Effects of isomaltulose ingestion on postexercise hydration state and heat loss responses in young men
Funding information:
This study was supported by a grant from Bourbon Corporation and Grant‐in‐Aid for Scientific Research (no. 18H03146) from the Japan Society for the Promotion of Science from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
Edited by: Michael White
Abstract
New Findings
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What is the central question of this study?
What are the effects of isomaltulose, an ingredient in carbohydrate–electrolyte beverages to maintain glycaemia and attenuate the risk of dehydration during exercise heat stress, on postexercise rehydration and physiological heat loss responses?
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What is the main finding and its importance?
Consumption of a 6.5% isomaltulose–electrolyte beverage following exercise heat stress restored hydration following a 2 h recovery as compared to a 2% solution or water only. While the 6.5% isomaltulose–electrolytes increased plasma volume and plasma osmolality, which are known to modulate postexercise heat loss, sweating and cutaneous vascular responses did not differ between conditions. Consequently, ingestion beverages containing 6.5% isomaltulose–electrolytes enhanced postexercise rehydration without affecting heat loss responses.
Abstract
Isomaltulose is a disaccharide carbohydrate widely used during exercise to maintain glycaemia and hydration. We investigated the effects of ingesting a beverage containing isomaltulose and electrolytes on postexercise hydration state and physiological heat loss responses. In a randomized, single‐blind cross‐over design, 10 young healthy men were hypohydrated by performing up to three 30 min successive moderate‐intensity (50% heart rate reserve) bouts of cycling, each separated by 10 min, while wearing a water‐perfusion suit heated to 45°C. The protocol continued until a 2% reduction in body mass was achieved. Thereafter, participants performed a final 15 min moderate‐intensity exercise bout followed by a 2 h recovery. Following cessation of exercise, participants ingested a beverage consisting of (i) water only (Water), (ii) 2% isomaltulose (CHO‐2%), or (iii) 6.5% isomaltulose (CHO‐6.5%) equal to the volume of 2% body mass loss within the first 30 min of the recovery. Changes in plasma volume (ΔPV) after fluid ingestion were greater for CHO‐6.5% compared with CHO‐2% (120 min postexercise) and Water (90 and 120 min) (all P ≤ 0.040). Plasma osmolality remained elevated with CHO‐6.5% compared with consumption of the other beverages at 30 and 90 min postexercise (all P ≤ 0.050). Urine output tended to be reduced with CHO‐6.5% compared to other fluid conditions (main effect, P = 0.069). Rectal and mean skin temperatures, chest sweat rate and cutaneous perfusion did not differ between conditions (all P > 0.05). In conclusion, compared with CHO‐2% and Water, consuming a beverage consisting of CHO‐6.5% and electrolytes during recovery under heat stress enhances PV recovery without modulating physiological heat loss responses.
1 INTRODUCTION
Evaporation of sweat represents the major avenue of heat loss during exercise in the heat. However, excessive sweating can induce significant fluid loss leading to marked reductions in blood volume (BV) and elevated blood osmolality, both of which are associated with impairment of the body's ability to dissipate heat (Kenny & McGinn, 2017; McKinley, Martelli, Pennington, Trevaks, & McAllen, 2018; Shibasaki, Wilson, & Crandall, 2006). To minimize fluid loss, commercial sports beverages that contain a mixture of carbohydrate, such as glucose, fructose and/or maltodextrin, with electrolytes are commonly consumed by athletes, workers, servicemen and others during and following exercise. This is because these beverages help maintain glycaemia and hydration during (Achten, Jentjens, Brouns, & Jeukendrup, 2007; König, Zdzieblik, Holz, Theis, & Gollhofer, 2016; Thomas, Erdman, & Burke, 2016) and following exercise (Evans, Shirreffs, & Maughan, 2009; Kamijo et al., 2012; Osterberg, Pallardy, Johnson, & Horswill, 2009), thereby improving performance and facilitating rehydration.
Isomaltulose is an alternative to other sugars commonly used in carbohydrate–electrolyte beverages such as commercially available sports beverages. It is a disaccharide with glucose and fructose linked by an α‐1,6‐glycosidic bond, which makes it more resistant to hydrolysis by gastrointestinal enzymes. Consequently, absorption rate is reduced and is paralleled by low glycaemic/insulinaemic responses relative to other carbohydrates such as sucrose and glucose (e.g. glycaemic index of 32, 65 and 103 for isomaltulose, sucrose and glucose, respectively) (Atkinson, Foster‐Powell, & Brand‐Miller, 2008; Sawale, Shendurse, Mohan, & Patil, 2017). It has recently been reported that hydrating with an isomaltulose–electrolyte beverage prior to performing exercise elevates blood glucose level while enhancing fat oxidation during exercise (Achten et al., 2007; König et al., 2016) and improving endurance performance (König et al., 2016) compared with maltodextrin or sucrose. However, despite its potential performance‐enhancing benefits, it is unclear whether the ingestion of beverages containing isomaltulose improves rehydration as compared to the ingestion of water only and if the benefits of an isomaltulose–electrolyte beverage are dependent on the concentration of isomaltulose used.
