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Revista MHSalud® (ISSN: 1659-097X) Vol. 7. No. 2. December, 2010.
ELIMINATION OF URINE IN RESPONSE TO WATER INTAKE IS CONSISTENT IN WELL-HYDRATED INDIVIDUALS
LA ELIMINACIÓN DE ORINA EN RESPUESTA A UNA INGESTA DE AGUA ES CONSISTENTE EN PERSONAS BIEN HIDRATADAS
Catalina Capitán-Jiménez & Luis Fernando Aragón-Vargas
School of Physical Education, Universidad de Costa Rica,
ktaucr@gmail.com
Translated by Elieth Salazar-Alpízar
ABSTRACT
A simple method has been recently
proposed to assess acute hydration status in humans; however, several
questions remain regarding its reliability, validity, and practicality.
Objective: Establish reliability of a simple method to assess
euhydration, that is, to analyze whether this method can be used as a
consistent indicator of a person´s hydration status. In addition,
the study sought to assess the effect exercise has on urine volume when
euhydration is maintained and a standardized volume of water is
ingested. Methods: Five healthy physically active men and five healthy
physically active women, 22.5 ± 2.3 years of age (mean ±
standard deviation) reported to the laboratory after fasting for 10
hours or more on three occasions, each one week apart. During the two
identical resting euhydration conditions (EuA and EuB), participants
remained seated for 45 minutes. During the exercise condition (EuExer),
participants exercised intermittently in an environmental chamber
(average temperature and relative humidity = 32 ± 3°C and 65
± 7%, respectively) for a period of 45 minutes and drank water
to offset loss due to sweating. The order of treatments was
randomized. Upon finishing the treatment period, they ingested a volume
of water equivalent to 1.43% body mass (BM) within 30 minutes. Urine
was collected and measured henceforth every 30 minutes for 3 hours.
Results: Urine volume eliminated during EuExer (1205 ± 399.5 ml)
was not different from EuB (1072.2±413.1 ml) or EuA (1068
± 382.87 ml) (p-value = 0.44). Both resting conditions
were practically identical (p-value = 0.98) and presented a strong
intraclass correlation (r = 0.849, p-value = 0.001). Conclusions: This
method, besides simple, proved to be consistent in all conditions;
therefore, it can be used with the certainty that measurements are
valid and reliable.
KEYWORDS: Reliability, rehydration, exercise, diuresis.
RESUMEN
Recientemente se ha propuesto un
método sencillo para evaluar el estado agudo de
hidratación en humanos, pero persisten varias preguntas con
respecto a su confiabilidad, validez y utilidad práctica.
Objetivo: establecer la confiabilidad de un método simple
para comprobar euhidratación, es decir, evaluar si este
método puede ser usado como un indicador consistente del estado
de hidratación de una persona. Además, se buscaba
evaluar el efecto que tiene el ejercicio sobre la producción de
orina, cuando se mantiene el estado de euhidratación y se
ingiere un volumen estandarizado de agua. Métodos: cinco
hombres y cinco mujeres saludables y físicamente activos, con
22,5 ± 2,3 años (media ± desviación
estándar), se presentaron al laboratorio después de un
ayuno de 10 horas o más; en tres ocasiones, separadas por una
semana de tiempo. En las dos condiciones idénticas de
reposo (EuA y EuB), permanecieron sentados durante 45 minutos. En
la condición de ejercicio (EuEjer) realizaron ejercicio
intermitentemente en una cámara de clima controlado
(temperatura media y humedad relativa = 32 ± 3° C y 65
± 7%, respectivamente) por un lapso de 45 minutos, bebiendo agua
para reponer las pérdidas por sudoración. El orden de los
tratamientos fue aleatorio. Al terminar el tratamiento ingirieron un
volumen de agua equivalente a 1,43% masa corporal (MC) en 30
minutos, y se recogieron y midieron los volúmenes de orina
eliminados posteriormente cada 30 minutos durante 3 horas. Resultados:
El volumen de orina eliminado para la condición EuEjer (1205
± 399.5 mL) no fue diferente de EuB (1072.2±413.1 mL)
ni de EuA (1068 ± 382.87mL) (p= 0.44); las dos condiciones
de reposo fueron prácticamente idénticas (p = 0,98), y
presentaron una correlación intraclase fuerte (r = 0.849, p =
0.001). Conclusiones: Este método, además de simple,
demostró ser consistente en sus mediciones, por lo que puede ser
utilizado con la certeza de que las mediciones son válidas y
confiables.
