This double-blinded, crossover randomized controlled trial study was designed to establish if combined ingestion of glucose and fructose (GLU + FRU) at the moderate rate 0.5 g·min−1 would result in higher rates of carbohydrate (CHO) oxidation compared with glucose (GLU) alone. Eight untrained females (VO2max: 25.8 ± 3.2 mL·kg−1·min−1) cycled on two different occasions for 60 min at 50% of maximal power output (60% ± 1 % VO2max) and consumed 12% CHO solution of either providing 0.33 g·min−1 glucose + 0.17 g·min−1 fructose (GLU + FRUC) or 0.5 g·min−1 of glucose (GLU) alone. Heart rate (HR) and rate of perceived exertion (RPE) were assessed during exercise and subjective exercise experience assessed two days after each trial. CHO oxidation was not significantly different (
There is growing interest from industry, clinicians, patients, and athletes in identifying nutritional approaches to optimise performance [
It is recognised that absorption of CHO is an important rate-limiting step in CHO metabolism during exercise [
Research into the response to different forms of carbohydrate has focused on male subjects who are trained in endurance sports [
The main aim of this study was to investigate whether there are differences in carbohydrate utilization when untrained female subjects consume either glucose alone or a combination of glucose and fructose following a protocol of submaximal exercise (60% VO2max) aimed at depleting muscle glycogen stores. We hypothesise that female subjects will have similar results to those previously observed in male subjects [
This was a crossover randomised double-blinded trial to measure responses to a 1-hour cycling intervention following administration of two carbohydrate (CHO) drinks in 2 separate trials.
Ten sedentary females between the ages of 18 and 30 years were recruited from the University of Nottingham campus for this study. Two subjects dropped out due to a new job commitment and illness, leaving only eight subjects for the study. The subjects were informed of the nature, risks, and potential benefits of the study orally and in writing, and then subjects provided written, informed consent. The procedures were reviewed and approved by the Nottingham University Medical School Research Ethics Committee. Untrained was defined as participation in less than 2 hours of regular strenuous activity per week for at least the last 6 months and having maximum oxygen consumption (VO2max) between 20 and 40 mL·kg−1·min−1. The mean time spent walking and sitting per week by the subjects was 5.6 ± 3.6 hrs and 75.3 ± 17.8 hrs, respectively. They participated in vigorous and moderate physical activity weekly for 0.1 ± 0.4 hrs and 0.7 ± 1.0 hrs, respectively. All subjects were at a low risk for cardiovascular disease according to ACSM’s Guidelines [
All subjects participated in 2 exercise trials and on each occasion they performed a standardised exercise test on a cycle ergometer while ingesting one of two drinks identical in appearance and taste and supplied in identical containers. Subjects were required to refrain from any strenuous exercise for at least 72 hours before the experiment, abstain from alcohol or caffeine 48 hours prior to the exercise, and fasted for at least 6 hours. A food record was provided to standardise food ingested before the experiment while an identical diet was prescribed for the day before the subsequent exercise. Figure
Schematic representation of the study protocol for trial 1 and trial 2.
A total of 1.2 L of lemon-flavoured carbohydrate (CHO) solutions were prepared for ingestion during each exercise bout by an independent member of the research staff blinded to both subjects and the research team. The GLU solution was 90 g glucose, 6 tsp of lemon juice in 1200 mL of water and lemon juice. 600 mL of this mixture was consumed before the start of the exercise bout, and the remaining volume was taken in 150 mL aliquots at 15, 30, 45, and 60 min of the exercise period. The GLU + FRU solution was 60 g glucose and 30 g fructose in 1200 mL of water and 6 tsp of lemon juice, again 600 mL was consumed before exercise and 150 mL every 15 min during the exercise. The glucose powder used in the drink supplements was maize-derived labelled “Glucose for Oral Use”, (Courtin and Warner, Ltd., London) and 100% glucose. The fructose powder was from 100% fruit sugar found naturally in most fruits and honey (“Fruisana” Pure Fructose, Surrey, UK). The CHO energy content was 1692 kJ/375 kcal per 100 g.