It has been reported that the ingestion of hypotonic carbohydrate–electrolyte solutions (e.g. 2–3% glucose and/or sucrose) after exercise can induce rapid (e.g. 45 min after ingestion) increases in plasma volume (PV) relative to the ingestion of carbohydrate‐free beverages or water only (Evans et al., 2009; Kamijo et al., 2012; Osterberg et al., 2009). However, hypotonic carbohydrate ingestion did not induce a full recovery from dehydration whereas the ingestion of beverages containing greater carbohydrate content (e.g. ≥6.5%) enhanced PV recovery approximately ∼120 min following ingestion relative to hypotonic solution (Kamijo et al., 2012). This response may in part be explained by a decrease in gastric emptying rate of ingested fluid (Clayton, Evans, & James, 2014) and thus a prolonged absorption of fluid in the higher concentration of carbohydrate (Kamijo et al., 2012). In addition, the ingestion of beverages containing a higher concentration of carbohydrates has been shown to induce an elevation in blood glucose and insulin which can subsequently increase renal tubular resorptions of sodium and water thereby enhancing PV and maintaining high plasma osmolality (Posm) during a state of hypohydration (Kamijo et al., 2012). In turn, changes in PV and Posm associated with ingestion of beverages containing carbohydrates such as isomaltulose may have a modulating effect on physiological heat loss responses thereby influencing core temperature (Tco) recovery following prolonged exercise (Kenny & McGinn, 2017; Kenny, Jay, & Journeay, 2007). This is because changes in PV and Posm can alter heat loss responses of cutaneous perfusion and sweating (Kenny & McGinn, 2017; McKinley et al., 2018; Shibasaki et al., 2006).
It has been demonstrated that following cessation of exercise in the heat, there is a sustained suppression of heat loss responses which is associated with a prolonged elevation in Tco lasting up to 2 h (Thoden, Kenny, Reardon, Jetfe, & Livingstone, 1994; Wilkins, Minson, & Halliwill, 2004), which has been attributed to non‐thermal factors (Kenny & Journeay, 2010). Non‐thermal factors such as those associated with the activation of baroreceptors (blood pressure regulation) and osmoreceptors (elevated osmolality) have been shown to impinge upon the activation of skin blood flow and sweating during the postexercise recovery period thereby affecting Tco recovery (Journeay, Reardon, Jean‐Gilles, Martin, & Kenny, 2004; McGinn, Paull, Meade, Fujii, & Kenny, 2014; Paull et al., 2016). Given that activity of these sensory receptors is directly stimulated by transient changes in PV and Posm, it is possible that ingesting beverages containing isomaltulose as compared to water only may modulate heat loss responses differently. In this context, previous studies suggested that potential increases in PV would enhance heat dissipation via increases in sweating and cutaneous perfusion (Journeay et al., 2004; McGinn et al., 2014) while elevated Posm attenuates sweating (Paull et al., 2016) during the postexercise recovery period. Consequently, there is a need to advance our understanding of the interactive influences of ingesting carbohydrate–electrolyte beverages containing isomaltulose on postexercise rehydration and heat loss.
Thus, the aim of the present study was to investigate the effect of ingesting carbohydrate–electrolyte beverages containing isomaltulose on postexercise rehydration and the heat loss responses of cutaneous perfusion and sweating. We evaluated the hypothesis that ingesting a carbohydrate–electrolyte beverage containing isomaltulose following a prolonged intermittent exercise bout under a high heat stress condition induced by whole‐body heating to cause a mild state of hypohydration (equivalent to 2% reduction in body bass; Maughan, 2003; Sawka et al., 2007) would improve fluid restoration as compared to water only. However, as previously demonstrated with other carbohydrates (Kamijo et al., 2012), this response would be dependent on the concentration of isomaltulose such that fluid restoration would be improved with 6.5% as compared to 2% isomaltulose. Further, we evaluated the hypothesis that relative to the ingestion of water only, ingesting a carbohydrate–electrolyte beverage with isomaltulose would also induce an increase in heat loss mediated by an increase in PV (Journeay et al., 2004; McGinn et al., 2014). However this effect would be greater with the ingestion of a drink containing a low concentration of isomaltulose (i.e. 2%), as the increase in Posm caused by the consumption of a higher concentration (i.e. 6.5%) would have an inhibiting influence on heat loss thereby negating any benefits induced by changes in PV (Paull et al., 2016).
2 MATERIALS AND METHODS
2.1 Ethical approval
The present study was approved by the human ethical committee of Niigata University (reference no. 2018‐0070) and was conducted in accordance with the latest version of the Declaration of Helsinki. It was registered in the University Hospital Medical Information Network (UMIN) clinical trial registry (ID: UMIN000033464). Verbal and written informed consent were obtained from all participants prior to the commencement of the experimental sessions.
2.2 Participants
Ten healthy and physically active young men participated in the present study (age: 21.2 ± 1.1 years, height: 1.72 ± 0.04 m, body mass: 64.4 ± 5.5 kg). We did not include female participants given the known sex‐related differences in the mechanisms governing postexercise heat loss responses (Gagnon, Jay, Reardon, Journeay, & Kenny, 2008). None of the participants took prescription medications, and all were non‐smokers. We determined a minimal sample size based on a previous study which reported an increase in PV with the ingestion of a beverage containing 6.5% carbohydrate (mixture of glucose and fructose) following exercise compared with a carbohydrate‐free beverage (Kamijo et al., 2012). Using an effect size 1.32 calculated from a postexercise increase in PV measured with the ingestion of a beverage containing 6.5% carbohydrate compared with carbohydrate‐free beverage (changes in PV (ΔPV) from pre‐exercise resting were 119 ± 63 and −33 ± 132 ml for the 6.5% carbohydrate drink and carbohydrate‐free beverage, respectively) (Kamijo et al., 2012), we determine a minimal sample size of 10 participants was required to determine a difference in PV with a power of 95% and an α‐error probability of 0.05.
