Is mild dehydration a risk for progression of childhood chronic kidney disease?

Children with chronic kidney disease (CKD) can have an inherent vulnerability to dehydration. Younger children are unable to freely access water, and CKD aetiology and stage can associate with reduced kidney concentrating capacity, which can also impact risk. This article aims to review the risk factors and consequences of mild dehydration and underhydration in CKD, with a particular focus on evidence for risk of CKD progression. We discuss that assessment of dehydration in the CKD population is more challenging than in the healthy population, thus complicating the definition of adequate hydration and clinical research in this field. We review pathophysiologic studies that suggest mild dehydration and underhydration may cause hyperfiltration injury and impact renal function, with arginine vasopressin as a key mediator. Randomised controlled trials in adults have not shown an impact of improved hydration in CKD outcomes, but more vulnerable populations with baseline low fluid intake or poor kidney concentrating capacity need to be studied. There is little published data on the frequency of dehydration, and risk of complications, acute or chronic, in children with CKD. Despite conflicting evidence and the need for more research, we propose that paediatric CKD management should routinely include an assessment of individual dehydration risk along with a treatment plan, and we provide a framework that could be used in outpatient settings.

Total water intake guidelines are sufficient for optimal hydration in United States adults

PURPOSE

Recent studies suggest that 24-h urine osmolality (UOsm) for optimal water intake should be maintained < 500 mmol·kg^-1. The purpose of this study was to determine the total water intake (TWI) requirement for healthy adults to maintain optimal hydration as indicated by 24-h urine osmolality < 500 mmol·kg^-1.

METHODS

Twenty-four-hour UOsm was assessed in 49 men and 50 women residing in the United States (age: 41 ± 14 y, body mass index: 26.3 ± 5.2 kg·m^-2). TWI was assessed from 7-day water turnover, using a dilution of deuterium oxide, corrected for metabolic water production. The diagnostic accuracy of TWI to identify UOsm < 500 mmol·kg^-1 was evaluated using receiver operating characteristic (ROC) analysis in men and women separately.

RESULTS

Twenty-four-hour UOsm was 482 ± 229 and 346 ± 182 mmol·kg^-1 and TWI was 3.57 ± 1.10 L·d^-1 and 3.20 ± 1.27 L·d^-1 in men and women, respectively. ROC analysis for TWI detecting 24-h UOsm < 500 mmol·kg^-1 in men yielded an area under the curve (AUC) of 77.4% with sensitivity, specificity, and threshold values of 83.3%, 64.5%, and 3.39 L·d^-1, respectively. The AUC was 82.4% in women with sensitivity, specificity, and threshold values of 85.7%, 72.1%, and 2.61 L·d^-1.

CONCLUSION

Considering threshold values in men and women of 3.4 L·d^-1 and 2.6 L·d^-1, respectively, maintaining TWI in line with National Academy of Medicine guidelines of 3.7 L·d^-1 in men and 2.7 L·d^-1 in women should be sufficient for most individuals in the United States to maintain 24-h UOsm < 500 mmol·kg^-1.

Towards a heuristic understanding of the storage effect

The storage effect is a general explanation for coexistence in a variable environment. Unfortunately, the storage effect is poorly understood, in part because the generality of the storage effect precludes an interpretation that is simultaneously simple, intuitive and correct. Here, we explicate the storage effect by dividing one of its key conditions-covariance between environment and competition-into two pieces, namely that there must be a strong causal relationship between environment and competition, and that the effects of the environment do not change too quickly. This finer-grained definition can explain a number of previous results, including (1) that the storage effect promotes annual plant coexistence when the germination rate fluctuates, but not when the seed yield fluctuates, (2) that the storage effect is more likely to be induced by resource competition than the apparent competition, and (3) why the storage effect arises readily in models with either stage structure or environmental autocorrelation. Additionally, our expanded definition suggests two novel mechanisms by which the temporal storage effect can arise-transgenerational plasticity and causal chains of environmental variables-thus suggesting that the storage effect is a more common phenomenon than previously thought.

The Effect of Stirrup Length on Impact Attenuation and Its Association With Muscle Strength

ABSTRACT

Keener, MM, Critchley, ML, Layer, JS, Johnson, EC, Barrett, SF, and Dai, B. The effect of stirrup length on impact attenuation and its association with muscle strength. J Strength Cond Res 35(11): 3056-3062, 2021-Horseback-riders have a high prevalence of low back injuries, which may be related to the repetitive low back impacts experienced in riding. The purposes of this study were to quantify the effect of 3 stirrup lengths and 2 riding styles on the peak acceleration experienced by the rider and the association between the peak acceleration and the rider's different elements of muscle strength. Thirteen female riders performed a sitting or rising trot at each of the 3 stirrup lengths (2-point length, mid-seat length, or dressage length), while the acceleration of the tibia, sacrum, seventh cervical vertebra (C7), and head were collected. Subjects completed a push-up, a vertical jump, and 4 core exercises to assess upper-body strength, lower-body strength, and core endurance, respectively. Peak acceleration of the sacrum, C7, and head were generally lower in the standing phase of the rising trot compared with the sitting phase of either the sitting or rising trot, particularly at the shortest stirrup length. Peak acceleration of the sacrum, C7, and head decreased as the stirrup length was shortened in the standing phase of the rising trot. Canonical correlations showed nonsignificant correlations between strength measurements and peak acceleration. Riding with more weight supported through the legs with a short stirrup length may decrease low back impacts and their associated injury risk. Technique training is likely needed to encourage riders to use lower-body and core strength for impact attenuation.

Sugarcane Workweek Study: Risk Factors for Daily Changes in Creatinine

INTRODUCTION

Agricultural workers laboring in thermally stressful environments are at increased risk for kidney injury and chronic kidney disease of unknown origin (CKDu), and their environmental and occupational exposures have been considered to be important risk factors. This study examined the effects of repeated kidney stress from the simultaneous strain of work and other factors experienced by workers in Guatemala during a typical workweek.

METHODS

We collected data from 107 sugarcane workers across 7 consecutive work shifts. Data included information on daily occupational, meteorological, environmental, and lifestyle factors. We used multivariable linear mixed models to evaluate associations of these factors with percent change in creatinine.

