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

Metabolic Flexibility During Exercise in Children with Overweight/Obesity Versus Children who are Lean

PURPOSE

This study examined metabolic flexibility with respect to fat metabolism during exercise in children who are lean (n=11; 10.9[0.9] y) and overweight/obese (OW/OB; n=9; 10.3[1.2] y).

METHOD

Participants were grouped based on body mass index percentiles for age and sex. Groups were mixed in age and sex. Participants completed two 20-minute exercise bouts on a cycle ergometer, separated by a 10-minute rest. Bout 1 consisted of 10 minutes at 50% VO2peak and 10 minutes at 75% VO2peak. Bout 2 was 20 minutes at 50% VO2peak. Absolute fat oxidation rate (FOR), FOR relative to body mass, FOR relative to fat-free mass, and proportional fat use were measured at 10 minutes of bout 1 and 5, 10, 15, and 20 minutes of bout 2.

RESULTS

Absolute FOR was higher in the OW/OB group (range: 117.8 [55.1]-206.2 [48.3] mg·min-1) than in the lean group (81.1 [32.2]-152.2 [38.2] mg·min-1); however, there were no significant main effects for group or significant interactions for proportional fat use, FOR relative to body mass, or FOR relative to fat-free mass.

CONCLUSION

Children in this age range who are overweight/obese do not display impaired metabolic flexibility with respect to fat metabolism during exercise.

Impact of slow versus rapid digesting carbohydrates on substrate oxidation in pre-pubertal children: A randomized crossover trial

BACKGROUND & AIMS

Consumption of rapid digesting sugars by children are under increased scrutiny because of their contribution to unhealthy weight gain. Previous studies in adults and children have suggested that altering the blend of carbohydrates (CHOs) consumed may cause shifts in substrate utilization. The purpose of this study was to examine the effects of consuming a slow digesting carbohydrate (SDC) and rapid digesting carbohydrate (RDC) on CHO and fat oxidation, glucose, and insulin responses at rest, during exercise, and post-exercise rest in pre-pubescent children.

METHODS

A randomized, double-blind, crossover design was used. Nineteen pre-pubescent children (n = 10 boys, n = 9 girls, mean ± standard error, age = 9.84 ± 0.37-yrs) participated. Visits to the laboratory began with a 30-min measurement of resting metabolism followed by consumption of either an RDC or SDC drink. Postprandial resting metabolism was recorded for 60-min, immediately followed by 60-min of submaximal cycling exercise while metabolism was recorded, which was immediately followed by another 60-min recording of post-exercise metabolism. Total CHO and fat oxidation, endogenous and exogenous CHO oxidation, blood glucose, and insulin were assessed.

RESULTS

Total CHO oxidation rate (g∙min^-1) was greater after the RDC drink at 60 min (p = 0.032). Endogenous CHO oxidation rate (g∙min^-1) was greater after the SDC drink at 15 min (p ≤ 0.010). Cumulative endogenous CHO oxidation (g) was greater after the SDC drink at 45 min (p = 0.009). Endogenous CHO oxidation accounted for a greater proportion of substrate oxidation after the first 60-min rest period (p = 0.028), while exogenous CHO oxidation accounted for a greater proportion of substrate oxidation for the RDC at all time points (p ≤ 0.019).

CONCLUSIONS

The present study provides novel data suggesting that an SDC promotes greater endogenous substrate utilization in pre-pubertal children, which may have beneficial health impacts on energy intake and carbohydrate regulation/metabolism during growth and development.

CLINICAL TRIALS REGISTRY NUMBER

NCT03185884, clinicaltrials.gov.

Carbohydrate Drink Use During 30 Minutes of Variable-Intensity Exercise Has No Effect on Exercise Performance in Premenarchal Girls

PURPOSE

This study examined the physiological, perceptual, and performance effects of a 6% carbohydrate (CHO) drink during variable-intensity exercise (VIE) and a postexercise test in premenarchal girls.

METHODS

A total of 10 girls (10.4 [0.7] y) participated in the study. VO2peak was assessed, and the girls were familiarized with VIE and performance during the first visit. The trial order (CHO and placebo) was randomly assigned for subsequent visits. The drinks were given before VIE bouts and 1-minute performance (9 mL/kg total). Two 15-minute bouts of VIE were completed (10 repeated sequences of 20%, 55%, and 95% power at VO2peak and maximal sprints) before a 1-minute performance sprint.

RESULTS

The mean power, peak power, heart rate (HR), %HRpeak, and rating of perceived exertion during VIE did not differ between trials. However, the peak power decreased, and the rating of perceived exertion increased from the first to the second bout. During the 1-minute performance, there were no differences between the trial (CHO vs placebo) for HR (190 [9] vs 189 [9] bpm), %HRpeak (97.0% [3.2%] vs 96.6% [3.0%]), rating of perceived exertion (7.8 [2.3] vs 8.1 [1.9]), peak power (238 [70] vs 235 [60] W), fatigue index (54.7% [10.0%] vs 55.9% [12.8%]), or total work (9.4 [2.6] vs 9.4 [2.1] kJ).

CONCLUSION

CHO supplementation did not alter physiological, perceptual, or performance responses during 30 minutes of VIE or postexercise sprint performance in premenarchal girls.

Carbohydrate supplementation: a critical review of recent innovations

PURPOSE

To critically examine the research on novel supplements and strategies designed to enhance carbohydrate delivery and/or availability.

METHODS

Narrative review.

RESULTS

Available data would suggest that there are varying levels of effectiveness based on the supplement/supplementation strategy in question and mechanism of action. Novel carbohydrate supplements including multiple transportable carbohydrate (MTC), modified carbohydrate (MC), and hydrogels (HGEL) have been generally effective at modifying gastric emptying and/or intestinal absorption. Moreover, these effects often correlate with altered fuel utilization patterns and/or glycogen storage. Nevertheless, performance effects differ widely based on supplement and study design. MTC consistently enhances performance, but the magnitude of the effect is yet to be fully elucidated. MC and HGEL seem unlikely to be beneficial when compared to supplementation strategies that align with current sport nutrition recommendations. Combining carbohydrate with other ergogenic substances may, in some cases, result in additive or synergistic effects on metabolism and/or performance; however, data are often lacking and results vary based on the quantity, timing, and inter-individual responses to different treatments. Altering dietary carbohydrate intake likely influences absorption, oxidation, and and/or storage of acutely ingested carbohydrate, but how this affects the ergogenicity of carbohydrate is still mostly unknown.