2.3 Test beverages
During the experimental session (see below), the participants consumed one of three test beverages: water only (Water), and beverages containing carbohydrates–electrolytes consisting of 2.0 or 6.5 g of the natural disaccharide carbohydrate (CHO) isomaltulose per 100 ml (CHO‐2.0% and CHO‐6.5%, respectively). These carbohydrate concentrations are commonly found in commercial sports beverages and oral rehydration solutions (Shirreffs, 2009). A list of nutrients contained in each beverage is presented in Table 1. Both the CHO‐2.0% and CHO‐6.5% beverages contained the same amount of electrolytes although the total energy differed between the beverages. Water contained no energy and minimal electrolytes. The osmolality for these beverages measured by freezing point depression method were 3, 126 and 269 mosmol kg−1 for Water, CHO‐2.0% and CHO‐6.5%, respectively.
| Water | CHO‐2% | CHO‐6.5% | |
|---|---|---|---|
| Energy (kcal) | 0 | 8 | 26 |
| Protein (mmol l−1) | 0.0 | 0.0 | 0.0 |
| Lipid (mmol l−1) | 0.0 | 0.0 | 0.0 |
| Carbohydrate as isomaltulose (mmol l−1) | 0.0 | 58.4 | 189.9 |
| Sodium (mmol l−1) | 0.7 | 20.6 | 20.6 |
| Calcium (mmol l−1) | 0.3 | 1.1 | 1.1 |
| Magnesium (mmol l−1) | 0.2 | 1.0 | 1.0 |
| Potassium (mmol l−1) | 0.0 | 4.5 | 4.5 |
| l‐Carnitine (mmol l−1) | 0.0 | 2.5 | 2.5 |
- CHO‐2%, 2% carbohydrate (isomaltulose); CHO‐6.5%, 6.5% carbohydrate.
2.4 Experimental protocol
Experiments were conducted in an open‐space laboratory under thermoneutral conditions (∼25°C and ∼50% relative humidity). Three experimental trials were separated by a minimum of 7 days. Participants were instructed to refrain from consuming alcohol and caffeine, and from participating in strenuous physical activity at least 24 h prior to each experimental trial. Additionally, the night before the experimental session they were encouraged to consume a standard meal consisting of ∼115 g of carbohydrate, ∼20 g of fats and ∼27 g of protein with an energy equivalent of ∼750 kcal. This included 500 ml of non‐caffeine Japanese tea. In addition, participants were asked to consume 500 ml of water before going to bed the night prior to each experiment. At least 2 h before the start of the experimental session, participants were instructed to consume a light breakfast consisting of a cup jelly and an energy bar (∼39 g of carbohydrate, 7.7 g of fat, 2 g of protein and 0.3 of salt; energy equivalent of 183 kcal) and 500 ml water.
Participants reported to the laboratory between 07.00 and 09.00 h. Upon arrival at the laboratory, urine samples were collected to measure urine specific gravity (USG) and assess the hydration status (Sawka et al., 2007). Participant’s body mass and height were then measured using a weighing scale (HW‐100KC; A & D, Tokyo, Japan) and a stadiometer (YS501‐P; Sanyu, Tokyo, Japan), respectively. Thereafter, the participant donned a water‐perfusion suit (Allen‐Vanguard, Ottawa, Canada) over their tight fitting shorts (i.e. half‐tights consisting of fabric with high moisture wicking properties), covering the whole body with the exception of the hands, feet and head. The participant then rested in a semi‐recumbent position during which time the water circulating in the suit was maintained at 34°C using a water bath heater (SP‐12N; Taitec, Koshigaya, Japan) for approximately 40 min. Instrumentation was also completed during this period. Blood samples were collected thereafter and participants remained resting for an additional 5 min resting period during which time baseline (BL) resting measurements were collected.
The temperature of water circulating in the suit was then increased to 45°C after which the participants were required to perform up to three 30 min bouts of moderate‐intensity (equivalent to 50% of their age‐predicted heart rate (HR) reserve) cycling, each separated by a 10 min rest period during which time the participant's body mass was assessed without the water‐perfusion suit. This protocol has been previously shown to induce a progressive state of dehydration (Kamijo et al., 2012) equivalent to a 2% reduction in body mass (i.e. equivalent to 2% dehydration, considered a state of mild hypohydration; Maughan, 2003; Sawka et al., 2007). Once a 2% reduction in body mass was achieved, the participant once again donned the water‐perfusion garment and performed a final 15 min bout of moderate‐intensity (i.e. 50% of their age‐predicted HR reserve) cycling to induce a further elevation in Tco and enhance cutaneous perfusion and sweating. At the cessation of exercise, the participant rested in a semi‐recumbent position for 2 h during which time the temperature of the water circulating in the suit was adjusted (∼45°C) to maintain mean skin temperature (Tsk) at ∼35°C (see results) as per previous studies assessing the regulation of postexercise heat loss responses (McGinn et al., 2014; Paull et al., 2016). We clamped skin temperature to minimize the confounding influences of changes in body temperature between conditions so that we could evaluate the potential influence of non‐thermal factors (e.g. those associated with the activation of baroreceptors and osmoreceptors) on heat loss responses. During this period, in keeping with a previous protocol (Kamijo et al., 2012), the participants were required to consume one of three randomly assigned beverages within the first 30 min following cessation of exercise. They consumed a volume equal to the body mass loss during the intermittent exercise bouts. They ingested 50, 25 and 25% of the total volume in 10 min intervals over the 30 min period. To minimize any influences on thermoregulatory responses, the temperature of the beverage was maintained at 37°C (Morris, Chaseling, Bain, & Jay, 2019).