RESULTS

We observed that increasing wet bulb globe temperature (β = 2.5%, 95% confidence interval [CI] = 0.3%, 4.7%) and increasing diastolic blood pressure (β = 6.2%, 95% CI = 0.9%, 11.6%) were associated with increases in creatinine across the shift, whereas consumption of water from chlorinated dormitory tanks as compared to artesian well water (β = -17.5%, 95% CI = -29.6%, -5.4%) and increasing number of rest breaks (β = -5.8%, 95% CI = -9.0%, -2.6%) were found to be protective against increases in creatinine. Workers reporting drinking tank water had lower concentrations of urine creatinine-corrected arsenic, lead, uranium, and glyphosate compared to workers reporting the use of well water or municipal water.

CONCLUSION

These results reinforce the need to focus on preventive actions that reduce kidney injury among this worker population, including strategies to reduce heat stress, managing blood pressure, and examining water sources of workers for nephrotoxic contaminants.

Validity and Reliability of a Water Frequency Questionnaire to Estimate Daily Total Water Intake in Adults

The purpose of this investigation was to assess the validity and reliability of a seven-day water frequency questionnaire (TWI-FQ) to estimate daily total water intake (TWI) in comparison to a water turnover objective reference value via deuterium oxide (D₂O). Data collection occurred over 3 weeks, with a wash-out period during week two. Healthy adults (n = 98; 52% female; 41 ± 14 y; BMI, 26.4 ± 5.5 kg·m^-2) retrospectively self-reported consumption frequencies of 17 liquids and 35 foods with specified volumes/amounts for weeks one and three via TWI-FQ. Standard water content values were utilized to determine the volume of water consumed from each liquid and food for calculation of mean daily TWI for each week. Diet records were completed daily during week two to estimate metabolic water production. To assess validity of the TWI-FQ, participants consumed D₂O at the start of each week and provided urine samples immediately before ingestion, the following day, and at the end of the week to calculate water turnover. Metabolic water was subtracted from water turnover to estimate TWI. TWI-FQ validity was assessed via Bland-Altman plot for multiple observations. Reliability was assessed via intraclass correlation and Pearson's correlation between weeks. TWI-FQ significantly underestimated D₂O TWI by -350 ± 1,431 mL·d^-1 (95% confidence interval (CI): -551, -149 mL·d^-1). TWI-FQ TWI was significantly correlated (r = 0.707, P <0.01) and not different (198 ± 1,180 mL·d^-1, 95% CI: -38, 435 mL·d^-1) between weeks. TWI-FQ intraclass correlation = 0.706 was significant [95% CI: 0.591, 0.793; F _{(97, 98)} = 5.799], indicating moderate test-retest reliability. While this tool would not be suitable for individual TWI assessment, the magnitude of bias may be acceptable for assessment at the sample-level.

Effects of a 14-Day Hydration Intervention on Individuals with Habitually Low Fluid Intake

BACKGROUND

Debate continues over whether or not individuals with low total water intake (TWI) are in a chronic fluid deficit (i.e., low total body water) [<xref ref-type="bibr" rid="ref1">1</xref>]. When women with habitually low TWI (1.6 ± 0.5 L/day) increased their fluid intake (3.5 ± 0.1 L/day) for 4 days 24-h urine osmolality decreased, but there was no change in body weight, a proxy for total body water (TBW) [<xref ref-type="bibr" rid="ref2">2</xref>]. In a small (n = 5) study of adult men, there were no observable changes in TBW, as measured by bioelectrical impedance, after increasing TWI for 4 weeks [<xref ref-type="bibr" rid="ref3">3</xref>]. However, body weight increased and salivary osmolality decreased indicating that the study may have been underpowered to detect changes in TBW. Further, no studies to date have measured changes in blood volume (BV) when TWI is increased.

OBJECTIVES

Therefore, the purpose of this study was to identify individuals with habitually low fluid intake and determine if increasing TWI, for 14 days, resulted in changes in TBW or BV.

METHODS

In order to identify individuals with low TWI, 889 healthy adults were screened. Participants with a self-reported TWI less than 1.8 L/day (men) or 1.2 L/day (women), and a 24-h urine osmolality greater than 800 mOsm were included in the intervention phase of the study. For the intervention phase, 15 participants were assigned to the experimental group and 8 participants were assigned to the control group. The intervention period lasted for 14 days and consisted of 2 visits to our laboratory: one before the intervention (baseline) and 14 days into the intervention (14-day follow-up). At these visits, BV was measured using a CO-rebreathe procedure and deuterium oxide (D2O) was administered to measure TBW. Urine samples were collected immediately prior, and 3-8 h after the D2O dose to allow for equilibration. Prior to each visit, participants collected 24-h urine to measure 24-h hydration status. After the baseline visit, the experimental group increased their TWI to 3.7 L for males and 2.7 L for females in order to meet the current Institute of Medicine recommendations for TWI.

RESULTS

Twenty-four-hour urine osmolality decreased (-438.7 ± 362.1 mOsm; p < 0.001) and urine volume increased (1,526 ± 869 mL; p < 0.001) in the experimental group from baseline, while there were no differences in osmolality (-74.7 ± 572 mOsm; p = 0.45), or urine volume (-32 ± 1,376 mL; p = 0.89) in the control group. However, there were no changes in BV (Fig. <xref ref-type="fig" rid="f01">1</xref>a) or changes in TBW (Fig. <xref ref-type="fig" rid="f01">1</xref>b) in either group.

CONCLUSIONS

Increasing fluid intake in individuals with habitually low TWI increases 24-h urine volume and decreases urine osmolality but does not result in changes in TBW or BV. These findings are in agreement with previous work indicating that TWI interventions lasting 3 days [<xref ref-type="bibr" rid="ref2">2</xref>] to 4 weeks [<xref ref-type="bibr" rid="ref3">3</xref>] do not result in changes in TBW. Current evidence would suggest that the benefits of increasing TWI are not related changes in TBW.