CONCLUSIONS

In conclusion, novel carbohydrate supplements and strategies alter carbohydrate delivery through various mechanisms. However, more research is needed to determine if/when interventions are ergogenic based on different contexts, populations, and applications.

TNF blockade contributes to restore lipid oxidation during exercise in children with juvenile idiopathic arthritis

BACKGROUND

Children with juvenile idiopathic arthritis (JIA) have impaired physical abilities. TNF-α plays a crucial role in this pathogenesis, but it is also involved in the use of lipids and muscle health. Objective of this study was to explore substrate oxidation and impact of TNF blockade on energy metabolism in children with JIA as compared to healthy children.

METHODS

Fifteen non-TNF-blockaded and 15 TNF-blockaded children with JIA and 15 healthy controls were matched by sex, age, and Tanner stage. Participants completed a submaximal incremental exercise test on ergocycle to determine fat and carbohydrate oxidation rates by indirect calorimetry.

RESULTS

The maximal fat oxidation rate during exercise was lower in JIA children untreated by TNF blockade (134.3 ± 45.2 mg.min- 1) when compared to the controls (225.3 ± 92.9 mg.min- 1, p = 0.007); but was higher in JIA children under TNF blockade (163.2 ± 59.0 mg.min- 1, p = 0.31) when compared to JIA children untreated by TNF blockade. At the same relative exercise intensities, there was no difference in carbohydrate oxidation rate between three groups.

CONCLUSIONS

Lipid metabolism during exercise was found to be impaired in children with JIA. However, TNF treatment seems to improve the fat oxidation rate in this population.

TRIAL REGISTRATION

In ClinicalTrials.gov, reference number NCT02977416 , registered on 30 November 2016.

Carbohydrate consumption and variable-intensity exercise responses in boys and men

PURPOSE

The effect of carbohydrate (CHO) supplementation on physiological and perceptual responses to steady-state exercise has been studied in children. However, little is known about these responses to variable-intensity exercise (VIE) and how these responses might differ from adults. This study examined the physiological and perceptual effects of CHO on VIE in boys and men.

METHODS

Eight boys (11.1 ± 0.9 years) and 11 men (23.8 ± 2.1 years) consumed CHO or a placebo (PL) beverage before and throughout VIE (three 12-min cycling bouts with intensity varying every 20-30 s between 25, 50, 75, and 125% peak work rate). Pulmonary gas exchange was assessed during the second 12-min bout. RPE was assessed twice per bout.

RESULTS

In CHO, blood glucose increased and then decreased more from pre-exercise to 12 min and was higher in this trial at the end of exercise in men versus boys. In boys, blood glucose in CHO was higher at 24 and 36 min of exercise than in PL. RER during the CHO trial was higher in both groups; the other physiological responses were unaffected by CHO. All RPE measures (whole body, legs and chest) increased over time, but were not different between groups or trials.

CONCLUSION

Blood glucose patterns during VIE were differentially affected by CHO in boys and men, but most physiological and perceptual responses to VIE were unaffected by CHO in either group. Knowledge of the underlying mechanisms of glucose regulation and effects on physical performance during this type of exercise in children is warranted.

Validity and reliability of a novel metabolic flexibility test in children with obesity

Existing methods for diagnosing diabetes and for identifying risk of diabetes development are completed under resting conditions and based on adult data. Studying additional methods to identify metabolic risk in children is warranted. Our objective was to examine the validity and reliability of a metabolic flexibility (MetFlex) test for screening glycemia and insulin resistance (IR) in children. We hypothesized higher MetFlex during exercise would be correlated with lower fasting glucose and homeostasis model assessment of insulin resistance (HOMA-IR) and higher whole body insulin sensitivity index (WBISI) and insulin secretion-sensitivity index-2 (ISSI-2). Thirty-four children with obesity (14 boys, 20 girls) attended two visits. At visit 1, an oral glucose tolerance test (OGTT) was followed by anthropometric and aerobic fitness (V̇o_2max) assessments. Insulin and glucose during the OGTT were used to calculate HOMA-IR, WBISI, and ISSI-2. At visit 2, a ¹³C-enriched carbohydrate drink was ingested before 60 min of exercise at 45% V̇o_2max. Breath measurements were collected to calculate area under the curve exogenous carbohydrate to measure MetFlex. Pearson's r correlation showed no significant association between MetFlex during exercise with fasting glucose ( r = -0.288, P = 0.110). MetFlex was associated with log-HOMA-IR ( r = -0.597, P = 0.024), log-WBISI ( r = 0.575, P = 0.051), and log-ISSI-2 ( r = 0.605, P = 0.037) in boys but not girls. When repeated ( n = 18), MetFlex was deemed a reliable test (intraclass correlation coefficient = 0.692). MetFlex during exercise was negatively associated with IR and β-cell function in boys. Further research is required to explore clinical utility of the MetFlex test and explain the lack of association in girls. NEW & NOTEWORTHY This study is the first to investigate the validity and reliability of a novel noninvasive metabolic flexibility (MetFlex) test for identifying insulin resistance in children with obesity. MetFlex was measured during exercise using [¹³C]glucose stable isotope methodology. Findings showed that MetFlex was negatively associated with insulin resistance in boys but not in girls with obesity. Future work is required to investigate these sex differences. MetFlex test results were deemed reliable when repeated on a separate day.

Physical Activity Duration but Not Energy Expenditure Differs between Daily and Intermittent Breakfast Consumption in Adolescent Girls: A Randomized Crossover Trial

Background

It is not known whether breakfast frequency affects physical activity (PA) in children or adolescents.

Objective

This study examined the effect of daily compared with intermittent breakfast consumption on estimated PA energy expenditure (PAEE) in adolescent girls.

