreased during rehydration and increased during exercise (except atriopeptin), but there were no differences between G and W. These data indicated that glycerol had little or no major effect on fluid-regulating factors during rehydration or exercise, and the improved exercise capacity in G was likely due to a greater plasma volume during exercise.     

Effect of voluntary dehydration on thermoregulatory responses to heat in men and women

The effects of dehydration prior to heat exposure on sweating and body temperature were tested in 8 men and 8 women, dehydration being 1.3 and 1.0% of body weight, respectively. The subjects were exposed to 40 degrees C for 60 min. Compared with controls (C), in the dehydrated men (D) there was a longer delay in the onset of sweating (C, 7.8, D, 11.6 min, p less than 0.05), a lower total sweat loss (C, 153, D, 127 g X m-2 X h-1, p less than 0.001), and a greater increase in Tre (C, 0.31, D, 0.43 degree C, p less than 0.002). In women, dehydration did not influence the control time course of sweating significantly, nor were these significant body temperature increases during heat exposure. Delay in the onset of sweating in women (C, 18.1, D, 18.7 min) was generally longer than in men (C, 7.8, D, 11.6 min), [F(1,14) = 7.41, p less than 0.05]. A significant correlation was found between the inertia time of sweating and delta Tre in both control and dehydration conditions in the men (r = 0.81, p less than 0.01). The rectal temperature increases in men were also related to the inertia time of electrical skin resistance (r = 0.83, p less than 0.01). It is concluded that dehydration affects sweating and body temperature in men more severely than in women.     

Influence of the menstrual cycle and oral contraceptives on thermoregulatory responses to exercise in young women

Thermoregulatory responses to exercise in relation to the phase of the menstrual cycle were studied in ten women taking oral contraceptives (P) and in ten women not taking oral contraceptives (NP). Each subject was tested for maximal aerobic capacity ( ) and for 50% exercise in the follicular (F) and luteal (L) phases of the menstrual cycle. Since the oral contraceptives would have prevented ovulation a quasi-follicular phase (q-F) and a quasi-luteal phase (q-L) of the menstrual cycle were assumed for P subjects. Exercise was performed on a cycle ergometer at an ambient temperature of 24° C and relative air humidity of 50%. Rectal (T _re), mean skin ( ), mean body ( ) temperatures and heart rate (f _c) were measured. Sweat rate was estimated by the continuous measurement of relative humidity of air in a ventilated capsule placed on the chest, converted to absolute pressure (PH₂O_chest). Gain for sweating was calculated as a ratio of increase inPH₂O_chest to the appropriate increase inT _re for the whole period of sweating (G) and for unsteady-state (G_u) separately. The did not differ either between the groups of subjects or between the phases of the menstrual cycle. In P, rectal temperature threshold for sweating (T _{re, td}) was 37.85° C in q-L and 37.60° C in q-F (P < 0.01) and corresponded to a significant difference fromT _re at rest. TheT _re, andf _c increased similarly during exercise in q-F and q-L. No menstrual phase-related differences were observed either in the dynamics of sweating or in G. In NP,T _{re, td} was shorter in L than in F (37.70 vs 37.47° C,P<0.02) with a significantly greater value fromT _re at rest. The dynamics and G for sweating were also greater in L than in F. The G_u was 36.8 versus 16.6 kPa · ° C^–1 (P<0.01) while G was 6.4 versus 3.8 kPa · ° C^–1 (P<0.05), respectively. TheT _re, andf _c increased significantly more in phase F than in phase L. It was concluded that in these women performing moderate exercise, there was a greater temperature threshold and larger gains for sweating in phase L than in phase F. Intake of oral contraceptives reduced the differences in the gains for sweating making the thermoregulatory responses to exercise more uniform.     

Effects of two cooling strategies on thermoregulatory responses of tetraplegic athletes during repeated intermittent exercise in the heat.

