Jenna M. Burchfield

Hydration Status over 24-H Is Not Affected by Ingested Beverage Composition

Objective: To investigate the 24-h hydration status of healthy, free-living, adult males when given various combinations of different beverage types. Methods: Thirty-four healthy adult males participated in a randomized, repeated-measures design in which they consumed: water only (treatment A), water+cola (treatment B), water+diet cola (treatment C), or water+cola+diet cola+orange juice (treatment D) over a sedentary 24-h period across four weeks of testing. Volumes of fluid were split evenly between beverages within each treatment, and when accounting for food moisture content and metabolic water production, total fluid intake from all sources was equal to 35 ± 1 ml/kg body mass. Urine was collected over the 24-h intervention period and analyzed for osmolality (Uosm), volume (Uvol) and specific gravity (USG). Serum osmolality (Sosm) and total body water (TBW) via bioelectrical impedance were measured after the 24-h intervention. Results: 24-h hydration status was not different between treatments A, B, C, and D when assessed via Uosm (590 ± 179; 616 ± 242; 559 ± 196; 633 ± 222 mOsm/kg, respectively) and Uvol (1549 ± 594; 1443 ± 576; 1690 ± 668; 1440 ± 566 ml) (all p > 0.05). A -difference in 24-h USG was observed between treatments A vs. D (1.016 ± 0.005 vs. 1.018 ± 0.007; p = 0.049). There were no differences between treatments at the end of the 24-h with regard to Sosm (291 ± 4; 293 ± 5; 292 ± 5; 293 ± 5 mOsm/kg, respectively) and TBW (43.9 ± 5.9; 43.8 ± 6.0; 43.7 ± 6.1; 43.8 ± 6.0 kg) (all p > 0.05). Conclusions: Regardless of the beverage combination consumed, there were no differences in providing adequate hydration over a 24-h period in free-living, healthy adult males. This confirms that beverages of varying composition are equally effective in hydrating the body.

Hydration status influences the measurement of arterial stiffness

Consensus guidelines have attempted to standardize the measurement and interpretation of pulse wave velocity (PWV); however, guidelines have not addressed whether hydration status affects PWV. Moreover, multiple studies have utilized heat stress to reduce arterial stiffness which may lead to dehydration. This study utilized two experiments to investigate the effects of dehydration on PWV at rest and during passive heat stress. In experiment 1, subjects (n = 19) completed two trials, one in which they arrived euhydrated and one dehydrated (1·2[1·0]% body mass loss). In experiment 2, subjects (n = 11) began two trials euhydrated and in one trial did not receive water during heat stress, thus becoming dehydrated (1·6[0·6]% body mass loss); the other trial subjects remained euhydrated. Using Doppler ultrasound, carotid‐to‐femoral (central) and carotid‐to‐radial (peripheral) PWVs were measured. PWV was obtained at a normothermic baseline, and at a 0·5°C and 1°C elevation in rectal temperature (via passive heating). In experiment 1, baseline central PWV was significantly higher when euhydrated compared to dehydrated (628[95] versus 572[91] cm s−1, respectively; P<0·05), but peripheral PWV was unaffected (861[117] versus 825[149] cm s−1; P>0·05). However, starting euhydrated and becoming dehydrated during heating in experiment 2 did not affect PWV measures (P>0·05), and independent of hydration status peripheral PWV was reduced when rectal temperature was elevated 0·5°C (−74[45] cm s−1; P<0·05) and 1·0°C (−70[48] cm s−1; P<0·05). Overall, these data suggest that hydration status affects measurements of central PWV in normothermic, resting conditions. Therefore, future guidelines should suggest that investigators ensure adequate hydration status prior to measures of PWV.

Obesity, but not hypohydration, mediates changes in mental task load during passive heating in females

Background The independent effects of hypohydration and hyperthermia on cognition and mood is unclear since the two stresses often confound each other. Further, it is unknown if obese individuals have the same impairments during hyperthermia and hypohydration that is often observed in non-obese individuals. Methods The current study was designed to assess the independent and combined effects of mild hypohydration and hyperthermia on cognition, mood, and mental task load in obese and non-obese females. Twenty-one healthy females participated in two passive heating trials, wherein they were either euhydrated or hypohydrated prior to and throughout passive heating. Cognition (ImPACT), mental task load (NASA-TLX), and mood (Brunel Mood Scale; BRUMS) were measured before and after a 1.0 °C increase in core temperature (TC). Results After a 1.0 °C TC elevation, hypohydration resulted in greater (p < 0.05) body mass loss (−1.14 ± 0.48 vs −0.58 ± 0.48 kg; hypohydrated and euhydrated, respectively) and elevation in serum osmolality (292 ± 4 vs 282 ± 3 mOsm; p < 0.05) versus euhydration. Hypohydration, independent of hyperthermia, did not affect mental task load or mood (p > 0.05). Hyperthermia, regardless of hydration status, impaired (∼5 A.U) measures of memory-based cognition (verbal and visual memory), and increased mental task load, while worsening mood (p < 0.05). Interestingly, obese individuals had increased mental task load while hyperthermic compared to the non-obese individuals (p < 0.05) even while euhydrated. Hypohydration did not exacerbate any heat-related effects on cognition between obese and non-obese females (p > 0.05). Conclusion These data indicate that hyperthermia independently impairs memory-based aspects of cognitive performance, mental task load, and leads to a negative mood state. Mild hypohydration did not exacerbate the effects of hyperthermia. However, obese individuals had increased mental task load during hyperthermia.

No Change in 24-Hour Hydration Status Following a Moderate Increase in Fluid Consumption.

Purpose: To investigate changes in 24-hour hydration status when increasing fluid intake. Methods: Thirty-five healthy males (age 23.8 ± 4.7 years; mass 74.0 ± 9.4 kg) were divided into 4 treatment groups for 2 weeks of testing. Volumes of 24-hour fluid ingestion (including water from food) for weeks 1 and 2 was 35 and 40 ml/kg body mass, respectively. Each treatment group was given the same proportion of beverages in each week of testing: water only (n = 10), water + caloric cola (n = 7), water + noncaloric cola (n = 10), or water + caloric cola + noncaloric cola + orange juice (n = 8). Serum osmolality (Sosm), total body water (TBW) via bioelectrical impedance, 24-hour urine osmolality (Uosm), and volume (Uvol) were analyzed at the end of each 24-hour intervention. Results: Independent of treatment, total beverage consumption increased 22% from week 1 to 2 (1685 ± 320 to 2054 ± 363 ml; p < 0.001). Independent of beverage assignment, the increase in fluid consumption between weeks 1 and 2 did not change TBW (43.4 ± 5.2 vs 43.0 ± 4.8 kg), Sosm (292 ± 5 vs 292 ± 5 mOsm/kg), 24-hour Uosm (600 ± 224 vs 571 ± 212 mOsm/kg), or 24-hour Uvol (1569 ± 607 vs 1580 ± 554 ml; all p > 0.05). Conclusions: Regardless of fluid volume or beverage type consumed, measures of 24-hour hydration status did not differ, suggesting that standard measures of hydration status are not sensitive enough to detect a 22% increase in beverage consumption.