Methods

Under a randomized crossover design, 27 girls [mean ± SD age: 12.4 ± 0.5 y, body mass index (in kg/m2): 19.3 ± 3.0] completed two 7-d conditions. A standardized breakfast (∼1674 kJ) was consumed every day before 0900 in the daily breakfast consumption (DBC) condition. The standardized breakfast was consumed on only 3 d before 0900 in the intermittent breakfast consumption (IBC) condition, alternating with breakfast omission on the remaining 4 d (i.e., only water consumed before 1030). Combined heart rate accelerometry was used to estimate PAEE throughout each condition. Statistical analyses were completed by using condition × time of day repeated-measures ANOVA. The primary outcome was PAEE and the secondary outcome was time spent in PA.

Results

Daily estimated PAEE from sedentary or light-, moderate-, or vigorous-intensity PA and total PAEE were not significantly different between the conditions. The condition × time of day interaction for sedentary time (P = 0.05) indicated that the girls spent an additional 11.5 min/d being sedentary in the IBC condition compared with the DBC condition from 1530 to bedtime (P = 0.04). Light PA was 19.8 min/d longer in the DBC condition than in the IBC condition (P = 0.05), which was accumulated from waking to 1030 (P = 0.04) and from 1530 to bedtime (P = 0.03). There were no significant differences in time spent in moderate PA or vigorous PA between the conditions.

Conclusions

Adolescent girls spent more time in light PA before 1030 and after school and spent less time sedentary after school when a standardized breakfast was consumed daily than when consumed intermittently across 7 d. However, breakfast manipulation did not affect estimated daily PAEE. This trial was registered at www.isrctn.com as ISRCTN74579070.

Energy substrate utilization with and without exogenous carbohydrate intake in boys and men exercising in the heat

Little is known about energy yield during exercise in the heat in boys compared with men. To investigate substrate utilization with and without exogenous carbohydrate (CHOexo) intake, seven boys [11.2 ± 0.2 (SE) yr] and nine men (24.0 ± 1.1 yr) cycled (4 × 20-min bouts) at a fixed metabolic heat production ( Ḣ p) per unit body mass (6 W/kg) in a climate chamber (38°C and 50% relative humidity), on two occasions. Participants consumed a 13C-enriched 8% CHO beverage (CARB) or placebo beverage (CONT) in a double-blinded, counterbalanced manner. Substrate utilization was calculated for the last 60 min of exercise. CHOexo oxidation rate (2.0 ± 0.3 vs. 2.5 ± 0.2 mg·kg fat-free mass−1·min−1, P = 0.02) and CHOexo oxidation efficiency (12.8 ± 0.6 vs. 16.0 ± 0.9%, P = 0.01) were lower in boys compared with men exercising in the heat. Total carbohydrate (CHOtotal), endogenous CHO (CHOendo), and total fat (Fattotal) remained stable in boys and men ( P > 0.05) during CARB, whereas CHOtotal oxidation rate decreased ( P < 0.001) and Fattotal oxidation rate increased over time similarly in boys and men during CONT ( P < 0.001). The relative contribution of CHOexo to total energy yield increased over time in both groups ( P < 0.001). In conclusion, endogenous substrate metabolism and the relative contribution of fuels to total energy yield were not different between groups. The ingestion of a CHO beverage during exercise in the heat may be as beneficial for boys as men to spare endogenous substrate.

Optimal composition of fluid-replacement beverages

The objective of this article is to provide a review of the fundamental aspects of body fluid balance and the physiological consequences of water imbalances, as well as discuss considerations for the optimal composition of a fluid replacement beverage across a broad range of applications. Early pioneering research involving fluid replacement in persons suffering from diarrheal disease and in military, occupational, and athlete populations incurring exercise- and/or heat-induced sweat losses has provided much of the insight regarding basic principles on beverage palatability, voluntary fluid intake, fluid absorption, and fluid retention. We review this work and also discuss more recent advances in the understanding of fluid replacement as it applies to various populations (military, athletes, occupational, men, women, children, and older adults) and situations (pathophysiological factors, spaceflight, bed rest, long plane flights, heat stress, altitude/cold exposure, and recreational exercise). We discuss how beverage carbohydrate and electrolytes impact fluid replacement. We also discuss nutrients and compounds that are often included in fluid-replacement beverages to augment physiological functions unrelated to hydration, such as the provision of energy. The optimal composition of a fluid-replacement beverage depends upon the source of the fluid loss, whether from sweat, urine, respiration, or diarrhea/vomiting. It is also apparent that the optimal fluid-replacement beverage is one that is customized according to specific physiological needs, environmental conditions, desired benefits, and individual characteristics and taste preferences.

Effect of fat- and carbohydrate-rich diets on metabolism and running performance in trained adolescent boys

OBJECTIVES

A randomized crossover trial was designed to analyze the impact of a short-term, isoenergetic fat-rich or carbohydrate (CHO)-rich diet on substrate oxidation rates during submaximal exercise and on performance in a 10,000-m running time trial in trained, mid- to late-pubertal boys.

METHODS

An incremental test was performed to determine the peak oxygen uptake (VO2peak). After 2 days on a fat-rich (24.2% ± 0.8% CHO, 60.4% ± 0.3% fat, and 15.5% ± 1.0% protein), CHO-rich (69.3% ± 1.2% CHO, 15.9% ± 2.1% fat, and 15.1% ± 1.1% protein), or habitual (56.1% ± 7.0% CHO, 27.5% ± 4.9% fat, and 16.5% ± 4.0% protein) diet, 19 trained adolescent boys (15.2 ± 1.5 years) performed a 10-minute constant run at 65% VO2peak to determine the respiratory exchange ratio (RER) during exercise and 10,000-m running on an outdoor track.

RESULTS

During the constant run, the RER and CHO contribution to energy expenditure were lower, and fat contribution higher, in the fat-rich diet than in the CHO-rich diet (P < 0.05), but the results were not different from those of the habitual diet. Performance in the 10,000-m run after consuming CHO- and fat-rich diets was similar to performance after a habitual diet (50.0 ± 7.0, 51.9 ± 8.3, and 50.9 ± 7.4 minutes, respectively), but consuming a CHO-rich diet enhanced performance compared with that after a fat-rich diet (P = 0.03).