Self-Reported Changes in Thirst and Alertness during Variable Prescribed Fluid Intake

BACKGROUND

Maintaining euhydration is beneficial for health, safety, and physical performance [<xref ref-type="bibr" rid="ref1">1</xref>]; however, it may also improve subjective feelings [<xref ref-type="bibr" rid="ref2">2</xref>, <xref ref-type="bibr" rid="ref3">3</xref>].

OBJECTIVE

The aim of this study was to evaluate the relationship between changes in self-reported thirst and alertness in people undergoing changes in drinking water volume.

METHODS

Subjects (mean ± SD) (n = 115, 59 males, 32 ± 10 years; 24.6 ± 4.4 kg·m-2) visited the lab 3 times over 10 days: V1, a baseline visit prior to participants were drinking ad libitum; V2, following 3 days of fluid restriction (1 L·d-1, 250 mL was consumed in the morning prior to the visit); and V3, the morning following a prescribed increase in water intake. The increase in water intake at V3 varied by group assignment: control group (CON) maintained 250 mL, while LOW and HIGH groups (n = 45 each) consumed 496 ± 82 mL and 878 ± 125 mL, respectively. At each visit, subjects indicated on an open-ended visual analog scale (VAS) how thirsty and alert they felt and were measured in millimeters (mm). Four, two-way ANOVAs (group × visit) for change in thirst and alertness between V1-V2 and V2-V3 were completed. A repeated-measures correlation (rrm) procedure was completed for change in alertness and thirst from V1 to V2 and V2 to V3 [<xref ref-type="bibr" rid="ref4">4</xref>]. The study was approved by the University of Wyoming's Institutional Review Board (protocol #20160524EJ01208), and all subjects provided written informed consent.

RESULTS

Groups were similar at baseline (V1) for fluid intake, thirst, and alertness (all p ≥ 0.17). Fluid restriction (V2) resulted in a main effect of visit for thirst and alertness (both p < 0.01), with no main effect of group. Thirst increased (35 ± 35 mm) and alertness decreased (-19 ± 31 mm) from V1 to V2. The prescribed increase in water intake (V3) revealed a significant interaction of visit and group for thirst and alertness (both p < 0.01) (Table <xref ref-type="table" rid="t01">1</xref>). Independent-samples t tests with a Bonferroni correction revealed that HIGH reduced thirst (-38 ± 37 mm) and increased alertness (18 ± 25 mm), while no change was observed for LOW (thirst, -7 ± 37 mm; alertness -1 ± 24 mm) and CON (thirst, -6 ± 23 mm; alertness 0 ± 23 mm; all p < 0.01) (Fig. <xref ref-type="fig" rid="f01">1</xref>). There was no difference between LOW and CON (both p > 0.92). Repeated-measures correlation analysis revealed an inverse relationship between change in alertness and thirst (rrm [114] = -0.53, 95% CI [-0.65, -0.38], p < 0.01).

CONCLUSION

A reduction in water intake resulted in an increase in thirst and decrease in alertness. Following 3 days of fluid restriction, 750-1,000 mL of water intake was needed to decrease thirst and increase alertness. Overall, an inverse relationship was observed between self-reported thirst and alertness.

Hippocrates Was Right: Now What? Water As a Part of Healthy Aging

BACKGROUND

Aging is defined as the progressive organism change leading to debility, disease, and death [<xref ref-type="bibr" rid="ref1">1</xref>]. We know that as we age, our risk increases for diseases such as diabetes, cancer, and chronic kidney disease, and many of our homeostatic processes change as well, such as cell signaling, metabolism, and proteostasis [<xref ref-type="bibr" rid="ref2">2</xref>]. The data clearly show that water intake decreases with aging, especially after age 60 years [<xref ref-type="bibr" rid="ref3">3</xref>]. However, the question becomes "Do we drink less because we age, or do we age because we drink less?" (Fig. 1).

SUMMARY

There are data to support both directions of this hypothesis. One example supporting that water intake decreases due to aging is that in older adults, the thirst response to hyperosmotic and hypovolemic stimuli is blunted in comparison to younger adults [<xref ref-type="bibr" rid="ref4">4</xref>]. However, we are increasingly gathering data to also support that low water intake can be a contributor to both disease and altered cellular processes, potentially accelerating aging related dysfunction. For example, in older adults, low water intake has been shown to be associated with working memory [<xref ref-type="bibr" rid="ref5">5</xref>], blood glucose regulation [<xref ref-type="bibr" rid="ref6">6</xref>], incidence of stroke [<xref ref-type="bibr" rid="ref7">7</xref>], and falls [<xref ref-type="bibr" rid="ref8">8</xref>] (i.e., dementia, diabetes, cardiovascular disease, and mobility). Additionally, processes such as metabolism [<xref ref-type="bibr" rid="ref9">9</xref>], cell signaling [<xref ref-type="bibr" rid="ref10">10</xref>], and muscle damage following exercise [<xref ref-type="bibr" rid="ref11">11</xref>] have also been linked to hydration. Key messages are that it is clear that water intake changes as we age, primarily due to changes in thirst. However, what is our duty as hydrationists is to begin to evaluate both directions of the aging, dysfunction, and water intake relationship (Fig. 1). Just as we have begun to realize the influence as water as a nutrient, we can hopefully demonstrate how a simple intervention like drinking to recommendations can be a part of healthy aging for all.

Estimating Differences in Risk of Chronic Kidney Disease Based on Water Intake in a National Sample

BACKGROUND

In agricultural communities in Central and South America, Egypt, India, and Sri Lanka, an unexplained form of chronic kidney disease affects agricultural workers. Termed chronic kidney disease of unknown origin (CKDu), it disproportionately affects young men in their 30s-40s and is unrelated to the traditional risk factors of diabetes, hypertension, and obesity [1-3]. Recent investigations suggest that agricultural work in the USA carries similar risks, as reduced kidney function has been found among those working in US agriculture [4-5]. However, researchers are yet to determine the etiology of the disease [6-8]. Central to the hypotheses of CKDu is the reduced blood flow to the kidneys due to inadequate hydration during periods of intense physical labor.

OBJECTIVES

The primary aim of the current investigation was to identify if a relationship between hydration and kidney function exists among the general population by using the data from the National Health and Nutrition Examination Survey (NHANES). We hypothesize that reduced hydration will be associated with reduced kidney function.