Athletes with spinal cord injury (SCI), and in particular tetraplegia, have an increased risk of heat strain and consequently heat illness relative to able-bodied individuals. Strategies that reduce the heat strain during exercise in a hot environment may reduce the risk of heat illness. To test the hypotheses that precooling or cooling during intermittent sprint exercise in a heated environment would attenuate the rise in core temperature in tetraplegic athletes, eight male subjects with SCI (lesions C(5)-C(7); 2 incomplete lesions) undertook four heat stress trials (32.0 +/- 0.1 degrees C, 50 +/- 0.1% relative humidity). After assessment of baseline thermoregulatory responses at rest for 80 min, subjects performed three intermittent sprint protocols for 28 min. All trials were undertaken on an arm crank ergometer and involved a no-cooling control (Con), 20 min of precooling (Pre), or cooling during exercise (Dur). Trials were administered in a randomized order. After the intermittent sprint protocols, mean core temperature was higher during Con (37.3 +/- 0.3 degrees C) compared with Pre and Dur (36.5 +/- 0.6 degrees C and 37.0 +/- 0.5 degrees C, respectively; P < 0.01). Moreover, perceived exertion was lower during Pre (13 +/- 2; P < 0.01) and Dur (12 +/- 1; P < 0.01) compared with Con (14 +/- 2). These results suggest that both precooling and cooling during intermittent sprint exercise in the heat reduces thermal strain in tetraplegic athletes. The cooling strategies also appear to show reduced perceived exertion at equivalent time points, which may translate into improved functional capacity.     

Evidence for thermoregulatory behavior during self-paced exercise in the heat

The primary objective of this investigation was to test the hypothesis that voluntary reductions in exercise intensity in heat improve heat exchange between the body and the environment, and are thus thermoregulatory behaviors. This was accomplished by observing the conscious selection of exercise intensity and the accompanying thermal outcomes of eleven moderately active males when exposed to an uncompensably hot (UNCOMP) and a compensable (COMP) thermal environment. Evidence for thermoregulatory behavior was defined relative to the specific, pre-determined definition. Self-selected exercise intensity (power output) was unanimously reduced in UNCOMP over time and relative to COMP in all the subjects. These voluntary responses were found to modify metabolic heat production over time and therefore heat exchange between the body and the environment. Likewise, the observed reductions in power output were, at least in part, due to a conscious action, that was found to be inversely related to the total body heat storage and thermal discomfort. There was no evidence for thermoregulatory behavior in COMP. These data uniquely indicate that voluntary reductions in exercise intensity improves heat exchange over time, and therefore contributes to the regulation of body temperature. These findings suggest that reductions in exercise intensity in heat are, by definition, thermoregulatory behaviors.     

Effect of positive-pressure breathing on cardiovascular and thermoregulatory responses to exercise

Five healthy male volunteers performed 20 min of both seated and supine cycle-ergometer exercise (intensity, 50% maximal O2 uptake) in a warm environment (Tdb = 30 degrees C, relative humidity = 40-50%) with and without breathing 10 cmH2O of continuous positive airway pressure (CPAP). The final esophageal temperature (Tes) at the end of 20 min of seated exercise was significantly higher during CPAP (mean difference = 0.18 +/- 0.04 degree C, P less than 0.05) compared with control breathing (C). The Tes threshold for forearm vasodilation was significantly higher (P less than 0.05) during seated CPAP exercise than C (C = 37.16 +/- 0.13 degrees C, CPAP = 37.38 + 0.12 degree C). The highest forearm blood flow (FBF) at the end of exercise was significantly lower (P less than 0.05) during seated exercise with CPAP (mean +/- SE % difference from C = -30.8 +/- 5.8%). During supine exercise, there were no significant differences in the Tes threshold, highest FBF, or final Tes with CPAP compared with C. The added strain on the cardiovascular system produced by CPAP during seated exercise in the heat interacts with body thermoregulation as evidenced by elevated vasodilation thresholds, reduced peak FBF, and slightly higher final esophageal temperatures.     