CONCLUSIONS

These findings indicate that a CHO-rich diet provides additional benefits to 10,000-m running performance in trained adolescent boys compared with a fat-rich diet.

Carbohydrate supplementation and prolonged intermittent high-intensity exercise in adolescents: research findings, ethical issues and suggestions for the future

In the last decade, research has begun to investigate the efficacy of carbohydrate supplementation for improving aspects of physical capacity and skill performance during sport-specific exercise in adolescent team games players. This research remains in its infancy, and further study would be beneficial considering the large youth population actively involved in team games. Literature on the influence of carbohydrate supplementation on skill performance is scarce, limited to shooting accuracy in adolescent basketball players and conflicting in its findings. Between-study differences in the exercise protocol, volume of fluid and carbohydrate consumed, use of prior fatiguing exercise and timing of skill tests may contribute to the different findings. Conversely, initial data supports carbohydrate supplementation in solution and gel form for improving intermittent endurance running capacity following soccer-specific shuttle running. These studies produced reliable data, but were subject to limitations including lack of quantification of the metabolic response of participants, limited generalization of data due to narrow participant age and maturation ranges, use of males and females within the same sample and non-standardized pre-exercise nutritional status between participants. There is a lack of consensus regarding the influence of frequently consuming carbohydrate-containing products on tooth enamel erosion and the development of obesity or being overweight in adolescent athletes and non-athletes. These discrepancies mean that the initiation or exacerbation of health issues due to frequent consumption of carbohydrate-containing products by adolescents cannot be conclusively refuted. Coupled with the knowledge that consuming a natural, high-carbohydrate diet -3-8 hours before exercise can significantly alter substrate use and improve exercise performance in adults, a moral and ethical concern is raised regarding the direction of future research in order to further knowledge while safeguarding the health and well-being of young participants. It could be deemed unethical to continue study into carbohydrate supplementation while ignoring the potential health concerns and the possibility of generating similar performance enhancements using natural dietary interventions. Therefore, future work should investigate the influence of pre-exercise dietary intake on the prolonged intermittent, high-intensity exercise performance of adolescents. This would enable quantification of whether pre-exercise nutrition can modulate exercise performance and, if so, the optimum dietary composition to achieve this. Research could then combine this knowledge with ingestion of carbohydrate-containing products during exercise to facilitate ethical and healthy nutritional guidelines for enhancing the exercise performance of adolescents. This article addresses the available evidence regarding carbohydrate supplementation and prolonged intermittent, high-intensity exercise in adolescent team games players. It discusses the potential health concerns associated with the frequent use of carbohydrate-containing products by adolescents and how this affects the research ethics of the field, and considers directions for future work.

Carbohydrate ingestion during team games exercise: current knowledge and areas for future investigation

There is a growing body of research on the influence of ingesting carbohydrate-electrolyte solutions immediately prior to and during prolonged intermittent, high-intensity exercise (team games exercise) designed to replicate field-based team games. This review presents the current body of knowledge in this area, and identifies avenues of further research. Almost all early work supported the ingestion of carbohydrate-electrolyte solutions during prolonged intermittent exercise, but was subject to methodological limitations. A key concern was the use of exercise protocols characterized by prolonged periods at the same exercise intensity, the lack of maximal- or high-intensity work components and long periods of seated recovery, which failed to replicate the activity pattern or physiological demand of team games exercise. The advent of protocols specifically designed to replicate the demands of field-based team games enabled a more externally valid assessment of the influence of carbohydrate ingestion during this form of exercise. Once again, the research overwhelmingly supports carbohydrate ingestion immediately prior to and during team games exercise for improving time to exhaustion during intermittent running. While the external validity of exhaustive exercise at fixed prescribed intensities as an assessment of exercise capacity during team games may appear questionable, these assessments should perhaps not be viewed as exhaustive exercise tests per se, but as indicators of the ability to maintain high-intensity exercise, which is a recognized marker of performance and fatigue during field-based team games. Possible mechanisms of exercise capacity enhancement include sparing of muscle glycogen, glycogen resynthesis during low-intensity exercise periods and attenuated effort perception during exercise. Most research fails to show improvements in sprint performance during team games exercise with carbohydrate ingestion, perhaps due to the lack of influence of carbohydrate on sprint performance when endogenous muscle glycogen concentration remains above a critical threshold of ∼200  mmol/kg dry weight. Despite the increasing number of publications in this area, few studies have attempted to drive the research base forward by investigating potential modulators of carbohydrate efficacy during team games exercise, preventing the formulation of optimal carbohydrate intake guidelines. Potential modulators may be different from those during prolonged steady-state exercise due to the constantly changing exercise intensity and frequency, duration and intensity of rest intervals, potential for team games exercise to slow the rate of gastric emptying and the restricted access to carbohydrate-electrolyte solutions during many team games. This review highlights fluid volume, carbohydrate concentration, carbohydrate composition and solution osmolality; the glycaemic index of pre-exercise meals; fluid and carbohydrate ingestion patterns; fluid temperature; carbohydrate mouthwashes; carbohydrate supplementation in different ambient temperatures; and investigation of all of these areas in different subject populations as important avenues for future research to enable a more comprehensive understanding of carbohydrate ingestion during team games exercise.

Carbohydrate intake reduces fat oxidation during exercise in obese boys

The recent surge in childhood obesity has renewed interest in studying exercise as a therapeutic means of metabolizing fat. However, carbohydrate (CHO) intake attenuates whole body fat oxidation during exercise in healthy children and may suppress fat metabolism in obese youth. To determine the impact of CHO intake on substrate utilization during submaximal exercise in obese boys, seven obese boys (mean age: 11.4 ± 1.0 year; % body fat: 35.8 ± 3.9%) performed 60 min of exercise at an intensity that approximated maximal fat oxidation. A CHO drink (CARB) or a placebo drink (CONT) was consumed in a double-blinded, counterbalanced manner. Rates of total fat, total CHO, and exogenous CHO (CHO(exo)) oxidation were calculated for the last 20 min of exercise. During CONT, fat oxidation rate was 3.9 ± 2.4 mg × kg fat-free mass (FFM)(-1 )× min(-1), representing 43.1 ± 22.9% of total energy expenditure (EE). During CARB, fat oxidation was lowered (p = 0.02) to 1.7 ± 0.6 mg × kg FFM(-1 )× min(-1), contributing to 19.8 ± 4.9% EE. Total CHO oxidation rate was 17.2 ± 3.1 mg × kg FFM(-1 )× min(-1) and 13.2 ± 6.1 mg × kg FFM(-1) × min(-1) during CARB and CONT, respectively (p = 0.06). In CARB, CHO(exo) oxidation contributed to 23.3 ± 4.2% of total EE. CHO intake markedly suppresses fat oxidation during exercise in obese boys.

