Effect of age and gender on sweat lactate and ammonia concentrations during exercise in the heat.

The dependence of sweat composition and acidity on sweating rate (SR) suggests that the lower SR in children compared to adults may be accompanied by a higher level of sweat lactate (Lac-) and ammonia (NH3) and a lower sweat pH. Four groups (15 girls, 18 boys, 8 women, 8 men) cycled in the heat (42 degrees C, 20% relative humidity) at 50% VO2max for two 20-min bouts with a 10-min rest before bout 1 and between bouts. Sweat was collected into plastic bags attached to the subject's lower back. During bout 1, sweat from girls and boys had higher Lac- concentrations (23.6 +/- 1.2 and 21.2 +/- 1.7 mM; P < 0.05) than sweat from women and men (18.2 +/- 1.9 and 14.8 +/- 1.6 mM, respectively), but Lac- was weakly associated with SR (P > 0.05; r = -0.27). Sweat Lac- concentration dropped during exercise bout 2, reaching similar levels among all groups (overall mean = 13.7 +/- 0.4 mM). Children had a higher sweat NH3 than adults during bout 1 (girls = 4.2 +/- 0.4, boys = 4.6 +/- 0.6, women = 2.7 +/- 0.2, and men = 3.0 +/- 0.2 mM; P < 0.05). This difference persisted through bout 2 only in females. On average, children's sweat pH was lower than that of adults (mean +/- SEM, girls = 5.4 +/- 0.2, boys = 5.0 +/- 0.1, women = 6.2 +/- 0.5, and men = 6.2 +/- 0.4 for bout 1, and girls = 5.4 +/- 0.2, boys = 6.5 +/- 0.5, women = 5.2 +/- 0.2, and men = 6.9 +/- 0.4 for bout 2). This may have favored NH3 transport from plasma to sweat as accounted for by a significant correlation between sweat NH3 and H+ (r = 0.56). Blood pH increased from rest (mean +/- SEM; 7.3 +/- 0.02) to the end of exercise (7.4 +/- 0.01) without differences among groups. These results, however, are representative of sweat induced by moderate exercise in the absence of acidosis.

Effect of age and gender on sweat lactate and ammonia concentrations during exercise in the heat

The dependence of sweat composition and acidity on sweating rate (SR) suggests that the lower SR in children compared to adults may be accompanied by a higher level of sweat lactate (Lac-) and ammonia (NH3) and a lower sweat pH. Four groups (15 girls, 18 boys, 8 women, 8 men) cycled in the heat (42°C, 20 percent relative humidity) at 50 percent VO2max for two 20-min bouts with a 10-min rest before bout 1 and between bouts. Sweat was collected into plastic bags attached to the subject's lower back. During bout 1, sweat from girls and boys had higher Lac- concentrations (23.6 ± 1.2 and 21.2 ± 1.7 mM; P < 0.05) than sweat from women and men (18.2 ± 1.9 and 14.8 ± 1.6 mM, respectively), but Lac- was weakly associated with SR (P > 0.05; r = -0.27). Sweat Lac- concentration dropped during exercise bout 2, reaching similar levels among all groups (overall mean = 13.7 ± 0.4 mM). Children had a higher sweat NH3 than adults during bout 1 (girls = 4.2 ± 0.4, boys = 4.6 ± 0.6, women = 2.7 ± 0.2, and men = 3.0 ± 0.2 mM; P < 0.05). This difference persisted through bout 2 only in females. On average, children's sweat pH was lower than that of adults (mean ± SEM, girls = 5.4 ± 0.2, boys = 5.0 ± 0.1, women = 6.2 ± 0.5, and men = 6.2 ± 0.4 for bout 1, and girls = 5.4 ± 0.2, boys = 6.5 ± 0.5, women = 5.2 ± 0.2, and men = 6.9 ± 0.4 for bout 2). This may have favored NH3 transport from plasma to sweat as accounted for by a significant correlation between sweat NH3 and H+ (r = 0.56). Blood pH increased from rest (mean ± SEM; 7.3 ± 0.02) to the end of exercise (7.4 ± 0.01) without differences among groups. These results, however, are representative of sweat induced by moderate exercise in the absence of acidosis.

Pilot study of fitness training and exercise testing in polyarticular childhood arthritis.