Metabolic and thermoregulatory responses to a simulated American football practice in the heat

Energy cost is a major factor influencing the tolerable thermal load, particularly during exercise in the heat. However, no data exist on the metabolic cost of football practice, although a value of 35% of maximal aerobic capacity (VO(2)max) has been estimated. The energy cost and thermoregulatory response of offensive linemen (OL) was measured wearing different American football ensembles during a simulated half of football practice in the heat. Five collegiate offensive linemen (133 kg, 20% fat, 42 ml x kg(-1) x min(-1) maximal oxygen uptake) completed each of four 60-minute test sessions in an environmental chamber (28 degrees C, 55% relative humidity [RH]) wearing shorts (S), helmet (H), helmet and shoulder pads (HS), and full gear (FUL). Core temperature in the digestive tract (TGI) was obtained using an ingestible sensor. During simulated football drills (e.g., repetitions of drive blocking), exercise intensity ranged from 30 to 81% VO(2)max but averaged 55%VO(2)max (6.7 METS) overall. Blood lactate remained >5 mmol x L(-1), and heart rate (HR) averaged 79%HRmax. Equipment had a significant effect on %VO(2)max but only during recovery between drills with HS (61.4 +/- 3.7%) compared with H (53.3 +/- 6.9%) and S (40.1 +/- 8.5%). The TGI was higher (p < 0.05) with HS compared with H at several time-points after 30 minutes. Football practice for OL elicits a significantly higher overall metabolic cost (>6 METS, >50%VO(2)max) than assumed in previous studies. The addition of shoulder pads increases core temperature and energy cost, especially during recovery between active drills in unacclimatized linemen.     

Comparison of thermoregulatory responses to heat between Japanese Brazilians and Japanese

1. 1. Eight male Japanese Brazilians and 11 male Japanese volunteered for this study. Each one sat on a chair for 45 min at 40°C (r.h. 50%).

2. 2. Then, they exercised using a bicycle ergometer in a semi-reclining position for 45 min at 40% of maximal oxygen uptake.

3. 3. Thermal and comfort sensation confirmed that Japanese Brazilians felt cooler and more comfortable in 40°C environment than the Japanese.

4. 4. Oxygen uptake, sweat rate and body weight loss for both groups were not significantly different.

5. 5. Forearm blood flow and heart rate for Brazilians were significantly lower than those for Japanese.

6. 6. Skin temperature at chest region for Brazilians was found to be significantly higher than that for Japanese.

7. 7. Thus the thermoregulatory responses observed in Japanese Brazilians may be largely attributed to the climate in their native places located on the Tropic of Capricorn.

8. 8. These results may indicate that environment condition is the important factor in determining the thermoregulatory responses.

Thermoregulatory responses of firemen to exercise in the heat

Twelve volunteer (VF) and 12 professional firemen (PF) wearing only brief trunks exercised on an electrically-braked cycle ergometer at three-five exercise intensities. After 45 min of exercise at 75 W, the exercise intensity was elevated in steps of 25 W every 15 min until the subject was exhausted. Air temperature was regulated to equal skin temperature (36 degrees-38 degrees C) and relative humidity was regulated at 52%. The two groups of firemen were comparable in terms of body mass, age and maximum oxygen consumption. Their oxygen consumption, rectal and skin temperatures, sweating and heart rate were measured during the tests. Blood lactate concentration was measured before, during and after the test. The physiological strain was higher in VF as indicated by higher heat storage, heart rate, skin and rectal temperatures. Sweat rate tended to be lower in VF than PF. The results indicated a better adaptation of the professional compared to the volunteer firemen to work in the heat, although the degree of heat acclimatization was considered to be equally minimal in both groups.     

Effects of mode of exercise recovery on thermoregulatory and cardiovascular responses.

To identify the effects of exercise recovery mode on cutaneous vascular conductance (CVC) and sweat rate, eight healthy adults performed two 15-min bouts of upright cycle ergometry at 60% of maximal heart rate followed by either inactive or active (loadless pedaling) recovery. An index of CVC was calculated from the ratio of laser-Doppler flux to mean arterial pressure. CVC was then expressed as a percentage of maximum (%max) as determined from local heating. At 3 min postexercise, CVC was greater during active recovery (chest: 40 +/- 3, forearm: 48 +/- 3%max) compared with during inactive recovery (chest: 21 +/- 2, forearm: 25 +/- 4%max); all P < 0.05. Moreover, at the same time point sweat rate was greater during active recovery (chest: 0.47 +/- 0.10, forearm: 0.46 +/- 0.10 mg x cm(-2) x min(-1)) compared with during inactive recovery (chest: 0.28 +/- 0.10, forearm: 0.14 +/- 0.20 mg x cm(-2) x min(-1)); all P < 0.05. Mean arterial blood pressure, esophageal temperature, and skin temperature were not different between recovery modes. These data suggest that skin blood flow and sweat rate during recovery from exercise may be modulated by nonthermoregulatory mechanisms and that sustained elevations in skin blood flow and sweat rate during mild active recovery may be important for postexertional heat dissipation.     