Health implications of fructose consumption: A review of recent data

This paper reviews evidence in the context of current research linking dietary fructose to health risk markers.Fructose intake has recently received considerable media attention, most of which has been negative. The assertion has been that dietary fructose is less satiating and more lipogenic than other sugars. However, no fully relevant data have been presented to account for a direct link between dietary fructose intake and health risk markers such as obesity, triglyceride accumulation and insulin resistance in humans. First: a re-evaluation of published epidemiological studies concerning the consumption of dietary fructose or mainly high fructose corn syrup shows that most of such studies have been cross-sectional or based on passive inaccurate surveillance, especially in children and adolescents, and thus have not established direct causal links. Second: research evidence of the short or acute term satiating power or increasing food intake after fructose consumption as compared to that resulting from normal patterns of sugar consumption, such as sucrose, remains inconclusive. Third: the results of longer-term intervention studies depend mainly on the type of sugar used for comparison. Typically aspartame, glucose, or sucrose is used and no negative effects are found when sucrose is used as a control group.Negative conclusions have been drawn from studies in rodents or in humans attempting to elucidate the mechanisms and biological pathways underlying fructose consumption by using unrealistically high fructose amounts.The issue of dietary fructose and health is linked to the quantity consumed, which is the same issue for any macro- or micro nutrients. It has been considered that moderate fructose consumption of ≤50g/day or ~10% of energy has no deleterious effect on lipid and glucose control and of ≤100g/day does not influence body weight. No fully relevant data account for a direct link between moderate dietary fructose intake and health risk markers.

Ingesting a 6% carbohydrate-electrolyte solution improves endurance capacity, but not sprint performance, during intermittent, high-intensity shuttle running in adolescent team games players aged 12-14 years

The main aim of this study was to investigate the influence of consuming a 6% carbohydrate-electrolyte (CHO-E) solution on the intermittent, high-intensity endurance performance and capacity of adolescent team games players. Fifteen participants (mean age 12.7 +/- 0.8 years) performed two trials separated by 3-7 days. In each trial, they completed 60 min of exercise composed of four 15-min periods of part A of the Loughborough Intermittent Shuttle Test, followed by an intermittent run to exhaustion (part B). In a double-blind, randomised, counterbalanced fashion participants consumed either the 6% CHO-E solution or a non-carbohydrate (CHO) placebo (5 ml kg(-1) BM) during the 5 min pre-trial and after each 15-min period of part A (2 ml kg(-1) BM). Time to fatigue was increased by 24.4% during part B when CHO was ingested (5.1 +/- 1.8 vs. 4.1 +/- 1.6 min, P < 0.05), with distance covered in part B also significantly greater in the CHO trial (851 +/- 365 vs. 694 +/- 278 m, P < 0.05). No significant between-trials differences were observed for mean 15-m sprint time (P = 0.35), peak sprint time (P = 0.77), or heart rate (P = 0.08) during part A. These results demonstrate, for the first time, that ingestion of a CHO-E solution significantly improves the intermittent, high-intensity endurance running capacity of adolescent team games players during an exercise protocol designed to simulate the physiological demands of team games.

Fructose modifies the hormonal response and modulates lipid metabolism during aerobic exercise after glucose supplementation

The metabolic response when aerobic exercise is performed after the ingestion of glucose plus fructose is unclear. In the present study, we administered two beverages containing GluF (glucose+fructose) or Glu (glucose alone) in a randomized cross-over design to 20 healthy aerobically trained volunteers to compare the hormonal and lipid responses provoked during aerobic exercise and the recovery phase. After ingesting the beverages and a 15-min resting period, volunteers performed 30 min of moderate aerobic exercise. Urinary and blood samples were taken at baseline (t(-15)), during the exercise (t(0), t(15) and t(30)) and during the recovery phase (t(45), t(75) and t(105)). Plasma insulin concentrations were higher halfway through the exercise period and during acute recuperation (t(15) and t(75); P<0.05) following ingestion of GluF than after Glu alone, without any differences between the effects of either intervention on plasma glucose concentrations. Towards the end of the exercise period, urinary catecholamine concentrations were lower following GluF (t(45); P<0.05). Plasma triacylglycerol (triglyceride) concentrations were higher after the ingestion of GluF compared with Glu (t(15), t(30), t(45) and t(105); P<0.05). Furthermore, with GluF, we observed higher levels of lipoperoxides (t(15), t(30), t(45) and t(105); P<0.05) and oxidized LDL (low-density lipoprotein; t(30); P<0.05) compared with after the ingestion of Glu alone. In conclusion, hormonal and lipid alterations are provoked during aerobic exercise and recovery by the addition of a dose of fructose to the pre-exercise ingestion of glucose.

Fat oxidation rate and the exercise intensity that elicits maximal fat oxidation decreases with pubertal status in young male subjects

The range of exercise intensities that elicit high fat oxidation rates (FOR) in youth and the influence of pubertal status on peak FOR are unknown. In a longitudinal design, we compared FOR over a range of exercise intensities in a small cohort of developing prepubertal male subjects. Five boys all at Tanner stage 1 (ages 11-12 yr) and nine men (ages 20-26 yr) underwent an incremental cycle ergometry test to volitional exhaustion. FOR curves were determined from indirect calorimetry during the final 30 s of each increment. The same protocol was duplicated annually in the boys as they progressed through puberty. The peak FOR was considerably higher (P<0.05) in boys at Tanner 1 (8.6+/-1.5 mg.kg lean body mass(-1).min(-1)) (mean+/-SD) compared with men (4.2+/-1.1 mg.kg lean body mass(-1).min(-1)). FOR dropped as boys developed through puberty (Tanner 2/3 peak rate=7.6+/-0.6 mg.kg lean body mass(-1).min(-1); Tanner 4 peak rate=5.4+/-1.8 mg.kg lean body mass(-1).min(-1), main effect of Tanner stage; P<0.05) to the levels found in men (not significant). The exercise intensity that elicited peak FOR was higher in the boys at Tanner 1 [56+/-6% peak aerobic power (VO2 peak)] than in men (31+/-4% VO2 peak) (P<0.001). This value tended to decrease by Tanner stage 4 (45+/-10% VO2 peak, main effect of Tanner stage; P=0.06). We conclude that, compared with men, prepubertal boys have higher relative FOR throughout a wide range of exercise intensities and that FOR drops as boys develop through puberty.

