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.

An explanation for unexpected population crashes in a constant environment.

Unexpected population crashes are an important feature of natural systems, yet many observed crashes have not been explained. Two difficulties in explaining population crashes are their relative rarity and the multi-causal nature of ecological systems. We approach this issue with experimental microcosms, with large numbers of replicates of red flour beetle populations (Tribolium castaneum). We determined that population crashes are caused by an interaction between stochasticity and successive episodes of density dependence: demographic stochasticity in oviposition rates occasionally produces a high density of eggs; so high that there are insufficient flour resources for subsequent larvae. This mechanism can explain unexpected population crashes in more general settings: stochasticity 'pushes' population into a regime where density dependence is severely overcompensatory. The interaction between nonlinearity and stochasticity also produces chaotic population dynamics and a double-humped one-generation population map, suggesting further possibilities for unexpected behaviour in a range of systems. We discuss the generality of our proposed mechanism, which could potentially account for previously inexplicable population crashes.

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.

Uneven but Conservative Pacing Is Associated With Performance During Uphill and Downhill Running.

PURPOSE: To investigate the relationship between pacing strategy and performance during uphill and downhill running-specifically, what distribution of energy corresponds to faster race finish times between and among participants. METHODS: Eighteen years of race data from a 10.2-mile running race with an uphill first half and a downhill second half were analyzed to identify relationships between pacing and performance. A pacing coefficient (PC), equal to a participant's ascent time divided by finishing time (FT), was used to define each participant's pacing strategy. The American College of Sports Medicine metabolic running equation was used to estimate energy expenditure during the ascent, descent, and total race. Statistical analyses compared participants' PC to their FT and finishing place within their age and gender category. Additionally, FT and finishing place were compared between groups of participants who exhibited similar pacing strategies. RESULTS: PCs were positively associated with faster FTs (r2 = .120, P < .001) and better finishing positions (r2 = .104, P < .001). PCs above .600 were associated with the fastest average FTs and best average finishing position within age and gender categories (all P ≤ .047). CONCLUSIONS: Participants performed the best when energy expenditure increased no more than 10.4% during the uphill portion compared to their overall average. It is not possible to state that overly aggressive uphill efforts resulted in premature fatigue and thus slower decent times and worse race performance. However, participants should still avoid overly aggressive uphill pacing, as performance was associated with larger PCs.

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 (ß = 2.5%, 95% confidence interval [CI] = 0.3%, 4.7%) and increasing diastolic blood pressure (ß = 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 (ß = -17.5%, 95% CI = -29.6%, -5.4%) and increasing number of rest breaks (ß = -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.

Combining urine color and void number to assess hydration in adults and children.

BACKGROUND/OBJECTIVES: To test the diagnostic ability of two combined practical markers for elevated urine osmolality (underhydration) in free-living adults and children. SUBJECTS/METHODS: One hundred and one healthy adults (females n = 52, 40 ± 14 y, 1.70 ± 0.95 m, 76.7 ± 17.4 kg, 26.5 ± 5.5 kg/m2) and 210 children (females = 105, 1.49 ± 0.13 m, 43.4 ± 12.6 kg, 19.2 ± 3.2 kg m-2) collected urine for 24-h. Urine was analyzed for urine osmolality (UOsm), color (UC), while the number of voids (void) was also recorded. Receiver Operating Characteristic (ROC) analysis was performed for UC, void, and combination of UC and void, to determine markers' diagnostic ability for detecting underhydration based on elevated UOsm (UOsm ≥ 800 mmol kg-1). RESULTS: Linear regression analysis revealed that UC was significantly associated with UOsm in both adults (R2 = 0.38; P < 0.001) and children (R2 = 0.45; P < 0.001). Void was significantly associated with UOsm in both adults (R2 = 0.13; P < 0.001) and children (R2 = 0.15; P < 0.001). In adults, when UC > 3 and void <7 were combined, the overall diagnostic ability for underhydration was 97% with sensitivity and specificity of 100% and 88%, respectively. In children, UC > 3 and void <5 had an overall diagnostic ability for underhydration of 89% with sensitivity and specificity of 100% and 62%, respectively. CONCLUSIONS: Urine color alone and the combination of urine color with void number can a valid and simple field-measure to detect underhydration based on elevated urine osmolality.

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.

Electrolyte Beverage Intake to Promote Hydration and Maintain Kidney Function in Guatemalan Sugarcane Workers Laboring in Hot Conditions.

OBJECTIVES: To evaluate impact of electrolyte supplementation on hydration status and health outcomes in Guatemalan agricultural workers performing heavy work under hot climatic conditions. METHODS: A 3-week pragmatic trial was conducted with a group of 50 workers during the 2017 to 2018 sugarcane harvest. Workers received an electrolyte hydration intervention during 2 of the 3 weeks. Blood and urine samples were collected each week. RESULTS: Increased electrolyte intake resulted in less muscle injury. Kidney function was maintained across the intervention period. Workers were adequately hydrated and average electrolyte levels remained in normal ranges. Mild indications of hyponatremia occurred at higher levels of fluid intake. CONCLUSIONS: This trial demonstrates the feasibility of maintaining workers' electrolyte levels under extremely hot and humid conditions while mitigating muscle injury. Electrolyte supplementation should be added to standard workplace water, rest, and shade interventions to protect workers.

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.

Water Intake, Body Water Regulation and Health.

The biological feedback provided by human water intake upon our physiology is grossly under-investigated [...].

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 (ß 1.41, 95% CI 0.88-2.03) and negatively associated with eGFR (ß -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.

Dietary Relationship with 24 h Urinary Iodine Concentrations of Young Adults in the Mountain West Region of the United States.

BACKGROUND: Iodine deficiency is not seen as a public health concern in the US. However certain subpopulations may be vulnerable due to inadequate dietary sources. The purpose of the present study was to determine the dietary habits that influence iodine status in young adult men and women, and to evaluate the relationship between iodine status and thyroid function. METHODS: 111 participants (31.6 ± 0.8 years, 173.2 ± 1.0 cm, 74.9 ± 1.7 kg) provided 24 h urine samples and completed an iodine-specific Food Frequency Questionnaire (FFQ) for assessment of urinary iodine content (UIC) as a marker of iodine status and habitual iodine intake, respectively. Serum Thyroid Stimulating Hormone (TSH) concentration was evaluated as a marker of thyroid function. Spearman correlational and regression analysis were performed to analyze the associations between iodine intake and iodine status, and iodine status and thyroid function. RESULTS: 50.4% of participants had a 24 h UIC < 100 µg/L). Dairy (r = 0.391, p < 0.000) and egg intake (r = 0.192, p = 0.044) were the best predictors of UIC, accounting for 19.7% of the variance (p ≤ 0.0001). There was a significant correlation between UIC and serum TSH (r = 0.194, p < 0.05) but TSH did not vary by iodine status category (F = 1.087, p = 0.372). DISCUSSION: Total dairy and egg intake were the primary predictors of estimated iodine intake, as well as UIC. Iodized salt use was not a significant predictor, raising questions about the reliability of iodized salt recall. These data will be useful in directing public health and clinical assessment efforts in the US and other countries.

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) [1]. 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) [2]. 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 [3]. 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. 1a) or changes in TBW (Fig. 1b) 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 [2] to 4 weeks [3] do not result in changes in TBW. Current evidence would suggest that the benefits of increasing TWI are not related changes in TBW.

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.

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.

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 Ucol. 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 Ucol changed from > 4 to < 4 (+ 1435 ± 812 mL) than those whose Ucol 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.

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.