METHODS

Data were retrieved from the NHANES dataset from 3 sample years 2005/2006, 2007/2008, and 2011/2012. Data were merged across all 3 periods with survey weights adjusted for combining across multiple years. Participants were excluded if they had missing data for hydration or kidney function, or if they were <19 year. Kidney function was categorized low risk, moderate risk, or high risk for impaired function based on estimated glomerular filtration rate and albumin creatinine ratio according to the National Kidney Foundation [9]. Hydration was classified based on total water intake (TWI) extracted from plain water intake and water from food. Participants were labeled as high if they met or exceeded sex-specific water recommendations, 3.7 and 2.7 L/day for men and women, respectively; otherwise they were labeled as low. A survey-weighted proportional odds logistic regression model was fitted to assess the association between water intake and kidney function, while controlling for other demographic, socio-economic, behavioral, and socio-economic risk factors [10-12].

RESULTS

Of the 13,056 participants initially sampled, 10,651 participants are included in the analysis after cleaning and including survey weights. 9,125 (85.67%) of participants were in the low-risk group, 1,128 (10.59%) were classified as medium-risk, while the remaining 398 (3.74%) were high risk (Fig. 1). Adjusting for survey weights, results suggest that the estimated rate of high-risk kidney function was 5% more for low water drinkers compared to high water drinkers (Fig. 2). There is strong evidence of a difference in CKD risk categories based on TWI (χ2(1) = 13.1, p value <0.0001) from a survey-weighted proportional odds logistic regression model, but only moderate evidence of a difference when controlled for sodium/potassium ratio, education, age, gender, ethnicity, income, BMI, blood pressure, diabetes, smoking, and alcohol consumption (χ2(1) = 3.3, p value = 0.067).

CONCLUSIONS

Not meeting recommended daily TWI was associated increased presentation of high-risk kidney function. Even though the NHANES data are not focused on areas where chronic kidney disease is prevalent, results from this are an indication that hydration does play a role in kidney function.

Impact of Nutrient Intake on Hydration Biomarkers Following Exercise and Rehydration Using a Clustering-Based Approach

We investigated the impact of nutrient intake on hydration biomarkers in cyclists before and after a 161 km ride, including one hour after a 650 mL water bolus consumed post-ride. To control for multicollinearity, we chose a clustering-based, machine learning statistical approach. Five hydration biomarkers (urine color, urine specific gravity, plasma osmolality, plasma copeptin, and body mass change) were configured as raw- and percent change. Linear regressions were used to test for associations between hydration markers and eight predictor terms derived from 19 nutrients merged into a reduced-dimensionality dataset through serial k-means clustering. Most predictor groups showed significant association with at least one hydration biomarker: (1) Glycemic Load + Carbohydrates + Sodium, (2) Protein + Fat + Zinc, (3) Magnesium + Calcium, (4) Pinitol, (5) Caffeine, (6) Fiber + Betaine, and (7) Water; potassium + three polyols, and mannitol + sorbitol showed no significant associations with any hydration biomarker. All five hydration biomarkers were associated with at least one nutrient predictor in at least one configuration. We conclude that in a real-life scenario, some nutrients may serve as mediators of body water, and urine-specific hydration biomarkers may be more responsive to nutrient intake than measures derived from plasma or body mass.

Association of Copeptin, a Surrogate Marker of Arginine Vasopressin, with Decreased Kidney Function in Sugarcane Workers in Guatemala

BACKGROUND

Vasopressin is elevated in response to heat and dehydration and has been postulated to have a role in the chronic kidney disease of unknown origin being observed in Central America. The aims of this study were to examine whether the vasopressin pathway, as measured by copeptin, is associated with the presence of kidney dysfunction, and to examine whether higher fluid intake is associated with lower circulating copeptin and thereby preserves kidney health among sugarcane workers exposed to hot conditions.

METHODS

Utilizing a longitudinal study of 105 workers in Guatemala, we examined relationships between hydration indices, plasma copeptin concentrations, and kidney function markers at 3 times during the 6-month harvest. We also examined whether baseline copeptin concentrations increased the odds of developing an estimated glomerular filtration rate (eGFR) <60 mL/min/1.73 m2.

RESULTS

Copeptin concentrations were positively associated with serum creatinine (β 1.41, 95% CI 0.88-2.03) and negatively associated with eGFR (β -1.07, 95% CI -1.43 to -0.70). In addition, as workers improved their hydration (measured by increases in fluid balance), copeptin concentrations were reduced, and this reduction was associated with an improvement in kidney function.

CONCLUSIONS

Results suggest that copeptin should be studied as a potential prognostic biomarker.

Educational Intervention Changes College Students' Attitudes toward Prescription Opioid Drug Use

Background: More than 47,000 people in the United States died from opioid drug overdoses in 2017. Among college students, opioid drugs are the second most abused drug. Objective: This study aimed to examine if an educational intervention impacted college students' attitudes towards prescription opioid drugs (POD). Methods: Two hundred forty-two participants (72 males, 21 ± 3 years) from an American university participated. After collecting demographic data (questionnaire 1; Q1), investigators recited a narrative in which the protagonist was injured and prescribed POD. Next, participants rated their agreement on 10 Likert prompts and two visual analog scales (VAS) before (Q2) and after (Q3) an educational intervention, then noted (Q4) which topics were most or least influential in any changed responses. Results: 7/10 Likert prompts (all p < 0.002) and both VAS (both p < 0.001) changed between Q2 and Q3. Educational intervention topics related to risk were most influential and topics related to alternative therapies were least influential. Conclusion/Importance: Educational interventions may be beneficial for college students. Any interventions that are employed should focus on risks associated with POD use.