Fat and carbohydrate metabolism during submaximal exercise in children

During exercise, the contribution of fat and carbohydrate to energy expenditure is largely modulated by the intensity of exercise. Age, a short- or long-term diet enriched in carbohydrate or fat substrate stores, training and gender are other factors that have also been found to affect this balance. These factors have been extensively studied in adults from the perspective of improving performance in athletes, or from a health perspective in people with diseases. During the last decade, lifestyle changes associated with high-energy diets rich in lipid and reduced physical activity have contributed to the increase in childhood obesity. This lifestyle change has emerged as a serious health problem favouring the early development of cardiovascular diseases, insulin resistance or type 2 diabetes mellitus. Increasing physical activity levels in young people is important to increase energy expenditure and promote muscle oxidative capacity. Therefore, it is surprising that the regulation of balance between carbohydrate and lipid use during exercise has received much less attention in children than in adults. In this review, we have focused on the factors that affect carbohydrate and lipid metabolism during exercise and have identified areas that may be relevant in explaining the higher contribution of lipid to energy expenditure in children when compared with adults. Low muscle glycogen content is possibly associated with a low activity of glycolytic enzymes and high oxidative capacity, while lower levels of sympathoadrenal hormones are likely to favour lipid metabolism in children. Changes in energetic metabolism occurring during adolescence are also dependent on pubertal events with an increase in testosterone in boys and estrogen and progesterone in girls. The profound effects of ovarian hormones on carbohydrate and fat metabolism along with their effects on oxidative enzymes could explain that differences in substrate metabolism have not always been observed between girls and women. Finally, although the regulatory mechanisms of fat and carbohydrate balance during exercise are quite well identified, there are a lack of data specific to children and most of the evidences reported in this review were drawn from studies in adults. Isotope tracer techniques and nuclear magnetic resonance will allow non-invasive investigation of the metabolism of the different substrate sources in skeletal muscle.

The endocrine response and substrate utilization during exercise in children and adolescents

Adolescence is a time of rapid growth caused by significant changes in hormone levels. For many, it is also a time of increased physical activity and sport that places a large demand on energy reserves. Exercise is known to cause perturbations in endocrine and metabolic systems in children and adolescents, yet careful characterization of these responses is only now being conducted. It does not appear that prepubertal youth have a different muscle composition than adults. However, these youth do have a lower anaerobic capacity and a greater reliance on aerobic metabolism during activity. Prepubertal adolescents may have an immature glucose regulatory system that influences glycemic regulation at the onset of moderate exercise. During heavy exercise, muscle and blood lactate levels are lower in children than in adults and there is a greater reliance on fat as fuel. The exercise intensity that causes maximal fat oxidation rate and the relative rate of fat oxidation decreases as adolescents develop through puberty. The mechanism for the attenuated lipid utilization with the advancement of puberty, and the impact that this may have on body composition, are unknown. Surprisingly, prepubertal adolescents have relatively high rates of exogenous glucose oxidation, perhaps because of their smaller endogenous carbohydrate reserves. Further study is needed to determine the optimal exogenous carbohydrate feeding regimen for peak performance in adolescence. Studies are also needed to determine whether physical activity, at an intensity targeted to maximize fat oxidation, help to lower body adiposity in overweight youth.

Energy substrate utilization during prolonged exercise with and without carbohydrate intake in preadolescent and adolescent girls

Little information is available on energy metabolism during exercise in girls, particularly the contribution of exogenous carbohydrate (CHOexo). The purpose of this study was to determine substrate utilization during exercise with and without CHOexo intake in healthy girls. Twelve-yr-old preadolescent (YG; n = 12) and 14-yr-old adolescent (OG; n = 10) girls consumed flavored water (WT) or 13C-enriched 6% CHO (CT) while cycling for 60 min at ∼70% maximal aerobic power (V̇o2max). Substrate utilization was calculated for the final 15 min of exercise. CHOexo decreased fat oxidation by ∼50% in YG but not in OG ( P < 0.001) and decreased endogenous CHO oxidation by ∼15% in OG but not in YG ( P = 0.006). Endogenous CHO oxidation was lower in YG than in OG regardless of trial ( P ≤ 0.01), whereas fat oxidation was higher in YG only during WT ( P < 0.001). CHOexo oxidation rate was similar between YG and OG (7.1 ± 0.5 and 6.8 ± 0.4 mg·kg−1·min−1, respectively, P = 0.67), contributing ∼19% to total energy expenditure. Serum estradiol levels in all girls correlated with fat ( r = −0.50 to −0.59, P = 0.03 to 0.005) and endogenous CHO oxidation ( r = 0.50 to 0.63, P = 0.03 to 0.005) but not with CHOexo oxidation ( r = −0.09, P = 0.71). We conclude that CHOexo influences endogenous substrate utilization in an age-dependent manner in healthy girls but that total CHOexo oxidation during exercise is not different between YG and OG. Our results also point to potential sex-related differences in energy substrate utilization even during childhood.

Relative fat oxidation is higher in children than adults

BACKGROUND

Prepubescent children may oxidize fatty acids more readily than adults. Therefore, dietary fat needs would be higher for children compared with adults. The dietary fat recommendations are higher for children 4 to 18 yrs (i.e., 25 to 35% of energy) compared with adults (i.e., 20 to 35% of energy). Despite this, many parents and children restrict dietary fat for health reasons.