2.5 Measurements
Blood samples were collected from a warmed fingertip at BL, the end of the final 15 min exercise bout, and at 30, 60, 90 and 120 min of recovery. All blood samples were collected at a semi‐recumbent posture throughout the experiment. Measurements of haemoglobin concentration (Hb), haematocrit (Hct), Posm, and plasma insulin concentration (Pins), as well as blood glucose (Glu) and lactate (Lac) concentrations were performed on each sample. Hb was measured using a spectrophotometric device (Hemocue Hb 201; HemoCue, Angelholm, Sweden). Hct was determined by the microhaematocrit method. PV, BV and cell volume (CV) was determined using procedures defined by Dill & Costill (1974). For the measurement of Posm and Pins, blood samples were centrifuged and the extracted plasma samples were frozen at −30°C until the analysis. Posm was measured using the freezing point depression method (the Fiske 210 micro osmometer, Advanced Instruments, Norwood, MA, USA) and Pins was measured using a sandwich enzyme‐linked immunosorbent assay method (YK060; Yanaihara, Fujinomiya, Japan). Glu (Glu‐Test Every; SKK, Nagoya, Japan) and Lac (Lactate pro 2 LT‐1730; Arkray, Kyoto, Japan) concentrations were analysed using portable analysers. Pins, Glu, and Lac were corrected for the changes in PV (Kraemer & Brown, 1986).
Tco was measured continuously using a thermistor probe (401J; Nikkiso‐thermo, Tokyo, Japan) inserted 12 cm past the anal sphincter. Skin temperatures were measured by thermistors (ITP082‐25; Nikkiso‐thermo) affixed to four skin sites with surgical tape. Tsk was calculated using the four skin temperatures as follows (Ramanathan, 1964): chest, 30%; upper arm, 30%; thigh, 20%; and lower leg, 20%. Mean body temperature (Tb) was calculated using the following formula (Stolwijk & Hardy, 1966): 0.8Tco + 0.2Tsk. Body core and skin temperatures were recorded at 1 s intervals using a data storage device (model N543; Nikkiso‐thermo).
Local chest sweat rate was measured continuously using the ventilated capsule method. A 5.3 cm2 plastic capsule was affixed using topical glue (Collodion; Kanto Chemical, Tokyo, Japan). Dry nitrogen gas was passed through each capsule over the skin surface at a rate of 1.0 l min−1. Water content from the effluent air was measured using a capacitance hygrometer (HMP60; Vaisala, Helsinki, Finland). Skin blood flow on the chest was measured continuously by laser‐Doppler velocimetry (FLO‐C1; Omegawave, Tokyo, Japan). A laser‐Doppler probe was located adjacent to the ventilated capsule. Cutaneous vascular conductance (CVC) was calculated from the ratio of skin blood flow to mean arterial pressure (MAP). Sweat rate and skin blood flow were recorded at 1 s intervals using a data logger system (MX100; Yokogawa, Tokyo, Japan). HR was recorded using a Polar coded WearLink and transmitter and RS800 interface (Polar Electro Oy, Finland). Systolic and diastolic blood pressures were measured at BL, the end of 15 min recumbent exercise, and every 15 min during the 2 h recovery period using an automated sphygmomanometer (Hem‐7511T; Omron Healthcare, Kyoto, Japan). MAP was subsequently calculated using a following equation: (systolic blood pressure − diastolic blood pressure)/3 + diastolic blood pressure. Urine volume was recorded using a graduated cylinder at 1 and 2 h into the postexercise recovery period. Total urine output was calculated as the sum of the two volumes measured at each of the two time points. Body mass was measured at the end of the 2 h postexercise recovery period.
2.6 Data and statistical analyses
All variables recorded continuously were averaged for 5 min at BL and for a final 5 min during the last 15 min exercise bout. Values were averaged each 10 min during the 2 h postexercise recovery period after 2% hypohydration. Given that MAP was recorded at 15 min during the recovery period, CVC was also analysed at 15 min intervals during this period (average of 5 min values before the blood pressure measurement).