Validation of a Vitamin D Specific Questionnaire to Determine Vitamin D Status in Athletes

The study objective was to validate a food frequency and lifestyle questionnaire (FFLQ) to assess vitamin D intake and lifestyle factors affecting status. Methods: Data collected previously during the fall (n = 86), winter (n = 49), and spring (n = 67) in collegiate-athletes (Study 1) and in active adults (n = 123) (Study 2) were utilized. Study 1: Vitamin D intake and ultraviolet B exposure were estimated using the FFLQ and compared to serum 25(OH)D concentrations via simple correlation and linear regression modeling. Study 2: Vitamin D intake from food was estimated using FFLQ and compared to vitamin D intake reported in 7-Day food diaries via paired t-test and Bland–Altman analysis. Results: Study 1: Serum 25(OH)D was not associated with vitamin D intake from food, food plus supplements, or sun exposure, but was associated with tanning bed use (r = 0.39) in spring, supplement use in fall (r = 0.28), and BMI (body mass index) (r = −0.32 to −0.47) across all seasons. Serum 25(OH)D concentrations were explained by BMI, tanning bed use, and sun exposure in fall, (R = 0.42), BMI in winter (R = 0.32), and BMI and tanning bed use in spring (R = 0.52). Study 2: Estimated Vitamin D intake from food was 186.4 ± 125.7 via FFLQ and 148.5 ± 228.2 IU/day via food diary. There was no association between intake estimated by the two methodologies (r = 0.12, p < 0.05). Conclusions: FFLQ-estimated vitamin D intake was not associated with serum 25(OH)D concentration or food-record-estimated vitamin D intake. Results highlight the difficulty of designing/utilizing intake methodologies for vitamin D, as its status is influenced by body size and both endogenous and exogenous (dietary) sources.

The renal molecular clock: broken by aging and restored by exercise

The mammalian circadian clock governs physiological, endocrine, and metabolic responses coordinated in a 24-h rhythmic pattern by the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. The SCN also dictates circadian rhythms in peripheral tissues like the kidney. The kidney has several important physiological functions, including removing waste and filtering the blood and regulating fluid volume, blood osmolarity, blood pressure, and Ca²⁺ metabolism, all of which are under tight control of the molecular/circadian clock. Normal aging has a profound influence on renal function, central and peripheral circadian rhythms, and the sleep-wake cycle. Disrupted circadian rhythms in the kidney as a result of increased age likely contribute to adverse health outcomes such as nocturia, hypertension, and increased risk for stroke, cardiovascular disease, and end organ failure. Regular physical activity improves circadian misalignment in both young and old mammals, although the precise mechanisms for this protection remain poorly described. Recent advances in the heart and skeletal muscle literature suggest that regular endurance exercise entrains peripheral clocks, and we propose that similar beneficial adaptations occur in the kidney through regulation of renal blood flow and fluid balance.

Urinary markers of hydration during 3-day water restriction and graded rehydration

PURPOSE

This investigation had three purposes: (a) to evaluate changes in hydration biomarkers in response to a graded rehydration intervention (GRHI) following 3 days of water restriction (WR), (b) assess within-day variation in urine concentrations, and (c) quantify the volume of fluid needed to return to euhydration as demonstrated by change in U_col.

METHODS

115 adult males and females were observed during 1 week of habitual fluid intake, 3 days of fluid restriction (1000 mL day^-1), and a fourth day in which the sample was randomized into five different GRHI groups: no additional water, CON; additional 500 mL, G_+0.50; additional 1000 mL, G_+1.00; additional 1500 mL, G_+1.50; additional 2250 mL, G_+2.25. All urine was collected on 1 day of the baseline week, during the final 2 days of the WR, and during the day of GRHI, and evaluated for urine osmolality, color, and specific gravity.

RESULTS

Following the GRHI, only G_+1.50 and G_+2.25 resulted in all urinary values being significantly different from CON. The mean volume of water increase was significantly greater for those whose U_col changed from > 4 to < 4 (+ 1435 ± 812 mL) than those whose U_col remained ≥ 4 (+ 667 ± 722 mL, p < 0.001).

CONCLUSIONS

An additional 500 mL of water is not sufficient, while approximately 1500 mL of additional water (for a total intake between 2990 and 3515 mL day^-1) is required to return to a urine color associated with adequate water intake, following 3 days of WR.

Examining the links between hydration knowledge, attitudes and behavior

PURPOSE

This study aimed to examine the psychological factors (knowledge, barriers and facilitators) that can contribute to hydration-related behaviors (i.e., fluid intake) in the general population and how these relate to physical health.

METHODS

A structured survey was developed to examine the links between hydration knowledge (29 items), attitudes about hydration (80 items), and fluid intake behavior (8 items) among US adults. Survey data from Phase 1 (n =301, US adults) psychometrically evaluated the items via item analysis (knowledge and fluid behavior) and factor analysis (attitudes). Phase 2 survey data (n =389, US adults and college students) refined and validated the new 16-item hydration knowledge measure, 4-item fluid intake behavior index, and 18-item attitude measure (barriers and facilitators of hydration-related behaviors) alongside indices of physical health (BMI and exercise behaviors).

RESULTS

Participants had a moderate level of hydration knowledge (Phase 1: 10.91 ± 3.10; Phase 2: 10.87 ± 2.47). A five-factor measure of attitudes which assessed both facilitators (social pressure and attention to monitoring) and barriers (lack of effort, physical barriers and lack of a fluid container) to hydration demonstrated strong internal consistency (αs from 0.75 to 0.90). Attitudes about hydration-most notably barriers to hydration-were associated with indicators of health and with fluid intake behaviors, whereas hydration knowledge was not.

CONCLUSIONS

Increasing hydration knowledge may be necessary for people who hold inaccurate information about hydration, but attitudes about hydration are likely to have a larger impact on fluid intake behaviors and health-related outcomes.

Water Intake, Water Balance, and the Elusive Daily Water Requirement

Water is essential for metabolism, substrate transport across membranes, cellular homeostasis, temperature regulation, and circulatory function. Although nutritional and physiological research teams and professional organizations have described the daily total water intakes (TWI, L/24h) and Adequate Intakes (AI) of children, women, and men, there is no widespread consensus regarding the human water requirements of different demographic groups. These requirements remain undefined because of the dynamic complexity inherent in the human water regulatory network, which involves the central nervous system and several organ systems, as well as large inter-individual differences. The present review analyzes published evidence that is relevant to these issues and presents a novel approach to assessing the daily water requirements of individuals in all sex and life-stage groups, as an alternative to AI values based on survey data. This empirical method focuses on the intensity of a specific neuroendocrine response (e.g., plasma arginine vasopressin (AVP) concentration) employed by the brain to regulate total body water volume and concentration. We consider this autonomically-controlled neuroendocrine response to be an inherent hydration biomarker and one means by which the brain maintains good health and optimal function. We also propose that this individualized method defines the elusive state of euhydration (i.e., water balance) and distinguishes it from hypohydration. Using plasma AVP concentration to analyze multiple published data sets that included both men and women, we determined that a mild neuroendocrine defense of body water commences when TWI is ˂1.8 L/24h, that 19⁻71% of adults in various countries consume less than this TWI each day, and consuming less than the 24-h water AI may influence the risk of dysfunctional metabolism and chronic diseases.

Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives

Heat acclimation or acclimatization (HA) occurs with repeated exposure to heat inducing adaptations that enhance thermoregulatory mechanisms and heat tolerance leading to improved exercise performance in warm-to-hot conditions. HA is an essential heat safety and performance enhancement strategy in preparation for competitions in warm-to-hot conditions for both individual and team sports. Yet, some data indicate HA is an underutilized pre-competition intervention in athletes despite the well-known benefits; possibly due to a lack of practical information provided to athletes and coaches. Therefore, the aim of this review is to provide actionable evidence-based implementation strategies and protocols to induce and sustain HA. We propose the following suggestions to circumvent potential implementation barriers: 1) incorporate multiple induction methods during the initial acclimation period, 2) complete HA 1-3 weeks before competition in the heat to avoid training and logistical conflicts during the taper period, and 3) minimize adaptation decay through intermittent exercise-heat exposure or re-acclimating immediately prior to competition with 2-4 consecutive days of exercise-heat training. Use of these strategies may be desirable or necessary to optimize HA induction and retention around existing training or logistical requirements.

Rapid thermal adaptation in a marine diatom reveals constraints and trade-offs

Rapid evolution in response to environmental change will likely be a driving force determining the distribution of species across the biosphere in coming decades. This is especially true of microorganisms, many of which may evolve in step with warming, including phytoplankton, the diverse photosynthetic microbes forming the foundation of most aquatic food webs. Here we tested the capacity of a globally important, model marine diatom Thalassiosira pseudonana, for rapid evolution in response to temperature. Selection at 16 and 31°C for 350 generations led to significant divergence in several temperature response traits, demonstrating local adaptation and the existence of trade-offs associated with adaptation to different temperatures. In contrast, competitive ability for nitrogen (commonly limiting in marine systems), measured after 450 generations of temperature selection, did not diverge in a systematic way between temperatures. This study shows how rapid thermal adaptation affects key temperature and nutrient traits and, thus, a population's long-term physiological, ecological, and biogeographic response to climate change.

Fluid consumption pattern and hydration among 8-14 years-old children

BACKGROUND/OBJECTIVES

Children consume various fluids to meet dietary water intake needs. However, the contribution of different fluid types on hydration is unclear. The purpose of this study was to develop fluid intake patterns and examine their association with hydration, as indicated by 24-h urine osmolality.

SUBJECTS/METHODS

Two hundred ten (105 girls) healthy children (height: 1.49 ± 0.13 m, weight: 43.4 ± 12.6 kg, body fat: 25.2 ± 7.8%) recorded their fluid intake for two consecutive days, and collected their urine for 24-h during the 2nd day, while conducting their normal daily activities. Urine samples were analyzed for specific gravity and osmolality. Factor analysis with principal components method was applied to extract dietary patterns from six fluid groups. Linear regression analysis evaluated the associations between the extracted dietary patterns and hydration based on 24-h urine osmolality.

RESULTS

The analysis revealed the following six components: 1, characterized by consumption of milk and fresh juice, but not packaged juice; 2, by regular soda and other drinks, but not water; 3, by fresh juice and other drinks; 4, by packaged juice, but not regular soda; 5, by water and milk; and 6, by fresh juice. Component 5 was negatively correlated with urine osmolality (P = 0.001) indicating better hydration, whereas component 2 was positively correlated with urine osmolality (P = 0.001).

CONCLUSIONS

A drinking pattern based on water and milk was associated with better hydration, as indicated by lower urine osmolality, whereas drinking regular soda and other drinks but not water was associated with inferior hydration.

Spot Sample Urine Specific Gravity Does Not Accurately Represent Small Decreases in Plasma Volume in Resting Healthy Males

BACKGROUND

Urine specific gravity (U_SG) is often used to assess hydration status, particularly around athletic competition, but it is unknown whether high U_SG is indicative of plasma volume (PV) reduction (i.e., hypohydration). We tested the hypothesis that if high U_SG is reflective of reduced PV, subsequent fluid ingestion would increase PV.

PURPOSE

The purpose of this study was to examine 24-hour changes in U_SG and PV in individuals presenting with high and low spot U_SG.

METHODS

Nineteen healthy males were provided food and water over 24 hours with a total water volume of 35 ml·kg^-1 body mass. Absolute PV and blood volume (BV), measured using the CO-rebreathe technique, along with U_SG were measured before and after a 24-hour intervention period. Based on a preintervention morning spot U_SG, subjects were post hoc assigned to groups according to U_SG (≤1.020 or >1.020; low and high U_SG, respectively).

RESULTS

Despite presenting with an elevated spot U_SG (1.026 ± 0.004), subsequent fluid ingestion over 24 hours did not lead to changes (∆) in PV (-75 ± 234 ml) or BV (-156 ± 370 ml) in the high U_SG group (p > 0.05). However, a spot U_SG after the 24-hour intervention in this group decreased (p = 0.018) to a level indicating improved hydration status (1.017 ± 0.007). In the low U_SG group, there were no changes in PV (-39 ± 274 ml), BV (-82 ± 396 ml), or U_SG (0.003 ± 0.007) over the 24-hour fluid intervention (all p > 0.05).

CONCLUSIONS

Despite a high preintervention U_SG and subsequent decrease after 24-hour fluid intake, measures of PV and BV were not indicative of this seemingly improved hydration status. This suggests that a highly concentrated spot sample U_SG and subsequent changes are not accurately representative of PV or BV.