OBJECTIVE: To 1) assess the safety and feasibility of laboratory-based exercise testing in juvenile idiopathic arthritis (JIA), 2) test the safety and feasibility of a 3-month exercise program in JIA, 3) assess pain during exercise in JIA, 4) compare ratings of perceived effort (RPE) with heart rate (HR) achieved, and 5) estimate the training effect on metabolic efficiency of gait as measured by submaximal exercise testing. METHODS: Nine children with JIA were enrolled in a 12-week circuit training program involving pool, stationary bicycle, treadmill, and Fitball. They underwent formal exercise testing before and after the program, underwent a full joint assessment, were administered the Childhood Health Assessment Questionnaire and Juvenile Arthritis Functional Status Index, and were assessed for overall quality of life and health-related quality of life. A visual analog scale was used to assess pain during testing and training, and the Borg scale was used to measure RPE. RESULTS: Children with JIA were able to participate in exercise testing without any significant problems. Children with severe hip disease dropped out of the exercise program due to pain during the exercise sessions and worsened arthritis symptoms. Target HR was achieved and correlated with RPE in the bicycle and treadmill sessions. Submaximal exercise testing showed an improvement with a small to moderate effect size. CONCLUSION: This study suggests that it is safe, feasible, and acceptable for children with arthritis, in the absence of severe hip involvement, to participate in formal exercise testing and structured fitness programs.

Minute-by-minute differences in co-activation during treadmill walking in cerebral palsy.

The primary purpose of this study was to determine, in children and adolescents with mild spastic cerebra palsy (CP); 1) minute-by-minute differences in lower limb antagonist muscle co-activation and stride length (SL) during treadmill walking following 12-15 minutes of treadmill walking practice, and 2) if the minute-by-minute pattern of co-activation is affected by site (thigh or lower leg) and lower limb dominance. A secondary purpose was to determine if overall there is a difference in co-activation between the dominant and non-dominant lower limbs. Eight independently ambulatory children and adolescents with mild spastic CP (9.2-15.7 yr) participated in the study. Minute-by-minute lower limb antagonist muscle co-activation and SL were measured during a 3-minute treadmill walk at 90% of individually determined fastest treadmill walking speed. Non-dominant thigh (quadriceps, hamstring muscles) co-activation decreased between minute 1 and a) minute 2 (6%), b) minute 3 (7.2%). Co-activation for the dominant lower leg (tibialis anterior, triceps surae muscles) decreased between minute 1 and minute 3 (11.3%). Non-dominant thigh co-activation was on average 27.3% higher than for the dominant thigh. Thigh co-activation was on average 27.7% higher than for the lower leg, independent of dominance or time. SL increased between minute 1 and minute 3 by 2.1%. Twelve to 15 minutes of treadmill walking practice may be sufficient time to obtain stable co-activation and SL values by minute 2 of a fast treadmill walk. Dominance and site affect the magnitude of co-activation.

Relationship between juvenile obesity, dietary energy and fat intake and physical activity.

OBJECTIVE: To determine the relationship of juvenile obesity to dietary fat, particularly saturated fat, and with dietary energy (controlling for activity patterns). DESIGN: Cross-sectional, evaluation of diet and activity patterns of obese and non-obese children and adolescents. SUBJECTS: A total of 181 children, aged 4-16 y. Subjects were divided into two groups: obese (body mass index, BMI, > 95th percentile for age and sex), 40 males and 51 females; and non-obese (BMI < 75th percentile for age and sex), 35 males and 55 females. MEASUREMENTS: Dietary intake was analyzed with a dietary history interview; activity patterns were analyzed with an activity interview and body fat was measured with bioelectrical impedance analysis. RESULTS: The obese subjects consumed significantly more total calories, total fat in grams and saturated fatty acids (SFA) in grams than did the non-obese subjects. Based on step-wise multiple regression, the total energy consumed, not total fat or SFA, had the strongest relationship to the subject's percentage body fat, controlling for activity levels. CONCLUSION: We suggest that, although obese children and adolescents consume more dietary energy and fat than non-obese children and adolescents, there is a stronger relationship between total energy consumed and juvenile adiposity than with dietary fat or type of dietary fat consumed.

Plantar pressure differences between obese and non-obese adults: a biomechanical analysis.

