Water is an essential nutrient, comprising 55% to 60% of total body mass and 73% of lean mass.1 Hydration physiology has been extensively studied in situations of acute water loss, such as prolonged exercise or heat exposure. It is well documented that a net loss of total body water due to sweat as well as insufficient water replacement during sport reduces exercise performance and alters thermoregulatory capacity.2–4 However, until recently, few studies had examined hydration physiology in the general population, where habitual water intake, and not excessive water loss, is the primary determinant of hydration. Recent evidence suggests that habitual low fluid intake is linked to health outcomes such as increased long-term risk of chronic kidney disease,5 kidney stones,6–8 and possibly even new-onset hyperglycemia.9 Thus, the ability to track water intake and hydration is of potential interest to the general population as well as to clinicians and other healthcare practitioners.
The ability to track changes in hydration biomarkers due to water intake is challenging because water turnover occurs constantly, and body water moves within and between intracellular and extracellular compartments. Proposed biomarkers of hydration status include measures of plasma and urine osmolality, as well as other associated urinary measures such as specific gravity, color, and volume. However, no single biomarker of hydration has yet been shown to be adapted to all situations.10 Recently, we published a study11 demonstrating significant differences in 24-hour urine variables between those who consume low (≤1.2 L/d of total water) versus high (≥2.0 L/d) daily fluid volumes, with low drinkers producing a significantly smaller volume of urine with a correspondingly higher urine osmolality, specific gravity, and darker color compared with high drinkers. Moreover, although plasma osmolality (POsm) has been demonstrated to track acute dehydration,12 we found no differences in POsm in average adults who were not subjected to exercise or heat exposure. This suggests the possibility of adaptive mechanisms to conserve total body water and, therefore, maintain POsm despite low fluid intake. A cascade of regulatory hormones, including arginine vasopressin, regulate total body water by modulating water reabsorption and excretion via the kidneys. Indeed, we observed increased circulating arginine vasopressin in the low drinkers in this study, supporting the hypothesis that habitual low fluid consumption may lead to a state of sustained antidiuresis. Urinary parameters therefore appeared well adapted to distinguish between habitual low and high drinkers in normal daily conditions. However, the “gold standard” of 24-hour urine collection is certainly impractical for widespread use. Shorter duration sampling would be more convenient and less costly, but the possible circadian fluctuation of urine concentration has not been well described. To address these challenges, we recently completed a study that sought to evaluate the responsiveness of hydration biomarkers in urine and blood to acute changes in water intake, as well as to assess diurnal variation of hydration biomarkers in urine.
Fifty-two participants (age, 24.8 ± 3.1 years; body mass index, 22.4 ± 1.6 kg/m2) were included and classified based on self-reported habitual fluid intake over 3 consecutive weekdays (30 low drinkers and 22 high drinkers). They then completed a 5-day inpatient parallel group crossover intervention. During the first 2 days of the inpatient intervention (baseline), participants followed a fluid regimen quantitatively comparable with their self-reported intake habits (1.0 L/d for low and 2.5 L/d for high drinkers). In the 3 days immediately after baseline, fluid intakes between groups were reversed (crossover). After the 5 days of inpatient intervention, the low group was encouraged to maintain an increased water intake for 1 month, after which they returned for a second 24-hour inpatient visit (follow-up). During all inpatient visits, the timing and volume of water ingested were carefully controlled and distributed throughout waking hours (from 7:00 AM to 11:00 PM), and standardized meals were provided to ensure consistent dietary solute load. Each day during the inpatient portions of the study, POsm was sampled twice daily (morning and evening). All urine produced over each 24-hour period was collected in 5 separate containers, corresponding to morning (AM; 07:00 AM–12:00 noon), early afternoon (PM1; 12:00 noon–4:00 PM), late afternoon (PM2; 4:00–8:00 PM), evening (EVE; 8:00–11:00 PM), and overnight (NIGHT; 11:00 PM–7:00 AM) collections. Urine collections were analyzed for osmolality (UOsm), specific gravity (USG), color (UCol), and volume (UVol). Following these measures, each of the 5 samples representing a 24-hour period was combined and measures were repeated on each complete 24-hour urine collection.
Daily fluid intake during the screening period was 0.71 ± 0.28 and 2.66 ± 0.66 L/d for low and high drinkers, respectively. During the inpatient intervention, water intake was fixed at either 1.0 or 2.5 L/d, depending on group assignment and study phase (baseline or crossover). During the 1 month after the inpatient screening, low drinkers consumed 1.86 ± 0.50 L/d of total fluids (Figure 1).
Baseline Analyses: Group Differences and Circadian Variation in Hydration Biomarkers
Plasma osmolality was not different between AM or PM assessments or between groups (mean [95% CI], low drinkers: AM, 291 [290, 293] mOsm/kg; PM, 290 [289, 293] mOsm/kg; high drinkers: AM, 288 [285, 290] mOsm/kg; PM, 289 [286, 291] mOsm/kg; P = .78).