Influence of a low-carbohydrate diet on thermoregulatory responses to prolonged exercise in men

Tympanic temperature (T_ty), mean skin temperature (T_sk) and dynamics of sweating were measured in male subjects, performing prolonged, bicycle-exercise. The exercise was performed under thermoneutral environment after the normal-mixed (C) and low-carbohydrate (LCHO) diet. No difference in changes of T_ty was observed after the C and LCHO diets. Sweating was triggered much faster after the LCHO diet. The delay in onset and inertia time of sweating was shorter after the LCHO diet. It is concluded that LCHO diet affects the thermoregulatory responses to exercise by acceleration of sweating, without noticeable differences in body temperature.     

Cardiopulmonary responses to exercise in obese girls and young women.

A study of exercise performance was carried out in 17 obese girls and young adults. During submaximal steady-state bicycle exercise oxygen intake (Vo2) for a given work output (W) was raised in obese subjects but minute ventilation at a fixed carbon dioxide output, gas exchange, blood gases, and cardiac output at a given VO2 were similar to the values previously found for normals. In obese subjects high levels of VO2 for fixed W were also obtained on the treadmill but when these were standardized for body weight (unlike the bicycle test) it was shown that the obese girls and women exercised within the normal (expected) range of aerobic energy expenditure. During maximal performance the absolute VO2 max was the same in obese and nonobese subjects but for a given body weight, lean body mass, and leg muscle (plus) bone volume, VO2max was reduced by 23.8, 16.3, and 24.5% respectively, in the former group. It was concluded that obesity though having minimal affect on responses to submaximal exercise is nevertheless associated with a marked reduction in physiological performance at or near maximal effort.     

Ventilatory and perceptual responses to cycle exercise in obese women.

The main purpose of this study was to examine the relative contribution of respiratory mechanical factors and the increased metabolic cost of locomotion to exertional breathlessness in obese women. We examined the relationship of intensity of breathlessness to ventilation (VE) when exertional oxygen uptake (VO2) of obesity was minimized by cycle exercise. Eighteen middle-aged (54+/-8 yr, mean+/-SD) obese [body mass index (BMI) 40.2+/-7.8 kg/m2] and 13 age-matched normal-weight (BMI 23.3+/-1.7 kg/m2) women were studied. Breathlessness at higher submaximal cycle work rates was significantly increased (by>or=1 Borg unit) in obese compared with normal-weight women, in association with a 35-45% increase in Ve and a higher metabolic cost of exercise. Obese women demonstrated greater resting expiratory flow limitation, reduced resting end-expiratory lung volume (EELV)(by 20%), and progressive increases in dynamic EELV during exercise: peak inspiratory capacity (IC) decreased by 16% (0.39 liter) of the resting value. VE/VO2 slopes were unchanged in obesity. Breathlessness ratings at any given VE or VO2 were not increased in obesity, suggesting that respiratory mechanical factors were not contributory. Our results indicate that in obese women, recruitment of resting IC and dynamic increases in EELV with exercise served to optimize operating lung volumes and to attenuate expiratory flow limitation so as to accommodate the increased ventilatory demand without increased breathlessness.     

Comparison of thermoregulatory responses between men and women immersed in cold water.