OBJECTIVE: To investigate plantar pressure differences between obese and non-obese adults during standing and walking protocols using a pressure distribution platform. SUBJECTS: Thirty-five males (age 42.4+/-10.8 y; 67-179 kg) and 35 females (age 40.0+/-12.6 y; 46-150 kg) divided into obese (body mass index (BMI) 38.75+/-5.97 kg/m2) and non-obese (BMI 24.28+/-3.00 kg/m2) sub-groups, respectively. MEASUREMENTS: Data collection was performed with a capacitive pressure distribution platform with a resolution of 2 sensors/cm2 (Emed F01, Novel GmbH, München). The measurement protocol included half and full body weight standing on the left, right and both feet, respectively, and walking across the platform, striking with the right foot. Pressures were evaluated for eight anatomical sites under the feet. RESULTS: For both men and women, the mean pressure values of the obese were higher under all anatomical landmarks during half body weight standing. Significant increases in pressure were found under the heel, mid-foot and metatarsal heads II and IV for men and III and IV for women. Foot width during standing was also significantly increased in obese subjects. For walking, significantly higher peak pressures were also found in both obese males and females. CONCLUSION: Compared to a non-obese group, obese subjects showed increased forefoot width and higher plantar pressures during standing and walking. The greatest effect of body weight on higher peak pressures in the obese was found under the longitudinal arch of the foot and under the metatarsal heads. The higher pressures for obese women compared to obese men during static weight bearing (standing) may be the result of reduced strength of the ligaments of the foot.

Exercise and the child born prematurely.

With the increase in survival rates of children born prematurely, issues related to their active pursuits and responses to exercise have been gaining increasing attention. In some preterm children with an extremely low birthweight, bronchopulmonary dysplasia or cerebral palsy exercise capacity may be limited, especially in tasks requiring good neuromotor coordination. Deficiencies in aerobic and anaerobic performance, strength and coordination may even occur in children without overt manifestations of a neuromuscular or pulmonary disease. However, as a rule, children born prematurely may engage in physical activities and competitive sports without limitations. Exercise is safe in almost all such children as long as precautions are taken to avoid exercise-induced bronchoconstriction. However, to date there are no studies that have determined the efficacy of training. A wide variety of activities should be encouraged in all children born prematurely at an early age, to support the development of skills and to compensate for the possible effects of their premature birth on coordination.

Competition performance and basal nocturnal catecholamine excretion in cross-country skiers.

UNLABELLED: Objective techniques to determine whether an athlete is optimally prepared for a competition are virtually nonexistent. Preparedness for sports competition is commonly judged through the experience of the athletes and their coaches. Evidence from overtraining studies suggests that catecholamine (Cat) excretion rates may correlate with performance in athletes. PURPOSE: We therefore attempted to determine whether a relationship existed between performances of world-class cross-country skiers and basal nocturnal Cat excretion (BNCE). METHOD: During the Cross-Country Skiing World Championships, we determined BNCE in nine cross-country skiers of the Swiss national team by measuring free Cat concentration (dopamine = D, norepinephrine = NE, epinephrine = E) in morning urine samples, using high performance liquid chromatography. Expert judgments of competition performance (ECP) were assessed by two experienced professional coaches of the national team by using an 11-step scale. RESULTS: The BNCE correlated significantly with ECP in cross-country skiers (r2 = 0.84 and P < for NE; r2 = 0.86 and P < 0.001 for D). Athletes who had their best absolute competition results (ACR) showed the highest NE and D concentrations. CONCLUSION: These data suggest that competitive cross-country skiers with higher D and NE excretion may reach better competition levels compared with those with lower levels. Measures of BNCE provide objective information about competition performance, which may benefit athletes in their precompetition preparation.

Organized sports for children and preadolescents.

Participation in organized sports provides an opportunity for young people to increase their physical activity and develop physical and social skills. However, when the demands and expectations of organized sports exceed the maturation and readiness of the participant, the positive aspects of participation can be negated. The nature of parental or adult involvement can also influence the degree to which participation in organized sports is a positive experience for preadolescents. This updates a previous policy statement on athletics for preadolescents and incorporates guidelines for sports participation for preschool children. Recommendations are offered on how pediatricians can help determine a child's readiness to participate, how risks can be minimized, and how child-oriented goals can be maximized.

Strength training by children and adolescents.

Pediatricians are often asked to give advice on the safety and efficacy of strength training programs for children and adolescents. This review, a revision of a previous American Academy of Pediatrics policy statement, defines relevant terminology and provides current information on risks and benefits of strength training for children and adolescents.