Height, body mass, and skinfolds were measured and body mass index (BMI) and percent body fat were calculated. Subject’s body mass index (BMI) was between 20 and 27 kg·m−2. Skinfold thicknesses (SFT) were measured at four standard sites: biceps, triceps, subscapular, and suprailiac with a skinfold caliper with manual read-out (accuracy 0.2 mm (range 0–19 mm) John Bull, British Indicators, UK) by a female investigator and procedures were done according to ACSM guidelines [
The test was undertaken at least one week prior to the initiation of the experiment while subjects cycled on a stationary bicycle ergometer (Tunturi, E630, Tunturi Fitness B.V, Almere, Finland). The seat height was adjusted to allow a slight bend in the knee with the leg at full extension and the foot parallel to the floor. The subject continued to wear the HR monitor and was fitted with a mouthpiece and head gear for collection of expired gases. A metabolic cart ( a levelling off of VO2 with increasing workload (increase of no more than 2 mL·kg−1 body weight per minute), heart rate within 10 beats·min−1 of predicted maximum (HR of 220 beats·min−1 age), and a respiratory exchange ratio (RER) greater than 1.05.
To calculate VO2max in this present study, a graph was plotted of the workload against VO2 showing the linear relationship. The workload at which subjects were cycling at 60% VO2max was then read from the graph and used as the
During the exercise tests, subjects wore a nose-clip and breathed through a mouthpiece which allowed room air to be inhaled and directly exhaled to a
Carbohydrate and fat oxidation (g/min) were calculated using stoichiometric equations from VO2 and VCO2 (L/min), with the assumption that protein oxidation during exercise was negligible according to Frayn [
The heart rate of each subject was recorded every 5 min during the cycling exercise and the same heart rate monitor was used for all subjects to avoid potential confounders.
Perceived exertion was measured using Borg’s Rating of Perceived Exertion (RPE) scale [
All subjects completed a telephone interview using the Subjective Exercise Experience Scale (SEES) [
A within-subject analysis of variance (ANOVA) for repeated measures on two factors (experimental trial × sampling time) was used to compare differences in substrate utilisation, heart rate, and rate of perceived exertion between the cycling trials with different nutritional solutions. When statistical significance was observed, post hoc analysis was undertaken with Tukey’s HSD to locate the difference. Wilcoxon signed rank test was used to analyse nonnormally distributed data. A paired-sample
All data are expressed as means ± SD. The mean age for the subjects was 24.0 ± 4.7 years. Descriptive statistics for age, weight, body mass index (BMI), body fat, maximal oxygen uptake (VO2max), and workload are illustrated in Table
Participant characteristics.
Characteristics | Mean | SD | Range |
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Age (years) | 24.3 | ±2.7 | 18–30 |
Height (cm) | 162.5 | ±0.1 | 1.53–1.67 |
Body mass (kg) | 58.4 | ±9.0 | 47–71 |
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72.2 | ±7.0 | 65.2–85.5 |
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94.6 | ±9.7 | 75–104 |
BMI | 22.5 | ±2.8 | 20–27 |
% body fat | 24.6 | ±3.5 | 28.7–41.3 |
% LBM | 75.4 | ±5.5 | 58.7–71.3 |
LBM (kg) | 43.9 | ±4.2 | 31.2–41.9 |
60% VO2max (mL·kg−1·min−1) | 15.5 | ±1.86 | 14.46–18.24 |
Predicted 60% VO2max (mL·kg−1·min−1) | 19.67 | ±1.94 | 15.81–22.57 |
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129 | ±12 | 110–150 |
Moderate Activity·wk−1 (METS) | 1320.5 | ±652.7 | 657–2376 |
HRmax (beats/min) | 196.4 | ±4.39 | 191–202 |
BMI: body mass index; LBM: lean body mass,
Note: predicted 60% VO2max is the mean value that was aimed for while the 60% VO2max was the mean value that was achieved among the study subjects.
The mean VO2 and RER values for GLU and GLU + FRUC are presented in Table
Effect of GLU and GLU + FRUC on VO2, RER, CHOtot, and FATtot in untrained women during submaximal exercise (0–60 min) and recovery period (61–90 min).
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0–15 | 16–30 | 31–45 | 46–60 | 61–75 | 76–90 | |
GLU+FRUC | ||||||
VO2 (L/min) |
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RER (g/min) |
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CHOtot (g/min) |
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FATtot (g/min) |
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GLUC | ||||||
VO2 (L/min) |
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RER (g/min) |
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CHOtot (g/min) |
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FATtot (g/min) |
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GLU: glucose; GLU + FRUC: glucose + fructose, VO2: mean oxygen uptake; RER: respiratory exchange ratio, CHOtot: total carbohydrate oxidation, and FATtot: total fat oxidation.