PALABRAS CLAVES: Confiabilidad, rehidratación, ejercicio, diuresis.
INTRODUCTION
Controlling and eliminating factors
that may impair performance is extremely important for athletes, even
more so now that sports are reaching such a competitive level that
every detail can make a difference between winning and losing.
Continued loss of sweat during training and competitions can cause
dehydration in athletes. Performance may decrease if this happens;
consequently, trying to offset loss due to sweating during exercise
with adequate fluid intake is of great importance. However, it is
difficult for trainers and athletes to see if a person is dehydrated
with the naked eye.
There are several methods to
determine whether a person is well hydrated or not ranging from the
simple to the most advanced, each with advantages and disadvantages.
Some of these methods have been summarized by Shirreffs (2003): change
in body weight before and after exercise (body mass; this method does
not assess the acute state, but only changes in hydration); color of
urine, urine osmolality, and urine specific gravity (USG) (all of these
are indirect indicators of acute hydration status). Other methods that
can be used are the change in blood plasma volume (another change
indicator) and the bioelectrical impedance analysis, which gives an
estimate of total water volume in the body. Also, the urine volume
response to a water load was recently studied (Capitán &
Aragon, 2009a).
Although there are different
techniques to assess hydration status, not all of them are useful in
the world of athletics because they may be very expensive, may require
specialized equipment or personnel, or are just not very practical
since they have to be conducted at specific times and under specific
conditions. The importance of being able to assess the hydration status
of athletes has resulted in many evaluations (Cheuvront & Sawka,
2005; Kovacs, Senden, & Brouns, 1999; Oppliger & Bartok, 2002;
Shirreffs, 2003), which are in search of a technique that allows both
trainers and researchers to obtain reliable data on the state of
hydration.
The body has several mechanisms to
maintain water balance. For instance, it is known that the kidneys play
an important role in maintaining fluid balance in the body (Ulate,
2007), which is done through urination. If a person is dehydrated the
kidneys reduce the production of urine in order to retain fluid,
whereas if the person ingests liquid being euhydrated urine output
increases to remove excess liquid. This could also occur when
rehydration is very aggressive (Sawka et al., 2007). Based on these
concepts, a method that has been recently investigated (Capitán
& Aragón, 2009a) is the measurement of urine output in
response to a standardized intake of water (1.43% of body mass, BM),
just the same way there is a test to measure blood glucose response to
an oral glucose load. The study revealed that this method clearly
distinguishes between euhydrated and dehydrated individuals to 1%, 2%,
or 3% of body mass (BM). However, this method, like many others
mentioned in this paper, has weaknesses. For instance,
reliability of the study is unknown (it has not been determined how
consistent measurements are in this new method), nor has it been
determined whether differences in urine production were really due to
hydration status or to the effect exercise has on urine production
(Capitán & Aragón, 2009b).
Therefore, the purpose of this study
was to establish the reliability of Capitán and
Aragón´s method, in order to assess whether it can be used
as a consistent indicator of a person´s hydration status. Another
aspect measured was if exercise in euhydrated individuals had any
effect on urine output.
METHODOLOGY
Participants. A convenience sample
was selected from a group of university students who responded to a
public announcement and met previously established requirements. Five
men and five women (22.5 ± 2.3 years, mean ± standard
deviation) consented to participate in this study. Subjects had the
following characteristics: healthy, physically active (performed
physical activity at least 4 times a week), had no kidney, endocrine or
heart problems, had not suffered from heat illness, and at the time of
the study were not taking diuretic drugs. This study was approved by
the University´s Scientific Ethics Committee.
Instruments. An e-Accura® scale, model DSB291, was used with an accuracy of 0.01 kg (10 g) to measure body weight.
Urine samples were collected in
plastic containers with a capacity of 750 ml and the amount of urine
was measured with a nutritionist scale (precision scale) (Model CS2000
OHAUS® Compact Scale), with an accuracy of 0.001 kg (1 g).