Use of orthoses lowers the O(2) cost of walking in children with spastic cerebral palsy.

PURPOSE: The aim of this study was to assess the effects of hinged ankle foot orthoses (AFO) on the metabolic and cardiopulmonary cost of walking and gross motor skills of children with cerebral palsy (CP). METHODS: Ten habitual users of hinged AFO with spastic diplegic CP (9.01 yr +/- 2.10) participated in the study. Expired gas and heart rate (HR) were measured during sitting and with AFO on and off during steady state treadmill walking at three speeds: 3 km.h(-1), comfortable walking speed (CWS), and 90% of their fastest walking speed (FWS). Comfortable and fastest ground walking speed and Gross Motor Function Measure scores were also assessed with AFO on and off and analyzed with ANOVA. Because not all children could walk at all speeds on the treadmill, an ANOVA was performed on data for children who walked at 3 km.h(-1) and CWS (N = 8 for HR; N = 9 for pulmonary ventilation and metabolic variables) and a t-test on data at 90% of FWS (N = 9 for HR; N = 8 for pulmonary ventilation and metabolic variables). RESULTS: When children wore their AFO net oxygen uptake (L.min(-1), absolute--sitting values) was significantly (P < 0.05) reduced by 8.9% at 3 km.h(-1) and by 5.9% at 90% of FWS. Net pulmonary ventilation (L.min(-1)) was significantly (P < 0.05) lower with AFO on by 10.3% but only at 3 km.h(-1). AFO did not affect net HR (beats.min(-1)) nor the respiratory exchange ratio at any speed, nor any physiologic variable at CWS, nor gross motor skills. CONCLUSIONS: Use of hinged AFO reduces the oxygen and ventilatory cost of walking in children with spastic diplegic CP.

Substrate utilization during exercise with glucose and glucose plus fructose ingestion in boys ages 10--14 yr.

We measured substrate utilization during exercise performed with water (W), exogenous glucose (G), and exogenous fructose plus glucose (FG) ingestion in boys age 10-14 yr. Subjects (n = 12) cycled for 90 min at 55% maximal O(2) uptake while ingesting either W (25 ml/kg), 6% G (1.5 g/kg), or 3% F plus 3% G (1.5 g/kg). Fat oxidation increased during exercise in all trials but was higher in the W (0.28 +/- 0.023 g/min) than in the G (0.24 +/- 0.023 g/min) and FG (0.25 +/- 0.029 g/min) trials (P = 0.04). Conversely, total carbohydrate (CHO) oxidation decreased in all trials and was lower in the W (0.63 +/- 0.05 g/min) than in the G (0.78 +/- 0.051 g/min) and FG (0.74 +/- 0.056 g/min) trials (P = 0.009). Exogenous CHO oxidation, as determined by expired (13)CO(2), reached a maximum of 0.36 +/- 0.032 and 0.31 +/- 0.030 g/min at 90 min in G and FG, respectively (P = 0.04). Plasma insulin levels decrease during exercise in all trials but were twofold higher in G than in W and FG (P < 0.001). Plasma glucose levels decreased transiently after the onset of exercise in all trials and then returned to preexercise values in the W and FG (approximately 4.5 mmol/l) trials but were elevated by approximately 1.0 mmol/l in the G trial (P < 0.001). Plasma lactate concentrations decreased after the onset of exercise in all trials but were lower by approximately 0.5 mmol/l in W than in G and FG (P = 0.02). Thus, in boys exercising at a moderate intensity, the oxidation rate of G plus F is slightly less than G alone, but both spare endogenous CHO and fat to a similar extent. In addition, compared with flavored W, the ingestion of G alone and of G plus F delays exhaustion at 90% peak power by approximately 25 and 40%, respectively, after 90 min of moderate-intensity exercise.

Nutritional considerations for the child athlete.

While nutritional issues are similar for all athletes irrespective of age, children have several physiological characteristics that distinguish them from adults and require specific nutritional considerations. These age- or maturation-related differences include: a greater need for protein intake, to support growth; a greater need for calcium intake, to support bone accretion; a higher energy cost of activities that include walking and running; lower losses of sodium and chloride in sweat; and a greater thermoregulatory strain at any given level of hypohydration. This review will focus on three areas: (a) a higher metabolic cost of locomotion, its causes, and possible relevance to the calculation of daily energy requirements for young athletes; (b) the effect of carbohydrate (glucose, glucose plus fructose, or glucose plus sucrose) ingestion on children's aerobic performance, substrate utilization, and immune responses; and (c) involuntary dehydration during exercise in hot climate and the means for its prevention.

