Seasonal changes in hydration in free-living Japanese children and adolescents.

Changes in hydration status occur throughout the day affecting physiological and behavioral functions. However, little is known about the hydration status of free-living Japanese children and the seasonality of this response. We evaluated hydration status estimated by urine osmolality (Uosm) in 349 children (189 boys and 160 girls, 9.5 ± 2.6 years, range: 6 to 15 years) upon waking at home and during a single school day in spring (April) and summer (July). Further, we assessed the efficacy of employing self-assessment of urine color (UC, based on an 8-point scale) by children to monitor their hydration status. Early morning Uosm was greater in the spring (903 ± 220 mOsm L-1; n = 326) as compared to summer (800 ± 244 mOsm L-1; n = 125) (P = 0.003, paired t-test, n = 104). No differences, however, were observed in Uosm during the school day (P = 0.417, paired t-test, n = 32). While 66% and 50% of children were considered underhydrated (Uosm ≧800 mOsm L-1) upon waking in the spring and summer periods respectively, more children were underhydrated (∼12%) during the school day. Self-reported UC was similar between seasons as assessed in the morning and school day (P ≧ 0.101, paired t-test), which differed from the pattern of responses observed with Uosm. We showed that a significant number of Japanese children are likely underhydrated especially in the spring period. Children do not detect seasonal changes in hydration from self-assessed UC, limiting its utility to manage hydration status in children.

Effects of ingesting beverages containing glycerol and sodium with isomaltulose or sucrose on fluid retention in young adults: a single-blind, randomized crossover trial.

We evaluated changes in hyperhydration and beverage hydration index (BHI, a composite measure of fluid balance after consuming a test beverage relative to water) during resting, induced by the consumption of beverages containing glycerol and sodium supplemented with fast-absorbing sucrose or slow-absorbing isomaltulose. In a randomized crossover, single-blinded protocol (clinical trials registry: UMIN000042644), 14 young physically active adults (three women) consumed 1 L of beverage containing either 7% glycerol + 0.5% sodium (Gly + Na), Gly + Na plus 7% sucrose (Gly + Na + Suc), Gly + Na plus 7% isomaltulose (Gly + Na + Iso), or water (CON) over a 40 min period. We assessed the change in plasma volume (ΔPV), BHI (calculated from cumulative urine output following consumption of water relative to that of the beverage), and blood glucose and sodium for 180 min after initiating ingestion. Total urine volume was reduced in all beverages containing glycerol and sodium compared to CON (all P ≤ 0.002). The addition of isomaltulose increased BHI by ∼45% (3.43 ± 1.0 vs. 2.50 ± 0.7 for Gly + Na, P = 0.011) whereas sucrose did not (2.6 ± 0.6, P = 0.826). The PV expansion was earliest for Gly + Na (30 min), slower for Gly + Na + Suc (90 min), and slowest for Gly + Na + Iso (120 min) with a concomitant lag in the increase of blood glucose and sodium concentrations. Supplementation of beverages containing glycerol and sodium with isomaltulose but not sucrose enhances BHI from those of glycerol and sodium only under a resting state, likely due to the slow absorption of isomaltulose-derived monosaccharides (i.e., glucose and fructose).

Transdermal iontophoretic application of l-NAME is available in sweating research induced by heat stress in young healthy adults.

Iontophoretic transdermal administration of NG-nitro-l-arginine methyl ester hydrochloride [l-NAME, a nitric oxide synthase (NOS) inhibitor] has been used as a non-invasive evaluation of NOS-dependent mechanisms in human skin. However, the availability has yet to be investigated in sweating research. Prior observations using invasive techniques (e.g., intradermal microdialysis technique) to administer l-NAME have implicated that NOS reduces sweating induced by heat stress but rarely influences the response induced by the administration of cholinergic muscarinic receptor agonists. Therefore, we investigated whether the transdermal iontophoretic administration of l-NAME modulates sweating similar to those prior observations. Twenty young healthy adults (10 males, 10 females) participated in two experimental protocols on separate days. Before each protocol, saline (control) and 1% l-NAME were bilaterally administered to the forearm skin via transdermal iontophoresis. In protocol 1, 0.001% and 1% pilocarpine were iontophoretically administered at l-NAME-treated and untreated sites. In protocol 2, passive heating was applied by immersing the lower limbs in hot water (43 °C) until the rectal temperature increased by 0.8 °C above baseline. The sweat rate was continuously measured throughout both protocols. Pilocarpine-induced sweat rate was not significantly different between the control and l-NAME-treated sites in both pilocarpine concentrations (P ≥ 0.316 for the treatment effect and interaction of treatment and pilocarpine concentration). The sweat rate during passive heating was attenuated at the l-NAME-treated site relative to the control (treatment effect, P = 0.020). Notably, these observations are consistent with prior sweating studies administrating l-NAME into human skin using intradermal microdialysis techniques. Based on the similarity of our results with already known observations, we conclude that transdermal iontophoresis of l-NAME is a valid non-invasive technique for the investigation of the mechanisms of sweating related to NOS during heat stress.