For all blood measurements, a two‐way repeated measures analysis of variance (ANOVA) was performed as the repeated factors of protocol stage (BL, the final 15 min exercise bout, and at 30, 60, 90 and 120 min postexercise) and test beverage (Water, CHO‐2.0%, CHO‐6.5%). For thermoregulatory variables, a two‐way repeated measures ANOVA was performed as the repeated factors of protocol stage (BL, the final 15 min exercise bout and 10 min intervals after exercise) and test beverage. Similarly, body mass was analysed by two‐way repeated measures ANOVA as the repeated factors of protocol stage (BL, after 2% hypohydration and after 2 h recovery) and test beverage. A one‐way repeated measures ANOVA with the repeated factor of test beverage was used to analyse variables between the conditions (e.g. beverage volume and exercise duration). The Greenhouse–Geisser correction was applied if the assumption of sphericity had been violated. The D'Agostino–Pearson test was performed to verify a normal distribution of measured variables. If a normal distribution was not observed, we then performed a Q–Q plot. For the variables that passed D'Agostino–Pearson test or Q–Q plot assessment, we performed a parametric test. When the Q–Q plot revealed a highly skewed response such as observed for sweat rate and Pins, we performed a non‐parametric test (Friedman's test) to assess these variables. Post hoc analysis was performed using Tukey's multiple comparisons test for the parametric variables and Dunn's multiple comparisons test for the non‐parametric variables. Due to technical difficulties (e.g. difficulty of blood sampling), some variables were only analysed in six to nine participants, which has been identified in the figure legends or tables. Because of the reduced sample size, we calculated a retrospective power (1 − β) and an effect size (partial eta‐squared) for the ANOVA (interaction) for primary variables of interest. These were as follows: 1.000 and 0.449 for ΔPV, 0.985 and 0.450 for absolute PV, 0.665 and 0.223 for BV, 0.237 and 0.100 for CV, 1.000 and 0.674 for Glu, 0.391 and 0.246 for CVC, respectively. Data are presented as means ± SD, and statistical significance was set at 0.05. All statistical analyses were performed using Prism (version 8.0.2, GraphPad Software, San Diego, CA, USA).
3 RESULTS
Baseline resting USG values were 1.017 ± 0.006, 1.014 ± 0.007 and 1.017 ± 0.007 for Water, CHO‐2% and CHO‐6.5%, respectively (main effect, P = 0.420). Exercise duration (75.0 ± 14.1, 73.5 ± 15.6 and 74.2 ± 15.7 min for the Water, CHO‐2% and CHO‐6.5% conditions, respectively; main effect, P = 0.803) did not differ between conditions. Similarly, the change in body mass recorded over the successive exercise–recovery cycles was similar between conditions such that a similar mild state of hypohydration was achieved across all conditions as defined by a 2% loss in body mass (i.e. −2.1 ± 0.2, −2.1 ± 0.1 and −2.1 ± 0.2% for Water, CHO‐2%, and CHO‐6.5%, respectively; main effect, P = 0.253). Consequently, the volume of water ingested during the first 30 min of the 2 h recovery was similar between conditions (Table 2).
| Water | CHO‐2% | CHO‐6.5% | ANOVA | |
|---|---|---|---|---|
| Body mass (kg) | ||||
| Pre exercise | 64.8 ± 5.8 | 64.4 ± 5.3 | 64.4 ± 5.4 | Time effect: P < 0.001 |
| Post exercise | 63.4 ± 5.7 | 63.1 ± 5.2 | 63.1 ± 5.4 | Drink effect: P = 0.268 |
| At 120 min during recovery | 63.5 ± 5.9 | 63.2 ± 5.4 | 63.3 ± 5.5 | Interaction: P = 0.662 |
| Beverage volume consumed (ml) | 1381 ± 117 | 1326 ± 129 | 1322 ± 92 | P = 0.212 |
| Total urine output (ml) | 367 ± 159 | 410 ± 242 | 242 ± 92 | P = 0.069 |
| Whole body sweat loss during final 15 min exercise and recovery period (ml) | 898 ± 353 | 788 ± 204 | 876 ± 273 | P = 0.616 |
- Values are presented as means ± SD. n = 10 for all variables. CHO‐2%, 2% carbohydrate (isomaltulose); CHO‐6.5%, 6.5% carbohydrate.
ΔPV, Posm, Glu, Pins and Lac before, at the end of and following the final 15 min exercise bout are presented in Figure 1. While there was no effect of beverage condition (P = 0.322), we observed a significant interaction of beverage and time (P < 0.001) on ΔPV such that a greater ΔPV during recovery occurred for CHO‐6.5% as compared to the ingestion of Water only (at 90 and 120 min; both P ≤ 0.040) and CHO‐2% (at 120 min, P = 0.017). A significant main effect of beverage (P = 0.011) was observed for Posm. Specifically, a higher osmolality was observed with CHO‐6.5% as compared to the other two beverages (interaction P = 0.035) which was evident at 30, 60 and 90 min of recovery (all P ≤ 0.050).