24-Hour Urine Biomarkers
At baseline, low drinkers produced (mean [95% CI]) 1.05 [0.92–1.19] L/24 hours of urine, with mean 24-hour UOsm that exceeded the criterion for euhydration in athletic adults13 (UOsm, 764 [707–820] mOsm/kg; USG, 1.020 [1.019–1.021]; UCol, 5.2 [4.8–5.6]), and were significantly different from high drinkers (UVol, 2.44 [2.32–2.57] L/24 hours; UOsm, 332 [276–389] mOsm/kg; USG, 1.010 [1.009–1.011]; UCol, 2.6 [2.2–2.9]; all P < .001).
Circadian Variation in Urine Biomarkers
In both low and high drinkers, the time of collection significantly influenced urinary biomarkers (Figure 2). In low drinkers, AM and NIGHT samples were significantly (P < .05) more concentrated than PM samples for UOsm, USG, and UCol. In high drinkers, AM and NIGHT samples had lower UVol and higher UOsm, USG, and UCol (P ≤ .01) than PM samples did. The short samples showed satisfactory correlations with the 24-hour sample (r 2 for UOsm: 0.69 to 0.78), with the values obtained from the PM1 and PM2 samples being most similar, on average, to the 24-hour value.
Crossover Analyses: Effect of a Change in Water Intake on Hydration Biomarkers
Low Drinkers: Effect of an Increased Daily Water Intake (+1.5 L/d)
In comparison with measures obtained at baseline, POsm did not change in response to an increase in water intake (crossover, mean [95% CI]: AM, 291 [289, 293] mOsm/kg; PM, 291 [289, 293] mOsm/kg; P = .82). All urine parameters responded significantly to a change in water intake (Figure 3): UVol increased significantly (2.38 [2.24, 2.52] L/24 hours; P < .001), whereas measures representing urine concentration decreased (UOsm, 352 [308, 397] mOsm/kg, P< .001; USG, 1.010 [1.008, 1.011], P < .001]; UCol, 2.7 [2.3, 3.0]).
High: Effect of a Decreased Daily Water Intake (−1.5 L/d)
Changes in high drinkers mirrored the changes seen in low drinkers. Compared with baseline values, POsm did not change (mean [95% CI]: AM, 290 [288, 293] mOsm/kg; PM, 290 [287, 292] mOsm/kg; P = .16). UVol decreased significantly (1.11 [0.98, 1.24] L/24 hours, P < .001), whereas urine concentration increased (UOsm, 720 [675, 765] mOsm/kg, P < .001; USG, 1.019 [1.018, 1.020], P < .001; UCol, 4.8 [4.5, 5.1], P < .001).
Follow-up Analysis (Low Drinkers Only): Effect of 1-Month Increased Water Intake on Biomarkers
During the 1-month follow-up, low drinkers increased their total fluid intake to 1.86 ± 0.50 L/d, representing an average increase of 1.1 L/d. During the inpatient follow-up visit, POsm was maintained (mean [95% CI]: AM, 288 [285, 291] mOsm/kg; PM, 291 [290, 292] mOsm/kg), and all urinary parameters were similar to those of the inpatient crossover period (UOsm, 380 [351, 409] mOsm/kg; USG, 1.011 [1.010, 1.012]; UCol, 3.7 [3.6, 3.8]).
The key finding of this study suggests that urinary hydration biomarkers are responsive to changes in water intake and that urine volume and concentration respond within 24 hours of initiating a change in daily intake. Urinary biomarkers such as osmolality, specific gravity, and color appear to be particularly well suited to track subtle changes in hydration in response to day-to-day changes in fluid intake volume. In contrast, plasma osmolality is tightly regulated and maintained across a broad range of daily fluid intake volumes.14 This distinction is important because plasma osmolality is widely regarded as a responsive biomarker to hypohydration due to sweating.12,15 Our findings contrast those of normal adults in sedentary conditions with the previously studied athletic populations and suggest that different biomarkers are relevant in different situations.
In sedentary individuals who do not lose excessive amounts of water to sweat, the relationship between fluid intake and urinary output is largely controlled by regulating antidiuretic activity in the kidneys. In situations of low fluid intake volume, increased antidiuresis minimizes the volume of water lost in urine, whereas excess water is eliminated via diuresis. Thus, urine concentration and volume have the ability to provide insight into the hormonal regulation of body water without the need to assess hormone concentrations directly. Given the recently identified links between fluid intake, urine output, and chronic kidney disease,5,16 there is at least epidemiological evidence that chronic antidiuretic activity as evidenced by low water intake or low urine output may worsen the rate of glomerular decline. Thus, biomarkers of urine concentration have potential applicability in the screening of and primary prevention for long-term kidney pathology.
Moreover, a limiting factor in the inclusion of hydration in common health assessments has historically been the need to collect full 24-hour urine samples to accurately measure 24-hour urine concentration. These results provide at least preliminary evidence that shorter, well-timed collections, such as those obtained over a 3- to 5-hour period, may be sufficient to approximate 24-hour urine concentration. These short collections may provide similar information with considerably less effort on the part of patients, healthcare practitioners, or the general population interested in monitoring urine concentration as a measure of adequate fluid intake. In conclusion, short, well-timed collections have the potential to reflect 24-hour fluid intakes as efficiently as 24-hour urine collections, but with a better practicality. Urinary biomarkers in short collections therefore have the potential to advance the use of hydration assessment as a routine component in primary prevention, particularly of chronic kidney disease, which currently represents a major healthcare burden at the worldwide level.
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