Influence of Heat Exposure on Motor Control Performance and Learning as Well as Physiological Responses to Visuomotor Accuracy Tracking Task.

This study aimed to determine whether heat exposure attenuates motor control performance and learning, and blunts cardiovascular and thermoregulatory responses to visuomotor accuracy tracking (VAT) tasks. Twenty-nine healthy young adults (22 males) were divided into two groups performing VAT tasks (5 trials × 10 blocks) in thermoneutral (NEUT: 25 °C, 45% RH, n = 14) and hot (HOT: 35 °C, 45% RH, n = 15) environments (acquisition phase). One block of the VAT task was repeated at 1, 2, and 4 h after the acquisition phase (retention phase). Heat exposure elevated skin temperature to ~3 °C with a marginally increased core body temperature. VAT performance (error distance of curve tracking) was more attenuated overall in HOT than in NEUT in the acquisition phase without improvement in magnitude alteration. Heat exposure did not affect VAT performance in the retention phase. The mean arterial blood pressure and heart rate, but not for sweating and cutaneous vascular responses to VAT acquisition trials, were more attenuated in HOT than in NEUT without any retention phase alternations. We conclude that skin temperature elevation exacerbates motor control performance and blunts cardiovascular response during the motor skill acquisition period. However, these alternations are not sustainable thereafter.

Effects of ingestion of isomaltulose beverage on plasma volume and thermoregulatory responses during exercise in the heat.

PURPOSE: Isomaltulose is a low glycemic and insulinaemic carbohydrate increasingly used as an alternative sweetener in commercial beverages. While isomaltulose beverages can improve hydration status compared to sucrose-based beverages, it remains unclear if ingestion of an isomaltulose beverage prior to exercise in the heat may improve plasma volume (PV) and thermoregulatory responses. METHODS: Twelve endurance-trained men consumed a 1L carbohydrate beverage containing either 6.5%-sucrose (SUC) or 6.5%-isomaltulose (ISO) 60 min prior to 5 successive, 15-min bouts of moderate-intensity (60% of their pre-determined maximum oxygen uptake) in the heat (32 °C, 50% relative humidity), each separated by a 5 min rest. A 6th bout was performed, wherein the participant adjusted running speed to maximize distance covered within the 15-min period. The change (Δ) in PV, heart rate (HR), body core (rectal and gastrointestinal) and skin temperatures, and whole-body sweat loss were assessed during each exercise bout. RESULTS: Ingestion of ISO induced a higher ΔPV at 4th bout only (P < 0.001) and lower HR (P = 0.032, main effect of beverage) during exercise compared to those of SUC. Body core and skin temperatures and whole-body sweat loss did not differ between conditions (all P ≥ 0.192, interaction effect). Running distance covered in final exercise bout tended to increase (~ 5%) in ISO versus SUC (P = 0.057, d = 0.64). CONCLUSIONS: Relative to a sucrose-based beverage, ISO ingestion prior to exercise in the heat reduced cardiovascular strain by preserving PV and attenuating HR, albeit with no corresponding benefit on thermoregulatory function. The former response may facilitate improvements in exercise performance.

Comparison of hydration efficacy of carbohydrate-electrolytes beverages consisting of isomaltulose and sucrose in healthy young adults: A randomized crossover trial.