Eleven women (age = 24.4 +/- 6.3 yr, mass = 65.0 +/- 7.8 kg, height = 167 +/- 8 cm, body fatness = 22.4 +/- 5.9%, mean +/- SD) were immersed to neck level in 18 degrees C water for up to 90 min for comparison of their thermal responses with those of men (n = 14) in a previous similarly conducted protocol. Metabolic rate increased about three times resting levels in men and women, whereas the rate of rectal temperature cooling (DeltaT(re)/Deltat) in women (0.47 degrees C/h) was about one-half that in men. With use of all data, DeltaT(re)/Deltat correlates with the ratio of body surface area to size and the metabolic rate of shivering correlates inversely to the square root of body fatness. No significant gender differences in total metabolic heat production normalized for body mass or surface area were found among subjects who completed 90 min of immersion (9 women and 7 men). Nor was there a gender difference in the overall percent contribution ( approximately 60%) of fat oxidation to total heat production. Blood concentrations of free fatty acids, glycerol, beta-hydroxybutyrate, and lactate increased significantly during the 90-min immersion, whereas muscle glycogen sampled from the right quadriceps femoris vastus lateralis decreased (free fatty acids, glycerol, and beta-hydroxybutyrate were higher in women). When the subjects were subgrouped according to similar body fatness and 60 min of immersion (6 women and 5 men), no significant gender differences emerged in DeltaT(re)/Deltat, energy metabolism, and percent fat oxidation. These findings suggest that no gender adjustments are necessary for prediction models of cold response if body fatness and the ratio of body surface area to size are taken into account and that a potential gender advantage with regard to carbohydrate sparing during cold water immersion is not supported.     

Analyses of thermoregulatory responses of feeder cattle exposed to simulated heat waves

Heat stress in feedlot cattle causes reduced performance, and in the most severe cases, death of the animals, thus causing the loss of millions of dollars in revenue to the cattle industry. A study was designed to evaluate dynamics of thermoregulation and feeding activities when feeder cattle were exposed to simulated heat waves, in comparison with repeated sinusoidal hot and thermoneutral environments. Nine beef steers were randomly assigned to an individual pen in one of three environmental chambers. Each chamber was subjected to each of three temperature regimes (Heatwave simulation from Rockport, Mo., 1995, Heatwave simulation from Columbia, Mo., 1999, and Controlled heat stress treatment of 32±7°C) for a period of 18 days, according to a Latin square treatment design, with a 10-day thermoneutral period (18±7°C) separating treatment periods. Respiration rate, core body temperature, heat production, feed intake, and feeding behavior were measured on each animal for the duration of the experiment. Differences were found in all treatments for all parameters except feeding behavior. It was shown that the two simulated heat waves elicited very different thermoregulatory responses. Based on these results the heat wave centered at Rockport, Mo. in 1995 was devastating because the animals were not acclimated to hot conditions, thus causing an acute response to heat stress. The responses of cattle to conditions at Columbia, Mo. showed some acclimation to heat prior to the peak stress days, and therefore a dampened response was seen. It appears the extreme conditions at Columbia, Mo., 1999 were made severe by environmental conditions not simulated during this study (low wind speed and intensive solar radiation). Overall, it was determined while a cyclic heat stress treatment is a representative model to test heat stress in cattle, further heat stress experiments should be conducted in an actual feedlot.Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. 9th Street Drive, West Palmetto, FL 34221, USA.     

Twenty-four-hour metabolic responses to resistance exercise in women

Seven nonobese adult females (40 +/- 8 years) were studied in a room calorimeter on a day that resistance exercise (REX) was performed (4 sets of 10 exercises) and on a nonexercise control day (CON). Twenty-four-hour energy expenditure (EE) on the REX day (mean +/- SD, 2,328 +/- 327 kcal.d(-1)) was greater than CON (2,001 +/- 369 kcal.d(-1), p < 0.001). The net increase in EE during and immediately after (30 minutes) exercise represented 76 +/- 12% of the total increase in 24-hour EE. Twenty four-hour RQ on the REX day (0.86 +/- 0.06) did not differ from CON (0.87 +/- 0.02). Twenty four-hour carbohydrate oxidation was elevated on the REX day, but 24-hour fat and protein oxidation were not different. Thus, in women, the increase in EE due to resistance exercise is largely seen during and immediately after the exercise. The increased energy demand is met by increased carbohydrate oxidation, with no increase in 24-hour fat oxidation.