Introduction
Adequate water intake is vital to health [
1]. Most frequently dehydration (i.e., measured total body water loss) is associated with changes to exercise performance, thermoregulation, or even immune function [
2,
3]. However, within the general population, the term “underhydration” has been defined as chronic low water intake accompanied by increased concentration of urine and fluid regulatory hormones, but not necessarily with a measurable decrement to total body water [
4]. Defining underhydration is multidimensional, because the volume of water which is adequate for one individual can be substantially different from another due to extrinsic (e.g., environment) and intrinsic factors (e.g., physical activity, sodium, and caloric intake) [
5]. Individualized water recommendations use urine concentration measurements [e.g., urine osmolality (
Uosm), urine-specific gravity (
Usg), and urine color (
Ucol)] as physiological criteria of water intake adequacy [
6,
7]. The rationale for their use is that observation of concentrated urine is an indication of negative free water clearance which assists with maintenance of total body water. Within the focus of underhydration, urinary evaluation provides more value to researchers compared to markers of dehydration (i.e., plasma osmolality and body mass change). Plasma osmolality is limited when evaluating changes in 24-h water intake, because it is tightly regulated across a wide range of water intake volumes [
8]. Body weight is less effective than urine evaluation, because it can fluctuate across days due to factors outside of water intake, such as caloric intake, particularly if measurements are greater than 1 day apart [
9,
10]. Therefore, researchers have employed the
Uosm threshold of 500 mOsm kg
−1 within the general population as a reasonable target in the indication of adequate water intake, because
Uosm accounts for factors such as body composition, dietary osmotic load, physical activity, and environment, and reflects the net result of urine concentrating and diluting mechanisms [
11].
While
Uosm and
Usg are capable of identifying different categories of water intake [
12‐
14], the required technical skills and equipment are often confined to clinical or research settings.
Ucol, on the other hand, is self-assessable [
15] and has been correlated tightly with
Uosm in previous studies [
16,
17]. Furthermore, in adults and children,
Ucol alone offers the potential to identify underhydration [
7,
15]. However, interpretation variation exists, because 24 h and single urine samples (i.e., spontaneous single samples) may be divergent [
18]. Within single
Ucol sample analysis, interpretation could also be altered depending on the use of first morning [
19] or early afternoon samples [
20] as representation of adequate fluid intake. In addition, although the directionality of the
Ucol and water intake relationship is apparent; the full value of
Ucol has yet to be realized, because the volume of water that should be consumed to return to euhydration following a decrease in habitual water intake and the consequential underhydration or dehydration has not been defined.
Efforts have been made to quantify the volume of water necessary to change
Ucol. A secondary, pooled analysis performed by Perrier et al. [
21] combined studies in which daily total water intake (TWI) was manipulated, showing that an increase in TWI of 1110 mL day
−1 [95% CI 914–1306] was required to lighten
Ucol by two units. Although useful, these results were limited, because (a) the volumes of TWI change that were pooled were generally large (i.e., > 1000 mL day
−1), (b) the pooled sample populations were heterogeneous, and (c) the study only evaluated changes in
Ucol of two units which is not necessarily equivalent to an individual moving from above the
Ucol adequate water intake threshold of “4” on an eight-point scale [
7], to below that threshold.
U
col thus appears to be particularly suited for applied self-evaluation outside of a research laboratory. However, inconsistencies in past investigations [
19,
22,
23] as well as differentiation between dehydration and underhydration [
4] point towards the need for additional and more specific research into the question of how to validly apply urinary markers to determine if, when, and how an individual has returned to adequate water intake following a period of insufficient intake. Therefore, the following investigation had three purposes: (a) to evaluate changes in hydration biomarkers in response to graded rehydration following 3 days of water restriction (WR), (b) assess within-day variation in urine concentration, and (c) quantify the volume of fluid needed to return to a urinary concentration associated with adequate water intake (i.e., a
Ucol of < 4 [
7]) as demonstrated by change in
Ucol. We hypothesized in reference to the above aims that (a) most participants would exhibit biomarkers indicative of underhydration following 3 days of WR and that there would be a progressive increase in the number of well hydrated individuals following the rehydration protocol, (b) urine concentration would be associated with the cumulative volume of water consumed over the progression of the day with the most concentrated occurring in the morning and least in the mid-day, and (c) at least 1000 mL of additional water would be needed to return and individual to
Ucol < 4.
Discussion
The main findings of the current investigation were (a) 1000 mL day−1 of water administered over 3 days was sufficient to induce urinary markers consistent with inadequate fluid intake (i.e., elevated urine osmolality) in most, but not all participants and that approximately 1500 mL day−1 of additional water was required during the GRHI to reduce urine concentration below that of CON, (b) fluctuations in Uosm coincided with the timing and the compounded volume of additional water given on the day of the GRHI, and (c) an increase of water intake between 1107 and 1763 mL day−1, in addition to the baseline intake of 1000 mL day−1, is required to reduce an individual’s Ucol from > 4 to < 4 within 24 h. These findings expand upon previous water intake investigations by allowing precise observation of the responses to graded rehydration and quantification of water volumes that are necessary to re-establish urinary concentration consistent with adequate water intake.