Isomaltulose is a low glycemic and insulinaemic carbohydrate now used as an alternative sweetener in beverages. However, it remains unclear if hydration status may be impacted differently with the consumption of beverages containing isomaltulose as compared to sucrose, a common beverage sweetener. Thirteen young adults (7 women) consumed 1 L of a carbohydrate beverage (with low electrolyte content) containing either 6.5%-sucrose, 6.5%-isomaltulose, or water within a 15 min period. For each beverage, beverage hydration index (BHI, a composite measure of fluid balance after consuming a test beverage relative to water) was calculated from urine volume produced over a 3 h period following ingestion of the carbohydrate beverages relative to water. The change in plasma volume (ΔPV), blood glucose, and lactate concentrations were assessed every 30 min post-beverage consumption. Isomaltulose ingestion attenuated urine production as compared to water and sucrose (P ≤ 0.005) over the 3 h post-ingestion period. However, no differences were observed between sucrose and water (P = 0.055). BHI was 1.53 ± 0.44 for isomaltulose (P ≤ 0.022 vs. sucrose and water) and 1.20±0.29 for sucrose (P = 0.210 vs. water). A transient reduction in ΔPV was observed following the ingestion of the isomaltulose beverage (at 30 min, P = 0.007 vs. sucrose). Thereafter, no differences in ΔPV between beverages were measured. Increases in blood glucose and lactate, indices of absorption and utility of glucose, were delayed in the isomaltulose as compared to sucrose beverage. In summary, we demonstrated a greater BHI with a carbohydrate-electrolyte beverage containing isomaltulose as compared to sucrose. This may in part be attributed to a delayed absorption of isomaltulose reducing diuresis.

Comparisons of isomaltulose, sucrose, and mixture of glucose and fructose ingestions on postexercise hydration state in young men.

PURPOSE: Isomaltulose is a low glycemic and insulinaemic carbohydrate available as a constituent in sports drink. However, it remains unclear whether postexercise rehydration achieved by isomaltulose drink ingestion alone differs as compared to other carbohydrates. METHODS: Thirteen young men performed intermittent exercise in the heat (35 °C and relative humidity 40%) to induce a state of hypohydration as defined by a 2% loss in body mass. Thereafter, participants were rehydrated by ingesting drinks equal to the volume of body mass loss with either a mixture of 3.25% glucose and 3.25% fructose, 6.5% sucrose (SUC), or 6.5% isomaltulose (ISO) within the first 30 min of a 3-h recovery. The change in plasma volume (ΔPV) from pre-exercise baseline, blood glucose, and plasma insulin concentration were assessed every 30-min. RESULTS: ΔPV was lower in ISO as compared to SUC until 90 min of the recovery (all P ≤ 0.038) with no difference thereafter (all P ≥ 0.391). The ΔPV were paralleled by concomitant changes in blood glucose levels that were greater in ISO as compared to other drinks after 90 min of the recovery (all P ≤ 0.035). Plasma insulin secretion, which potentially enhances renal sodium reabsorption and fluid retention, did not differ between the trials (interaction, P = 0.653). ISO induced a greater net fluid volume retention as compared to SUC (P = 0.010). CONCLUSION: We showed that rehydration with an isomaltulose drink following exercise-heat stress induces comparable recovery of PV and a greater net fluid retention as compared to other drinks, albeit this response is delayed. The delayed water transport along with glucose absorption may modulate this response. This trial was registered in 25th Sep 2019 at https://www.umin.ac.jp/ as UMIN000038099. (249/250).

Effects of Isomaltulose Ingestion on Thermoregulatory Responses during Exercise in a Hot Environment.

Isomaltulose is a low glycemic and insulinemic carbohydrate available as a constituent of sports drinks. However, it remains unclear whether thermoregulatory responses (sweating and cutaneous vasodilation) after isomaltulose drink ingestion differ from those of sucrose and water during exercise in a hot environment. Ten young healthy males consumed 10% sucrose, 10% isomaltulose, or water drinks. Thirty-five minutes after ingestion, they cycled for fifteen minutes at 75% peak oxygen uptake in a hot environment (30 °C, 40% relative humidity). Sucrose ingestion induced greater blood glucose concentration and insulin secretion at the pre-exercise state, compared with isomaltulose and/or water trials, with no differences during exercise in blood glucose. Change in plasma volume did not differ between the three trials throughout the experiment, but both sucrose and isomaltulose ingestions similarly increased plasma osmolality, as compared with water (main beverage effect, p = 0.040)-a key response that potentially delays the onset of heat loss responses. However, core temperature thresholds and slopes for heat loss responses were not different between the trials during exercise. These results suggest that ingestion of isomaltulose beverages induces low glycemic and insulinemic states before exercise but does not alter thermoregulatory responses during exercise in a hot environment, compared with sucrose or water.