Validation Testing Demonstrates Efficacy of a 7-Day Fluid Record to Estimate Daily Water Intake in Adult Men and Women When Compared with Total Body Water Turnover Measurement

Background: Mean daily water intake from fluids (WATER-FL) has proven to be difficult to measure because of a range of nonvalidated data collection techniques. Few questionnaires have been validated to estimate WATER-FL against self-reported diaries or urinary hydration markers, which may limit their objectivity.Objectives: The goals of this investigation were 1) to assess the validity of a 7-d fluid record (7dFLR) to measure WATER-FL (WATER-FL-7dFLR) through comparison with WATER-FL as calculated by measuring deuterium oxide (D₂O) disappearance (WATER-FL-D₂O), and 2) to evaluate the reliability of the 7dFLR in measuring WATER-FL.Methods: Participants [n = 96; 51% female; mean ± SD age: 41 ± 14 y; mean ± SD body mass index (in kg/m²): 26.2 ± 5.1] completed body water turnover analysis over 3 consecutive weeks. They completed the 7dFLR and food diaries during weeks 2 and 4 of the observation. The records were entered into nutritional software to determine the water content of all foods and fluids consumed. WATER-FL-D₂O was calculated from water turnover (via the D₂O dilution method), minus water from food and metabolic water. The agreement between the 2 methods of determining WATER-FL were compared according to a Bland-Altman plot at week 2. The test-retest reliability of 7dFLR between weeks 2 and 4 was assessed via intraclass correlation (ICC).Results: The mean ± SD difference between WATER-FL-7dFLR and WATER-FL-D₂O was -131 ± 845 mL/d. In addition, no bias was observed (F_[1,94] = 0.484; R² = 0.006; P = 0.488). When comparing WATER-FL-7dFLR from weeks 2 and 4, no significant difference (mean ± SD difference: 71 ± 75 mL/d; t_[79] = 0.954; P = 0.343) and an ICC of 0.85 (95% CI: 0.77, 0.90) was observed.Conclusions: The main findings of this study were that the use of the 7dFLR is an effective and reliable method to estimate WATER-FL in adults. This style of questionnaire may be extremely helpful for collecting water intake data for large-scale epidemiologic studies.

Water intake and urinary hydration biomarkers in children

BACKGROUND/OBJECTIVES

The aims of the study were as follows: (1) examine fluid intake and urinary hydration markers of children in Greece, (2) determine the calculated relative risk of hypohydration in children who did not meet the recommendations for daily water intake provided by the Institute of Medicine and the European Food Safety Authority compared with those who did and (3) analyze the efficacy of the recommendations as a method to achieve euhydration in children.

SUBJECTS/METHODS

One hundred and fifty Greek boys and girls (age 9-13) recorded their fluid intake for 2 consecutive days. A 24-h urine collection was obtained during the second day. Fluid intake records were analyzed for total water intake from fluids (TWI-F), and urine samples were analyzed for osmolality, color, specific gravity and volume. Urine osmolality ⩾800 mmol/kg H₂O was defined as hypohydration.

RESULTS

Water intake from fluids was 1729 (1555-1905) and 1550 (1406-1686) ml/d for boys and girls, respectively. Prevalence of hypohydration was 33% (44% of boys, 23% of girls). Children who failed to meet TWI-F recommendations demonstrated a risk of hypohydration that was 1.99-2.12 times higher than those who met recommendations (P⩽0.01). Boys between 9 and 13 years displayed urine osmolality of 777 (725-830) mmol/kg, and urine specific gravity of 1.021 (1.019-1.022), which was higher than those in girls between 9-13 years (P⩽0.015), and >27% were classified as hypohydrated despite meeting water intake recommendations.

CONCLUSIONS

Failure to meet TWI-F guidelines increased calculated relative risk of hypohydration in children. Boys between 9 and 13 years are at greater hazard regardless of meeting guidelines and may require greater water intake to avoid elevated urine concentration and ensure adequate hydration.

Fluid Balance of Adolescent Swimmers During Training

Swimming, either competitively or leisurely, is a unique activity that involves prolonged exercise while immersed in stable water temperatures. This environment could have an influence on the hydration status of swimmers independently of fluid balance. Forty-six healthy adolescent swimmers (26 males and 20 females; 12.8 ± 2.3 years; 50.6 ± 13.4 kg) were studied during a typical training session in an indoor swimming pool. First morning, prepractice and postpractice urine samples were tested for osmolality and specific gravity, whereas all athletes consumed fluids ad libitum. Sixty-seven percent of the athletes were hypohydrated (urine osmolality [Uosm] ≥700 mmol·kg(-1)) based on their first morning urine sample, which increased to 78% immediately before training. During the 2-hour swimming practice, the minimal sweat loss (0.39 ± 0.27 L) combined with ad libitum fluid availability resulted in unchanged body weight (0.1 ± 0.3 kg). Additionally, thirst was similar (before practice: 46 ± 26, after practice: 55 ± 33 mm on a 100-mm visual analog scale) at pretraining and posttraining time points (p > 0.05). Interestingly, postpractice Uosm was reduced significantly compared with the prepractice value (630 vs. 828 mmol·kg(-1); p = 0.001), without any significant change in body weight (0.1 ± 0.3 kg; p > 0.05). In conclusion, the present data indicated that more than two-thirds of the young swimmers appeared in their practice suboptimally hydrated. Although no changes in body mass were observed during the swimming practice, the decrease in urine hydration markers after swimming might less accurately reflect hydration state.