METHODS

This study assessed whether rates of fat oxidation are similar between prepubescent children and adults. Ten children (8.7 +/- 1.4 yr, 33 +/- 13 kg mean +/- SD) in Tanner stage 1 and 10 adults (41.6 +/- 8 yr, 74 +/- 13 kg) were fed a weight maintenance diet for three days to maintain body weight and to establish a consistent background for metabolic rate measurements (all foods provided). Metabolic rate was measured on three separate occasions before and immediately after breakfast and for 9 hrs using a hood system (twice) or a room calorimeter (once) where continuous metabolic measurements were taken.

RESULTS

During all three sessions whole body fat oxidation was higher in children (lower RQ) compared to adults (mean RQ= 0.84 +/- .016 for children and 0.87 +/- .02, for adults, p < 0.02). Although, total grams of fat oxidized was similar in children (62.7 +/- 20 g/24 hrs) compared to adults (51.4 +/- 19 g/24 hrs), the grams of fat oxidized relative to calorie expenditure was higher in children (0.047 +/- .01 g/kcal, compared to adults (0.032 +/- .01 p < 0.02). Females oxidized more fat relative to calorie expenditure than males of a similar age. A two way ANOVA showed no interaction between gender and age in terms of fat oxidation.

CONCLUSION

These data suggest that fat oxidation relative to total calorie expenditure is higher in prepubescent children than in adults. Consistent with current dietary guidelines, a moderate fat diet is appropriate for children within the context of a diet that meets their energy and nutrient needs.

Influence of age and pubertal status on substrate utilization during exercise with and without carbohydrate intake in healthy boys

Substrate utilization during exercise is known to differ between children and adults, but whether these differences are related to pubertal status is unclear. The objective of this study was to investigate the effects of pubertal status on endogenous (CHOendo) and orally ingested exogenous (CHOexo) carbohydrate and fat oxidation rates during exercise. Twenty boys at the same chronological age (12 y) were divided into three pubertal groups (pre-pubertal, PP: n = 7; early-pubertal, EP: n = 7; mid- to late-pubertal, M-LP: n = 6) and consumed either a placebo or13C-enriched 6% CHO drink while cycling for 60 min at ~70% of their maximal aerobic power (VO2 max). Another group of 14-year-old boys (pubertal, n = 9) completed all procedures. Substrate utilization was calculated for the final 15 min of exercise using indirect calorimetry and stable isotope methodology. CHOexodecreased fat (p < 0.001) and increased total CHO (p < 0.001) oxidation, irrespective of group. Fat oxidation was higher (p = 0.01) in younger boys than in older boys, but similar (p ≥ 0.33) among PP, EP, and M-LP boys. CHOexocontributed to ~30% of energy expenditure (EE) in PP and EP, but to only 24% in M-LP (p = 0.02), which was identical to the older boys (24%). CHOexooxidation rate as a percentage of EE was inversely related to testosterone levels (r = −0.51, p = 0.005, n = 29). It was concluded that reliance on CHOexoduring exercise is particularly sensitive to pubertal status, with the highest oxidation rates observed in pre- and early-pubertal boys, independent of chronological age.

Reducing the risk of heat-related decrements to physical activity in young people

The purpose of this review is to highlight differences in thermoregulatory responses during activity of children and adolescents compared with adults. Some differences are due to movement inefficiency and physical size such as body surface area to body mass ratio, and body composition. Identified physiological differences in sweat rates appear to alter with maturation, at least in boys, but the research remains incomplete. A number of findings from hydration studies with young people exercising in the heat are also discussed. The research on clothing is adult-based, but key concepts from this research also apply to children. The final section addresses the limited research on acclimatization of children to hot conditions. Specific recommendations for children who are active in the heat conclude this review.

Physical activity, sport, and pediatric diabetes

The benefits derived from regular physical activity include improved cardiovascular fitness, increased lean mass, improved blood lipid profile, enhanced psychosocial well-being, and decreased body adiposity. The benefits for children with diabetes may also include blood glucose control and enhanced insulin sensitivity. However, for these children, engagement in vigorous physical activity and sport must be properly controlled through modifications in insulin therapy and nutritional intake so that the benefits of exercise outweigh the risks. The following review describes the various physiological and metabolic factors which occur both during exercise and during sport while describing specific recommendations to control glucose excursions by proper insulin management and diet.

The reproducibility of an endurance performance test in adolescent cyclists

The purpose of the study was to measure the reproducibility of a performance test in well-trained adolescent cyclists. Eight male and one female cyclist [mean age 15.7 (0.7) y] participated in the study. Lactate threshold (LT) and peak VO2 were assessed. The performance test was repeated on three separate days and consisted of 30 min of steady state (SS) cycling at 80% of individual LT. Immediately after the SS cycling a time trial (TT) started with the cyclists having to complete a fixed amount of work as fast as possible. Reliability was assessed for the TT with the coefficient of variation (CV) as the (SD/mean)*100 for each participant, intraclass correlation coefficients (ICC) and typical error (SD of the difference in mean /radical2). The group mean (SD) times for the TT were TT1 1889 (306), TT2 1857 (283) and TT3 1953 (279) s respectively. Individual CV varied from 0.25% to 10%. The ICC for TT1/2 and 2/3 were r = 0.78 and 0.93 (P<0.05). The typical errors, expressed as a CV% on the log transformed performance times, were 7.3 and 3.7% for TT1/2 and TT2/3 respectively. The largest individual CVs were observed between TT1 and TT2. The differences in CV and SD among the three TTs indicate that trial two and three were more reliable than TT1, suggesting a habituation trial is needed. It is concluded that the present performance test is reliable in adolescent cyclists with lower variation between trials 2 and 3.