Following 3 days of WR, a majority of individuals displayed not only signs of inadequate water intake (i.e., elevated urine concentration), but those of dehydration (e.g., decrease of body mass). During the period of WR, individuals consumed only 1000 mL day
−1 of plain water, and the remainder of their TWI (~ 700 mL day
−1) came from food sources (e.g., fruit, soups, etc.). Our measurements did not include an estimation for metabolic water production. TWI was below both the U.S. National Academy of Medicine (3000–3700 mL day
−1) [
5] and the European Food Safety Authority (EFSA, 2000–2500 mL day
−1) [
34] adequate intake recommendations for females and males. The finding that most but not all individuals displayed signs of underhydration due to this WR provides two items of significance. First, this supports both above adequate intakes, because 1700 mL day
−1 of TWI is not adequate among a selected sample of healthy individuals. However, it also demonstrates the variability in water needs across individuals. Even though the mean intake during baseline was nearly double the volume provided during WR, underhydration was not apparent in all individuals, which may be indicative of the extrinsic and intrinsic factors related to total water needs (e.g., environmental heat exposure or lean body mass, respectively).
The graded rehydration was important, because several past investigations specific to the relationship between water intake and hydration biomarkers have identified “high” and “low” drinkers, and then evaluated changes that occurred when TWI volumes are switched [
12,
18,
35] (i.e., a change in TWI of > 1000 mL day
−1). Although this approach is beneficial for evaluation of the largest and smallest TWI groups, its applicability is limited. It would be helpful to know if moderate increases in water intake could shift hydration biomarkers towards values associated with adequate water intake, because water interventions that impose large changes in TWI could be met with resistance by users. In this investigation, the group which received the smallest increase in water intake during the GRHI (
G+0.50) realized no benefit due to the additional water, as evaluated by urinary biomarkers. This is similar to past research which found no differences in urinary or hematological biomarkers of hydration state after increasing chronic water intake over the course of 2 weeks from 35 to 40 mL kg
−1 body mass (1685 ± 320 to 2054 ± 363 mL day
−1) [
36]. Although there may be physiological benefits of increasing water intake by volumes even as small as 100 mL, such as suppression of vasopressin secretion or exercise performance enhancement [
37,
38], it is clear that urinary biomarkers of water intake cannot differentiate these small changes due to the relatively small samples sizes that were investigated.
In contrast to the lower volumes of GRHI that did not induce changes in urinary markers, the higher volumes resulted in significant changes. As mentioned above, approximately 1100 mL of additional water was previously recommended to decrease
Ucol by two units [
21]. Our findings support these previous studies, because the low end of the 95% CI associated with moving from a
Ucol > 4 to < 4 was 1107 mL and the upper end of the 95% CI was 1763 mL. While this is a large range encompassing 600 mL (just over two 8 oz glasses), it reflects the range of
Ucol that was induced by the WR. In the previous study, the 1100 mL was associated only with a decrease of
Ucol by two units. This would only be applicable to the current investigation if that participant were moving from
Ucol = 5–3. The present findings also can be applied to an individual moving from
Ucol = 7 to
Ucol = 3. However, it is clear that even 2250 ml of additional water was not sufficient for some, as evidenced by the 35% of
G+2.25 whose
Ucol remained ≥ 4 following GRHI. Because the GRHI took place over a single 24 h period, it is possible that a second day of increased water intake within many of the intervention groups may have allowed those participants to return to
Ucol < 4. Although from a different geographical population, the finding that free-living individuals with habitual TWI > 2500 mL day
−1 were associated with optimal hydration [
39] supports the theory that a longer duration of
G+1.50 or
G+2.25 GRHI would have been successful in returning a greater proportion to more desirable urine concentrations.
Factors such as (1) the relatively small gradations between the GRHI groups and (2) a sample population with a wide range of ages, body masses, and an equal distribution of both sexes make this investigation strong. However, there are also several limitations that are acknowledged. For example, the evidence of underhydration induced by WR over 3 days is a unique protocol. It is not clear if the urinary values associated with the WR or the GRHI would be similar if the alterations of fluid balance were induced over a shorter duration or as the result of chronic or habitual low intake. While the evidence of underhydration was strong, it was also assumed as evidenced by body mass measurements that dehydration was imparted by the WR protocol. However, the lack of increase in body mass following the GRHI, even in groups receiving higher volumes of water (e.g.,
G+1.50), makes it plausible that the − 0.4-kg change in body mass that was observed did not represent a loss of total body water and thus increased water intake would not influence body water. In this case, it is likely that the reduced urinary concentration markers were indicative of additional water excretion only and not changes in hydration status. This may be important, because both hydration status (i.e., total body water volume) and hydration process (i.e., daily water turnover) have been implicated in relationships with overall health [
40]. It appears as if the present intervention is most relevant to the latter. A second limitation of this investigation is the self-report nature of the dietary logs. Finally, the limitations of self-report food logs are well documented, and thus, the reported values for macronutrient intake reported within this investigation should be viewed as an estimation.
The results of this investigation can be applied to a number of situations. For example, health care workers who demonstrate poor hydration practices [
41] can be coached to evaluate their
Ucol. In the case of noticing a darker
Ucol at the end of a work shift, the volume of additional water that should be consumed to achieve ideal urine concentration is now understood. Moreover, as increased water intake has been shown to significantly reduce the incidence of urinary tract infection (UTI) recurrence [
42], these results may help young women who suffer from recurrent UTI to manage their fluid intake appropriately for secondary UTI prevention. Overall, the data of the present study provide a starting point regarding the optimization of water intake within populations known to exhibit increased urinary concentration, as well as within individuals seeking to improve their hydration status within a 24-h period.