Precision, Accuracy, and Performance Outcomes of Perceived Exertion vs. Heart Rate Guided Run-training

Johnson, EC, Pryor, RR, Casa, DJ, Ellis, LA, Maresh, CM, Pescatello, LS, Ganio, MS, Lee, EC, and Armstrong, LE. Precision, accuracy, and performance outcomes of perceived exertion vs. heart rate guided run-training. J Strength Cond Res 31(3): 630-637, 2017-The purpose of this investigation was to compare run-prescription by heart rate (HR) vs. rating of perceived exertion (RPE) during 6 weeks to determine which is superior for consistent achievement of target intensities and improved performance. Forty untrained men participated in this laboratory-controlled and field-controlled trial. Participants were divided into heart rate (HRTG) and rating of perceived exertion training groups (RPETG). All underwent maximal-graded exercise testing and a 12-minute run test before and after training. Intensity was prescribed as either a target HR or RPE that corresponded to 4 relative intensity levels: 45, 60, 75, and 90% V[Combining Dot Above]O2 reserve (V[Combining Dot Above]O2R). Mean exercise intensity over the 6 weeks did not differ between HRTG (65.6 ± 7.2%HRR) and RPETG (61.9 ± 9.0%HRR). V[Combining Dot Above]O2max (+4.1 ± 2.5 ml·kg·min) and 12 minutes run distance (+240.1 ± 150.1 m) improved similarly in HRTG and RPETG (p > 0.05). HRTG displayed lower coefficients of variation (CV) (5.9 ± 4.1%, 3.3 ± 3.8%, and 3.0 ± 2.2%) and %error (4.1 ± 4.7%, 2.3 ± 4.1% and 2.6 ± 3.2%) at 45, 60, and 75% V[Combining Dot Above]O2R compared with RPETG (CV 11.1 ± 5.0%, 7.7 ± 4.1% and 5.6 ± 3.2%; all p < 0.005) %error (15.7 ± 9.2%, 10.6 ± 9.2% and 6.7 ± 3.2%; all p < 0.001), respectively. Overall, HR-prescribed and RPE-prescribed run-training resulted in similar exercise intensity and performance outcomes over 6 weeks. Differences in the CV and %error suggest use of HR monitoring for individuals that are new to running as it improves precision and accuracy but does not increase performance improvements across 6 weeks.

Hormonal and Thirst Modulated Maintenance of Fluid Balance in Young Women with Different Levels of Habitual Fluid Consumption

Background: Surprisingly little is known about the physiological and perceptual differences of women who consume different volumes of water each day. The purposes of this investigation were to (a) analyze blood osmolality, arginine vasopressin (AVP), and aldosterone; (b) assess the responses of physiological, thirst, and hydration indices; and (c) compare the responses of individuals with high and low total water intake (TWI; HIGH and LOW, respectively) when consuming similar volumes of water each day and when their habitual total water intake was modified. Methods: In a single-blind controlled experiment, we measured the 24 h total water intake (TWI; water + beverages + food moisture) of 120 young women. Those who consumed the highest (HIGH, 3.2 ± 0.6 L·day⁻¹, mean ± SD) and the lowest (LOW, 1.6 ± 0.5 L·day⁻¹) mean habitual TWI were identified and compared. Outcome variables were measured during two ad libitum baseline days, a four-day intervention of either decreased TWI (HIGH) or increased TWI (LOW), and one ad libitum recovery day. Results: During the four-day intervention, HIGH and LOW experienced differences in thirst (p = 0.002); also, a statistically significant change of AVP occurred (main effect of TWI and day, p < 0.001), with no effect (TWI or day) on aldosterone and serum osmolality. Urine osmolality and volume distinguished HIGH from LOW (p = 0.002) when they consumed similar 24 h TWI.

Observed Dietary Practices of Recreational Ultraendurance Cyclists in the Heat

Dietary approaches for optimizing exercise performance have been debated in the literature for years. For endurance athletes, various position stands focus on recommendations for high-carbohydrate diets to maximize performance in events. However, theories of low-carbohydrate diets and their ability to provide more fuel may prove beneficial to ultraendurance athletes. Therefore, the purpose of this research was to observe the food and fluid consumption of successful recreational ultraendurance cyclists on the day before (ED-1), Event Day, and the day after (ED+1), a 162 km endurance event in a hot environment, and subsequently compare dietary intakes to recommendations and other observed dietary practices. Twenty men (age, 48 ± 8 years; mass, 85.1 ± 13.4 kg; height, 178.2 ± 7.4 cm) recorded all dietary items during ED-1, Event Day, and ED+1. Diet composition and the relationships between carbohydrate and caloric intake with finish time were examined. Results show athletes consumed a high-carbohydrate diet on ED-1 (384 g·d), Event Day (657 g·d), and ED+1 (329 g·d). However, there were no significant associations between carbohydrate (p > 0.05), or caloric intake (p > 0.05), and finish time. This study results great variation in diets of recreational endurance cyclists, although most consume within nationally recognized dietary recommendation ranges. Because there is great variation and lack of correlation with performance, these findings suggest that current high-carbohydrate dietary recommendations for general endurance athletes may not be sport specific or individualized enough for recreational ultraendurance cyclists, and that individualized dietary macronutrient composition manipulations may improve performance outcomes.

Urine colour change as an indicator of change in daily water intake: a quantitative analysis

Purpose

Urine colour (U_Col) is simple to measure, differs between low-volume and high-volume drinkers, and is responsive to changes in daily total fluid intake (TFI). However, to date, no study has quantified the relationship between a change in TFI and the resultant change in U_Col. This analysis aimed to determine the change in TFI needed to adjust 24-h U_Col by 2 shades on an 8-colour scale, and to evaluate whether starting U_Col altered the relationship between the change in TFI and change in U_Col.

Methods

We performed a pooled analysis on data from 238 healthy American and European adults (50 % male; age, 28 (sd 6) years; BMI 22.9 (sd 2.6) kg/m²), and evaluated the change in TFI, urine volume (U_Vol), and specific gravity (U_SG) associated with a change in U_Col of 2 shades.

Results

The mean [95 % CI] change in TFI and U_Vol associated with a decrease in U_Col by 2 shades (lighter) was 1110 [914;1306] and 1011 [851;1172] mL/day, respectively, while increasing U_Col by 2 shades (darker) required a reduction in TFI and U_Vol of −1114 [−885;−1343] and −977 [−787;−1166] mL/day. The change in U_Col was accompanied by changes in U_SG (lighter urine: −.008 [−.007;−.010]; darker urine: +.008 [.006;.009]). Starting U_Col did not significantly impact the TFI change required to modify U_Col by 2 shades.

Conclusions

Our results suggest a quantifiable relationship between a change in daily TFI and the resultant change in U_Col, providing individuals with a practical means for evaluating and adjusting hydration behaviours.