A main effect of beverage and a significant interaction of beverage and time (both P < 0.001) were observed in Glu such that a higher Glu with CHO‐2% and CHO‐6.5% relative to Water was measured during the first 60 min of recovery (all P < 0.001). Glu returned to levels comparable to those measured in the ingestion of Water only in the CHO‐2% condition (P = 0.123) while it remained elevated for the CHO‐6.5% condition relative to both the Water and 2.5% beverage conditions at the end of recovery (all P ≤ 0.017). Pins was significantly higher with CHO‐6.5% relative to Water at 90 min (P = 0.015), and showed a similar trend at 120 min of recovery (P = 0.069) and relative to CHO‐2% at 120 min (P = 0.097). A main effect of beverage only (P = 0.023) was observed in Lac wherein we observed higher values with CHO‐2% and CHO‐6.5% relative to Water (both P ≤ 0.002). Absolute (ml) changes in BV, CV, and PV that were calculated based on pre‐exercise baseline resting BV as 100 ml were not significantly different between the trials (Table 3). In addition, Hct and Hb, which were used to calculate absolute and relative changes in BV, CV and PV, did not differ between conditions although an interaction was observed in Hct (Table 3).
| Time into postexercise (min) | |||||||
|---|---|---|---|---|---|---|---|
| BL | Ex | 30 | 60 | 90 | 120 | ANOVA | |
| BV (ml) | |||||||
| Water | 100 ± 0 | 92 ± 1 | 95 ± 2 | 95 ± 1 | 95 ± 2 | 95 ± 2 | Time effect: P < 0.001 |
| CHO‐2% | 100 ± 0 | 92 ± 3 | 96 ± 2 | 96 ± 2 | 96 ± 2 | 96 ± 2 | Drink effect: P = 0.328 |
| CHO‐6.5% | 100 ± 0 | 93 ± 2 | 95 ± 3 | 96 ± 3 | 97 ± 2 | 98 ± 2 | Interaction: P = 0.066 |
| CV (ml) | |||||||
| Water | 52 ± 5 | 52 ± 5 | 51 ± 5 | 51 ± 5 | 52 ± 5 | 51 ± 5 | Time effect: P = 0.919 |
| CHO‐2% | 52 ± 4 | 52 ± 5 | 52 ± 4 | 52 ± 5 | 52 ± 4 | 52 ± 4 | Drink effect: P = 0.709 |
| CHO‐6.5% | 51 ± 3 | 51 ± 4 | 52 ± 4 | 52 ± 4 | 51 ± 3 | 51 ± 3 | Interaction: P = 0.475 |
| PV (ml) | |||||||
| Water | 48 ± 5 | 41 ± 4 | 44 ± 4 | 44 ± 5 | 43 ± 5 | 43 ± 5 | Time effect: P < 0.001 |
| CHO‐2% | 48 ± 4 | 40 ± 4 | 44 ± 4 | 44 ± 4 | 44 ± 5 | 44 ± 4 | Drink effect: P = 0.535 |
| CHO‐6.5% | 49 ± 3 | 41 ± 3 | 43 ± 4 | 44 ± 3 | 46 ± 3 | 47 ± 3 | Interaction: P < 0.001 |
| Hct (%) | |||||||
| Water | 52 ± 4 | 56 ± 4 | 54 ± 4 | 54 ± 4 | 55 ± 5 | 56 ± 5 | Time effect: P < 0.001 |
| CHO‐2% | 53 ± 4 | 56 ± 4 | 54 ± 4 | 54 ± 4 | 54 ± 4 | 54 ± 4 | Drink effect: P = 0.357 |
| CHO‐6.5% | 51 ± 3 | 55 ± 4 | 55 ± 4 | 54 ± 3 | 53 ± 3 | 52 ± 3 | Interaction: P = 0.045 |
| Hb (g dl−1) | |||||||
| Water | 15.3 ± 1.1 | 16.6 ± 1.2 | 16.1 ± 1.1 | 16.0 ± 1.2 | 16.0 ± 1.2 | 16.1 ± 1.3 | Time effect: P < 0.001 |
| CHO‐2% | 15.4 ± 1.2 | 16.7 ± 1.2 | 16.0 ± 1.2 | 16.1 ± 1.1 | 15.9 ± 1.2 | 15.9 ± 1.2 | Drink effect: P = 0.968 |
| CHO‐6.5% | 15.3 ± 1.1 | 16.6 ± 1.2 | 16.1 ± 1.1 | 16.0 ± 1.2 | 16.0 ± 1.2 | 16.1 ± 1.3 | Interaction: P = 0.073 |
- Values are presented as means ± SD. n = 9 for BV, CV, PV and Hb; n = 10 for Hct. BL, baseline; Ex, end of exercise; W, water; BV, blood volume; CV, cell volume; PV, plasma volume; Hct, haematocrit; Hb, haemoglobin concentration; CHO‐2%, 2% carbohydrate (isomaltulose); CHO‐6.5%, 6.5% carbohydrate.
Figure 2 depicts the Tco, Tsk, HR, MAP, chest SR and chest CVC response prior to, at the end of and following the final 15 min exercise bout. No significant differences were measured in Tco (P ≥ 0.299 for both main effect of drink and interaction), Tsk (both P ≥ 0.329), Tb (data not shown, both P ≥ 0.465), chest SR (all P ≥ 0.078, Friedman's test), and chest CVC (both P ≥ 0.210) (Figure 2). Similarly, no differences were measured in whole‐body sweat rate including sweat loss measured during and following the final 15 min exercise bout (Table 2). A main effect of drink on MAP was observed (P = 0.027) such that a higher MAP was observed in CHO‐2% (P = 0.008) and CHO‐6.5% (P < 0.001) compared with Water. While we observed a higher HR in the Water condition relative to CHO‐2% during exercise (P = 0.036, interaction: P = 0.031), no differences were observed over the 2 h recovery period. Finally, total urine output tended to be reduced with CHO‐6.5% relative to the other two beverage conditions (main effect, P = 0.069, Table 2).