Oxidation of combined ingestion of glucose and sucrose during exercise

The first purpose of the study was to examine whether combined ingestion of glucose and sucrose at an intake rate of 1.2 g/min would lead to higher oxidation rates compared with the ingestion of an isocaloric amount of glucose or sucrose alone. The second aim of the study was to investigate whether a mixture of glucose and sucrose when ingested at a high rate (2.4 g/min) would result in exogenous CHO oxidation rates higher than 1.2 to 1.3 g/min. Eight trained cyclists (maximal oxygen consumption: 64 +/- 2 mL . kg -1 . min -1 , mean +/- SE) performed 5 exercise trials in random order. Each trial consisted of 120 minutes of cycling at 50% maximum power output (63% +/- 2% maximal oxygen consumption), whereas subjects received a solution providing either 1.2 g/min of glucose (GLU), 1.2 g/min of sucrose (SUC), 0.6 g/min of glucose + 0.6 g/min of sucrose (M-GLU+SUC), 1.2 g/min of glucose + 1.2 g/min of sucrose (H-GLU+SUC), or water (WAT). Peak exogenous CHO oxidation rates in the H-GLU+SUC trial (1.20 +/- 0.07 g/min) were significantly higher ( P < .01) compared with the GLU, M-GLU+SUC, and SUC trials (0.77 +/- 0.04, 0.90 +/- 0.07, 0.98 +/- 0.04 g/min, respectively). Furthermore, peak exogenous CHO rates in M-GLU+SUC and SUC trials were significantly higher ( P < .05) compared with the GLU trial. In conclusion, combined ingestion of moderate amounts of glucose and sucrose (144 g) during cycling exercise resulted in approximately 21% higher exogenous CHO oxidation rates compared with the ingestion of an isocaloric amount of glucose. Furthermore, when a mixture of glucose and sucrose was ingested at high rates (2.4 g/min), exogenous CHO oxidation rates reached peak values of approximately 1.20 g/min.

Carbohydrate intake during exercise and performance

It is generally accepted that carbohydrate (CHO) feeding during exercise can improve endurance capacity (time to exhaustion) and exercise performance during prolonged exercise (>2 h). More recently, studies have also shown ergogenic effects of CHO feeding during shorter exercise of high intensity ( approximately 1 h at >75% of maximum oxygen consumption). During prolonged exercise the mechanism behind this performance improvement is likely to be related to maintenance of high rates of CHO oxidation and the prevention of hypoglycemia. Nevertheless, other mechanisms may play a role, depending on the type of exercise and the specific conditions. The mechanism for performance improvements during higher-intensity exercise is less clear, but there is some evidence that CHO can have central effects. In the past few years, studies have investigated ways to optimize CHO delivery and bioavailability. An analysis of all studies available shows that a single CHO ingested during exercise will be oxidized at rates up to about 1 g/min, even when large amounts of CHO are ingested. Combinations of CHO that use different intestinal transporters for absorption (e.g., glucose and fructose) have been shown to result in higher oxidation rates, and this seems to be a way to increase exogenous CHO oxidation rates by 20% to 50%. The search will continue for ways to further improve CHO delivery and to improve the oxidation efficiency resulting in less accumulation of CHO in the gastrointestinal tract and potentially decreasing gastrointestinal problems during prolonged exercise.

Oxidation of combined ingestion of glucose and fructose during exercise

The purpose of the present study was to examine whether combined ingestion of a large amount of fructose and glucose during cycling exercise would lead to exogenous carbohydrate oxidation rates >1 g/min. Eight trained cyclists (maximal O(2) consumption: 62 +/- 3 ml x kg(-1) x min(-1)) performed four exercise trials in random order. Each trial consisted of 120 min of cycling at 50% maximum power output (63 +/- 2% maximal O(2) consumption), while subjects received a solution providing either 1.2 g/min of glucose (Med-Glu), 1.8 g/min of glucose (High-Glu), 0.6 g/min of fructose + 1.2 g/min of glucose (Fruc+Glu), or water. The ingested fructose was labeled with [U-(13)C]fructose, and the ingested glucose was labeled with [U-(14)C]glucose. Peak exogenous carbohydrate oxidation rates were approximately 55% higher (P < 0.001) in Fruc+Glu (1.26 +/- 0.07 g/min) compared with Med-Glu and High-Glu (0.80 +/- 0.04 and 0.83 +/- 0.05 g/min, respectively). Furthermore, the average exogenous carbohydrate oxidation rates over the 60- to 120-min exercise period were higher (P < 0.001) in Fruc+Glu compared with Med-Glu and High-Glu (1.16 +/- 0.06, 0.75 +/- 0.04, and 0.75 +/- 0.04 g/min, respectively). There was a trend toward a lower endogenous carbohydrate oxidation in Fruc+Glu compared with the other two carbohydrate trials, but this failed to reach statistical significance (P = 0.075). The present results demonstrate that, when fructose and glucose are ingested simultaneously at high rates during cycling exercise, exogenous carbohydrate oxidation rates can reach peak values of approximately 1.3 g/min.

Oxidation rate of exogenous carbohydrate during exercise is higher in boys than in men

To determine whether the relative utilization of exogenous carbohydrate (CHO(exo)) differs between children and adults, substrate utilization during 60 min of cycling at 70% peak O(2) uptake was studied in 12 pre- and early pubertal boys (9.8 +/- 0.1 yr) and 10 men (22.1 +/- 0.5 yr) on two occasions. Subjects consumed either a placebo or a (13)C-enriched 6% CHO(exo) beverage (total volume per trial: 24 ml/kg). Substrate utilization was calculated for the final 30 min of exercise. During both trials, total fat oxidation was higher (5.4 +/- 0.5 vs. 3.0 +/- 0.4 mg x kg(-1) x min(-1), P < 0.001) and total CHO oxidation lower (27.4 +/- 1.5 vs. 34.8 +/- 1.2 mg x kg(-1) x min(-1), P < 0.001) in boys than in men, respectively. During the CHO(exo) trial, CHO(exo) oxidation was higher (P < 0.001) in boys (8.8 +/- 0.5 mg x kg(-1) x min(-1)) than in men (6.2 +/- 0.5 mg x kg(-1) x min(-1)) and provided a greater (P < 0.001) relative proportion of total energy in boys (21.8 +/- 1.4%) than in men (14.6 +/- 0.9%). These results suggest that, although endogenous CHO utilization during exercise is lower, the relative oxidation of ingested CHO is considerably higher in boys than in men. The greater reliance on CHO(exo) in boys may be important in preserving endogenous fuels and may be related to pubertal status.

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.