Validating a practical approach to determine weight control in obese children and adolescents.

OBJECTIVE: To study the usefulness of a mathematical index for assessing changes in body composition of obese children and adolescents who undergo a weight control program. DESIGN: A short-term longitudinal (mean of 19 months) cohort study. SUBJECTS: Sixty-seven obese children and adolescents (38 M, 29 F, age 6-16 (mean 11)y) who took part in a clinic-based weight control program. MEASUREMENTS: Percentage body fat was assessed at the start of the program by underwater weighing (UWW) and by bioelectrical impedance (BIA). Response to the program was assessed by a mathematical index (MI), based on observed and expected changes in height and weight, and by changes in percentage fat as measured by BIA. RESULTS: Adiposity, as assessed by BIA at the start of the program, was highly correlated to that obtained by UWW (r = 0.96 for fat-free mass). Changes in the MI over the program were correlated fairly well (r = -0.81, SEE = 3.57 kg) with changes in percentage fat as assessed by BIA. CONCLUSION: Using change in BIA as criterion, the MI is valid for assessing changes in percentage body fat of obese children and adolescents over time. This index is of use to clinicians who lack body composition equipment and need a quick method to analyze the effectiveness of a weight control program in obese children and adolescents.

Glucose ingestion and substrate utilization during exercise in boys with IDDM.

This study was intended to compare exogenous [(13)C]glucose (Glu(exo)) oxidation in boys with insulin-dependent diabetes mellitus (IDDM) and healthy boys of similar age, weight, and maximal O(2) uptake. In a control trial with water intake (CT) and in a (13)C-enriched glucose trial (GT), subjects cycled for 60 min (58.8 +/- 0.9% maximal O(2) uptake) while the utilization of total glucose, total fat, and Glu(exo) was assessed. In CT, total glucose was 84.7 +/- 9.2 vs. 91.3 +/- 6.6 g/60 min (not significantly different) and total fat was 13.3 +/- 2.2 vs. 11.1 +/- 1.7 g/60 min (not significantly different) in IDDM vs. healthy boys, respectively. In GT, Glu(exo) was 10.4 +/- 1.7 vs. 14.8 +/- 1.1 g/60 min, corresponding to 9.0 +/- 1.0 vs. 12.4 +/- 0.5% of the total energy supply in IDDM and healthy boys, respectively (P < 0.05). Endogenous glucose was spared in both groups by 12.6 +/- 3.5% (P < 0.05). Blood glucose and plasma insulin concentrations were two- to threefold higher in IDDM vs. healthy boys in both trials. In conclusion, Glu(exo) is impaired in exercising boys with IDDM, even when plasma insulin levels are elevated.

Anaerobic performance in 5- to 7-yr-old children of low birthweight.

PURPOSE: This study was intended to determine whether anaerobic muscle performance is deficient in 5- to 7-yr-old children of extremely low birthweight (ELBW, 500-999 g) and very low birthweight (VLBW, 1000-1499 g). METHODS: Fourteen ELBW and 20 VLBW children were compared with 24 normal birthweight (NBW, >2500 g) term controls. Peak (PP) and mean (MP) muscle power were determined by the Wingate anaerobic test. Bioimpedance analysis and anthropometry were done to assess fat-free mass (FFM) and lean cross-sectional area of the thigh and calf. RESULTS: The ELBW group had significantly lower MP and PP, compared with the VLBW and, in particular, with the NBW group. This lower performance was apparent also when values were corrected for total body mass (MP) and FFM (MP and PP), but not when corrected for cross-sectional area of thigh and calf. CONCLUSION: The lower anaerobic muscle performance in ELBW children may be partly due to their smaller muscle mass, but may also reflect a low percentage of fast-twitch muscle fibers, low muscle phosphagen content, or deficiency in motor control.

Perceived exertion with glucose ingestion in adolescent males with IDDM.