The mean CHO and fat oxidation at different time points are shown in Table
Simultaneously, fat oxidation declined in the initial 15 min following exercise, then rose to reach a peak at the end of the 60 min exercise, and declined during the recovery period (Figure
There was no significant difference in heart rate between GLU and GLU + FRUC trials during exercise (
Heart rate (HR) during 60 min exercise with ingestion of GLU or GLU + FRUC. Values are presented as means ± SE,
Total carbohydrate (CHO) oxidation during 60 min exercise and 30 min recovery with GLU or GLU + FRUC. Values are presented as means ± SE;
Total fat oxidation during 60 min exercise and 30 min recovery with GLU of GLU + FRUC. Values are presented as means ± SE;
No significant difference in perceived exertion (
Rate of perceived exertion (RPE) during 60 min exercise with GLU or GLU + FRUC. Values are presented as means ± SE;
Mean values for SEES: physical wellbeing (PWB), psychological distress (PD), fatigue (FT) with ingestion of GLU + FRUC, and GLU are presented in Table
Paired
PWB | PD | FT | |
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GLU + FRUC |
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GLU |
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0.743 | 40.036 | 0.233 |
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Trial 1 |
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Trial 2 |
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0.743 | 0.623 | 0.85 |
PWB: physical wellbeing; PD: psychological distress; FT: fatigue. Values are presented as means ± SE;
Subjective Exercise Experience Scale questionnaire comparing GLU and GLU + FRUC ingestion; (a) and visit 1 and visit 2; (b). Values are means ± SE. *Significant difference between reports of psychological distress between GLU and GLU + FRUC.
The main finding of this study was that with the amounts of CHO provided (0.5 g·min−1 or 30 g·h−1) the rate of CHO oxidation was similar between GLU and GLU + FRUC. According to Jeukendrup [
In the study by Currell and Jeukendrup [
On the contrary, Hulston et al. [
The result of this present study may be explained by the possibility that the participants did not benefit from the GLU + FRUC mixture because adequate quantities of glucose was not ingested to saturate the glucose transporters in the intestine [
The outcome of this present study and findings by Hulston et al. [
Walker et al. [
CHO ingestion also enhances immediate recovery after exercise. Ivy et al. [
Jeukendrup et al. [
The rate of perceived exertion and cardiovascular response to exercise were not statistically significant between the beverages. The fatigue and wellbeing subscales of the SEES were similar for both nutritional solutions while the distress component was significantly lower after GLU + FRUC. Exercises that are perceived more positively are likely to foster exercise adoption and adherence [
Comparable findings of no difference in RPE were also reported by Glass and Chvala [
Compared to males, there is a dearth of physical activity information for females and it is necessary to consider psychological exercise experience in females to gain insight into the underrepresentation of females in leisure activity [
Respondents on the telephone have been found to be slightly more likely to choose one of the extreme categories in the case of questions involving response scales [
Generalising the findings of this study to the broader population may be difficult due to the narrow demographic of the current sample and the low-to-moderate fitness level of the subjects. However, this sample is quite representative of the general population in terms of fitness level. The result of this study may be due to lower rates of CHO ingestion compared with previous studies. However, the contribution of exogenous glucose to energy provision during prolonged high-intensity endurance exercise is limited to approximately 1.0-1.1 g·min−1, even when ingested in quantities up to ~3.0 g·min−1 [
Also, a relatively small sample size could have contributed to the overall result and a larger number of subjects could have detected smaller differences between both groups. According to Bengtsson et al. [
The present study has shown that combined ingestion of moderate amounts of glucose and fructose during 60 min cycling resulted in peak CHO oxidation rates of 0.8 g·min−1 and did not increase total CHO oxidation compared to glucose alone in untrained females. The recovery period after exercise also revealed similar total CHO and fat oxidation in both trials. The heart rate response to exercise, perceived exertion, and postexercise experience of fatigue and wellbeing were similar in both trials. However, experience of distress was higher in the glucose alone compared to glucose plus fructose. The present findings support the suggestion that, in order to achieve high CHO oxidation rates (~1.7 g·min−1), it may be necessary to consume a mixture of glucose plus fructose at higher intake rates (1.2 g·min−1 glucose + 1.2 g·min−1 fructose).
The study protocol was approved by the Nottingham University Medical School Research Ethics Committee (approval no. B/03/2010).
The authors declare that there is no conflict of interests regarding the publication of this paper.