4 DISCUSSION
We showed that ingesting a beverage containing 6.5% isomaltulose and electrolytes following cessation of exercise under a heat stress condition enhanced PV recovery as compared to a beverage containing 2% isomaltulose and electrolytes or water only. However, the ingestion of the 6.5% isomaltulose–electrolyte drink response was also paralleled by a higher Posm. In contrast to our hypothesis, we did not observe an effect of rehydrating with a drink containing isomaltulose and electrolytes on heat loss responses of cutaneous perfusion and sweating despite the resultant increases in PV. In fact, we observed a similar heat loss response across all drink conditions over the 2 h recovery period despite marked differences in both PV and Posm, key non‐thermal factors previously shown to modulate heat loss responses. Taken together, our findings demonstrate that ingestion of carbohydrate–electrolyte beverages containing 6.5% isomaltulose during recovery in heat‐stress conditions improves hydration state without altering heat loss.
We observed an elevated PV during postexercise recovery during the late stages of the 2 h recovery with the ingestion of 6.5% isomaltulose only. Our findings indicate that the ingestion of this beverage during the early stages of recovery (i.e. first 30 min) can help re‐establish near normal resting hydration status. Our findings are consistent with those of Kamijo et al. (2012) who reported a concentration‐dependent PV recovery at 120–180 min after rehydration under non‐heat stress conditions using a solution containing 6.5 g of carbohydrate comprising a mixture of glucose and fructose with electrolytes compared with beverages containing 3.3 g carbohydrates plus electrolytes and a carbohydrate‐free control. Since in the present study we used water as a control drink, it could be argued that the electrolyte content in the CHO‐6.5% beverage may have in part influenced the change in PV, a response independent of the amount of carbohydrate in the beverage. However, we showed that the elevation in PV only occurred with the ingestion of the beverage containing 6.5% isomaltulose despite the fact that both the CHO‐2% and CHO‐6.5% drinks included the same amount of electrolytes (Table 1, Figure 1). Our findings therefore indicate that the concentration of isomaltulose was likely an important determinant of the change in PV following an exercise‐induced state of mild hypohydration.
Several possible mechanisms may explain our observation of an enhanced PV recovery with the ingestion of a 6.5% isomaltulose–electrolyte beverage. Firstly, the ingestion of a 6.5% isomaltulose–electrolyte beverage induced a sustained elevation of Posm over the course of the 2 h recovery period as compared with the other two beverages. This response would play an important role in minimizing diuresis thereby enhancing the restoration of body fluid. In support of this possibility, we observed a slight reduction in urine output with the ingestion of the CHO‐6.5% beverage as compared to the other two beverages (Table 2). Secondly, it has been suggested that carbohydrate fluid intake enhances renal sodium ion reabsorption, which is associated with elevated insulin concentration (Kamijo et al., 2012) since insulin enhances sodium and fluid reabsorption in the tubules in the kidney (Tiwari, Riazi, & Ecelbarger, 2007). We observed concentration‐dependent changes in Glu and Pins during the recovery period (Figure 2), implying that Pins might play a role in the restoration of postexercise PV recovery observed with the consumption of the 6.5% isomulatulose–electrolyte beverage. Taken together, we showed that sustained elevations in Pins and Posm appear to be critical factors for promoting PV recovery after 6.5% isomaltulose–electrolyte beverage ingestion.
We observed a sustained elevation in Posm in the early‐ to mid‐stages of recovery (i.e. ≤90 min) following the ingestion of the 6.5% isomaltulose–electrolyte beverage relative to the other two beverages. The increase in PV was observed in the late stage of recovery only. This was paralleled by a return of heat loss responses back to near pre‐exercise resting levels (Figure 2). However, we observed a significantly greater MAP after the ingestion of both a 2% and a 6.5% isomaltulose solution relative to water only (Figure 2). The activation of osmoreceptors (elevated Posm) and baroreceptors (associated with changes in blood pressure and therefore baroreceptor loading status) has been shown to alter the regulation of postexercise heat loss responses of skin blood flow and sweating resulting in a prolonged elevated state of hyperthermia, a response which is worse during recovery in the heat (Franklin, Green, & Cable, 1993). However, both non‐thermal modulators have been shown to have differing levels of influence on heat dissipation (Paull et al., 2016). A recent study by Suzuki et al. (2014) showed that an elevated Posm following fructose ingestion impairs cutaneous vasodilatation and sweating as reflected by elevated body core temperature thresholds during a passive heat stress. In addition, a hyperosmolality (approximately 10 mosmol kg−1) induced by 3.0% NaCl infusion has been shown to attenuate postexercise sweating in the heat (Paull et al., 2016). However, in contrast to these previous observations, we showed no effect of 6.5% isomaltulose fluid ingestion on heat loss responses. It is unclear why we did not observe a reduction in heat loss responses. It may be that the 30 min fluid ingestion period in the early stages of recovery induced only a slow transient change in Posm compared with the more rapid increases in Posm that occurred with direct infusion of NaCl (Paull et al., 2016). It is plausible therefore that the changes in Posm observed in the present study may be insufficient to modulate postexercise heat loss responses.