PURPOSE: The rating of perceived exertion (RPE) is an indicator of exercise effort in adolescents that may be influenced by certain pediatric conditions. The purpose of this study was to determine the influence of insulin-dependent diabetes mellitus (IDDM) and glucose intake on RPE. METHODS: Eight male adolescents with IDDM and eight healthy controls of similar age, weight, and VO2peak cycled for 60 min at 60%VO2peak on two occasions spaced 1-4 wk apart. During a control trial (CT), subjects drank water, and in a glucose trial (GT), glucose at a rate of approximately 1.5 g x kg(-1) x h(-1). Heart rate, ventilation, and RPE (Borg 6-20 scale) were assessed at 5, 25, 35, and 55 min and blood glucose and lactate levels before and at 30 and 60 min. RESULTS: RPE in both trials was 15-25% higher in IDDM versus healthy subjects (F = 8.83; df = 1,14; eta-squared = 0.39; P = 0.01). In CT, it increased from 10.6 +/- 0.4 at 5 min to 15.2 +/- 0.6 at 55 min in IDDM and from 9.3 +/- 0.9 at 5 min to 13.0 +/- 0.8 at 55 min in healthy adolescents. In GT, RPE increased similarly to CT in the IDDM group but was 1-2 points lower in the healthy group. Blood glucose levels were 4.8 +/- 1.8 mmol x L(-1) and 1.8 +/- 0.4 mmol x L(-1) higher by the end of exercise in GT than in CT for the IDDM and healthy groups, respectively. There were no differences in heart rate, ventilation, or lactate levels between the groups or trials. CONCLUSIONS: For exercise performed at a similar moderate intensity, RPE in IDDM is higher by 2-3 points than in controls. Compared with water, glucose intake is associated with lower RPE in healthy, but not in IDDM, adolescents.

New and old in pediatric exercise physiology.

Researchers in pediatric exercise physiology sometimes overlook previously published findings that are similar, or even identical, to their own. The intent of this review is to provide examples for the above pattern. These examples relate to the following topics: 1) is the O2 uptake plateau a necessary criterion for the establishment of maximal O2 uptake in children? 2) children's greater utilization of fat during prolonged exercise, 3) children's lower maximal blood lactate levels, 4) the higher metabolic energy cost of locomotion in children and the possible causes for this phenomenon, and 5) the lower cardiac output and stroke volume in children, at any given O2 uptake.

Substrate utilization in boys during exercise with [13C]-glucose ingestion.

The influence of glucose ingestion on substrate utilization during prolonged exercise in children and adolescents is currently unknown. In the present study we determined the effect of intermittent exogenous glucose (GLUexo) ingestion on substrate utilization during prolonged exercise, in adolescent boys ages 13 17 years. Healthy untrained volunteers performed four 30-min exercise bouts on a cycle ergometer, separated by 5-min rest periods (approximately equal to 60% maximum O2 consumption), on two occasions spaced 1-4 weeks apart. Two trials were performed, a control trial (CT), in which subjects ingested water intermittently during the exercise, and a glucose trial (GT), in which subjects ingested a 13C-enriched GLUexo drink (approximately egual to glucose kg body mass(-1)), also intermittently during the exercise. Total free fatty acids (FATtotal), glucose (GLUtotal) and carbohydrate (CHOtotal) oxidation was determined from indirect calorimetry, while GLUexo oxidation was calculated from the 13C/12C ratio in expired air after 5-10 min and 25-30 min of exercise in each bout. Heart rate and rating of perceived exertion (RPE) were determined at the same time intervals. The oxidation of CHOtotal was 169.1 (12.9) g x 120 min(-1) and 203.1 (15.9) g x 120 min(-1) (P < 0.01) and that of FATtotal was 31.0 (4.2) g x 120 min(-1) and 17.1 (2.5) g x 120 min(-1) (P < 0.01) in CT and GT, respectively. GLUexo oxidation in GT was 57.8 (4.3) g x 120 min(-1), or 34.2 (2.2)% of that ingested. Endogenous glucose oxidation was 169.1 (12.9) g x 120 min(-1) and 145.3 (11.9) g x 120 min(-1) (P < 0.01) in CT and GT, respectively. Insulin and glucose concentrations were higher in GT than in CT by 226% and 37%, respectively (both P < 0.05). Free fatty acids and glycerol concentrations were lower in GT than in CT, by 27% and 79%, respectively (both P < 0.05). Heart rate was similar between trials, but RPE was lower in GT vs CT at both 115 and 135 min. Thus, under these experimental conditions, GLUexo intake spares endogenous carbohydrate and fat by 16% and 45%, respectively, contributes to approximately 25% of the total energy demand of exercise, and lowers the RPE.