As noted above, changes in baroreceptor loading status have been show to modulate heat loss responses. For example, reversing the postexercise reduction in MAP, and therefore baroreceptor loading status, during the postexercise recovery period through the application of positive pressure to the lower limbs has been shown to enhance heat loss responses of skin blood flow (Journeay et al., 2004; McGinn et al., 2014; Paull et al., 2016) and sweating (Jackson & Kenny, 2003; Journeay et al., 2004) during recovery in the heat. However, as is the case with an elevation in Posm, we observed no effect on postexercise heat loss responses under any condition, including the ingestion of 2.0% and 6.5% isomaltulose solution, which caused an increase in MAP with a marginally greater increase measured in the 6.5% isomaltulose condition. The mechanism(s) underlying the isomaltulose‐mediated increase in MAP remains unclear although it has been reported that plasma hyperosmolality induced by 3% NaCl infusion elevates MAP during passive heating (Shibasaki, Aoki, Morimoto, Johnson, & Takamata, 2009). Taken together, it is likely that the lack of an effect of a change in baroreceptor loading status on heat loss responses may be attributed to the relatively small increase in MAP (2–4 mmHg). This is a likely possibility given that prior studies showed elevated heat dissipation with manoeuvres (e.g. application of lower body positive pressure) causing larger increases in postexercise MAP (i.e. 10–15 mmHg) (Jackson & Kenny, 2003; Journeay et al., 2004). In the context of the 6.5% isomaltulose condition, it is possible, however, that the elevation in Posm may have negated the activation of heat loss responses caused by baroreceptor loading (associated with an increase in MAP) (Paull et al., 2016). Further studies are required to assess the separate and interactive influences of these mechanisms on postexericse hydration state and heat loss with different concentrations of carbohydrate beverage.
4.1 Perspectives and significance
Isomaltulose is a unique carbohydrate known for its slow absorption that results in slow elevations in blood glucose and insulin compared with other carbohydrate beverages (Kawai, Okuda, & Yamashita, 1985; van Can, IJzerman, van Loon, Brouns, & Blaak, 2009). Several studies have reported important physiological benefits associated with the consumption of isomaltulose during exercise including the maintenance of blood glucose, increased fat oxidation, and improved exercise performance relative to maltodextrin or sucrose (Achten et al., 2007; König et al., 2016). The present study advances our understanding of these physiological benefits, by showing that isomaltulose assists in re‐establishing hydration status following an exercise heat stress without compromising recovery of body core temperature.
4.2 Limitations
We did not directly compare changes in postexercise hydration and the physiological heat loss responses associated with the consumption of an isomaltulose–electrolyte beverage to other standard carbohydrate–electrolyte (e.g. glucose) drinks. This is because our study was designed to investigate these effects in regard to the differences in concentration of isomaltulose. However, further studies are required to assess if the responses observed in the present study differ from those associated with the consumption of other carbohydrate–electrolyte beverages, such as those containing glucose and sucrose, which are commonly used in commercial sports drinks.
After we confirmed the participants achieved a mild state of hypohydration (i.e. 2% loss of body mass), the participants conducted a final 15 min exercise bout under a high heat stress condition (i.e. exercising while wearing a water‐perfusion suit circulated with water maintained at 45°C). This additional exercise would have caused a further increase in whole‐body sweating resulting in a further hypohydration. However, we did not measure body mass loss after this final exercise bout since body weight measurement with minimum clothing in a thermoneutral condition would likely have caused a rapid attenuation of heat loss responses thereby making it difficult to investigate the potential influence of fluid consumption on these variables during the postexercise recovery period. We are therefore unaware of the exact body mass loss and the magnitude of hypohydration after the final exercise bout. Finally, it is important to note that we clamped postexercise skin temperature using the water‐perfusion suit to minimize potential differences in thermal status (e.g. core and skin temperatures) between the conditions and to isolate the relative influence of non‐thermal factors on heat loss responses. However, this approach limits our ability to assess any potential differences in body temperature between the beverage conditions that may have otherwise occurred in recovery. Further studies are required to assess the separate and combined influences of thermal and non‐thermal factors between beverage conditions.
In conclusion, we show that ingesting beverages containing 6.5% isomaltulose with electrolytes following cessation of exercise restores hydration status as compared to the ingestion of a beverage containing a 2% isomaltulose solution or water only. However, we show that beverage type did not affect the postexercise heat loss responses of cutaneous perfusion and sweating and therefore core temperature.
ACKNOWLEDGEMENTS
We thank our volunteer participants for participating in this study. We appreciate Dr Tomoyuki Yokoyama for his medical support. This study was a graduate research project of Y.S. and J.O. for their bachelor's degree.
COMPETING INTERESTS
K.S., Y.E. and D.M. are employees of Bourbon Corporation. The views expressed in this manuscript do not represent those of Bourbon Corporation.
AUTHOR CONTRIBUTIONS
Conception and design of research were undertaken by T.A., K.S., Y.E. and D.M. Data were collected by T.A., Y.S. and J.O. Data and sample analysis were performed by T.A., Y.S., J.O., N.F., K.S. and Y.E. The manuscript was drafted by T.A., N.F. and G.P.K. All authors contributed to data interpretation, editing and revision of manuscript critically. All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.