A hand-held Atago® refractometer,
model URC-NE, d 1.000-1.050, was used to analyze urine specific gravity
and a Polar® heart rate monitor, model A1, was used to control
exercise intensity during dehydration.
PROCEDURES
Predehydration. Each participant
reported to the laboratory at 7 am on 3 separate occasions having
fasted from food and drink for at least 10 hours.
In the laboratory each participant provided a urine sample and urine
specific gravity was measured to estimate the initial hydration status
(Initial USG). This urine sample was discarded.
After completely empting their
bladders, subjects were weighed nude (weight after fasting, WAF) and
that weight was used to determine the volume of liquid to ingest. Once
they provided the urine sample and were weighed, participants ate a
standardized breakfast of 750 kcal (corresponding to 24.6% lipid, 20.7%
protein, and 54.7% carbohydrates including 250 ml natural orange
juice), and rested for 30 minutes. This liquid helped ensure
euhydration.
Exercise. After having breakfast,
participants were weighed nude and dry (baseline body weight, BBW) and
later exercised intermittently for 45 minutes. The cycle included
15 minutes in the stationary bicycle and 15 minutes on the treadmill,
stopping every 15 minutes to be weighed. In order to stay euhydrated,
participants ingested the amount of water equivalent to the weight
difference due to sweat loss with respect to the baseline body weight
(BBW). Only one exercise session was performed (EuEXER).
Exercise was conducted in a
controlled environment chamber. Average temperature was 32 ±
3°C and relative humidity was 65 ± 7%; exercise intensity
was moderate (75% - 80% of the maximum heart rate, calculated by the
formula 220 - age).
Rest sessions. Two rest sessions were
performed (EuA and EuB), for which the protocol did not require
exercise (i.e. 0%). After eating breakfast, participants were weighed
nude and dry (baseline body weight) and were later asked to remain
sitting outside the controlled environment room for 45 minutes, where
they were again weighed nude and dry every 15 minutes. In order to stay
euhydrated, participants ingested the amount of water equivalent to the
weight difference due to sweat loss with respect to the baseline body
weight.
Average room temperature was 25
± 1°C and relative humidity was 60 ± 2%. Heart rate
remained between 35% - 40% of the maximum heart rate, calculated by the
formula HRmax = 220 - age.
Postexercise. Once the rest and
exercise periods were finished, subjects showered with cold water,
ingested no liquid and completely emptied their bladders in a
container, if necessary. This urine sample was weighed and taken
as loss of fluid due to exercise. After showering, they were
weighed nude and dry (rehydration weight, RHW).
Rehydration. Once bathed and weighed,
participants ingested a volume of bottled water (Crystal® brand,
sodium = 7.0 mg/L) equivalent to 1.43% of the weight they had when they
arrived at the laboratory (WAF), as stipulated by Capitán and
Aragon´s method. This percentage of liquid was the same in
all conditions.
The total volume of fluid was ingested in three equal aliquots, separated by 10 minutes each.
Urine collection. Once the rehydration protocol was finished, urine was
collected every 30 minutes for 3 hours. Each urine sample collected was
weighed and specific gravity was measured before properly disposing of
the sample.
Statistical analysis. Data was
analyzed using descriptive statistics (mean and standard deviation) for
age, baseline body weight, and volume of water ingested. Inferential
statistics were applied with the SPSS statistical package, version 16,
to analyze variance and post hoc tests.
Three one-way analyses of variance
(ANOVA) were conducted, one for each of the following variables: urine
specific gravity, baseline body weight and volume of water ingested, to
determine whether participants began the study in the same conditions
in all treatments.
A two-way ANOVA was performed with
repeated measures on both factors (3 treatments x 7 measurements) to
determine whether there were differences between treatments.
In addition, a two-way ANOVA was
performed with repeated measures (3 treatments x 2 measurements) to
determine whether there were differences in weight before and after
exercise in order to verify if participants remained euhydrated during
the exercise and rest periods.
An intraclass correlation and a paired t-test were conducted between the two resting conditions to determine reliability.
A statistical power analysis was used to determine whether the sample size could give an acceptable result.
RESULTS
Initial conditions. There were no
significant differences in the initial hydration status (p-value =
0.429), which was measured with the specific gravity of the first urine
of the day, or in body weight of participants after fasting (p-value =
0.179), nor were significant differences found in the volume of water
ingested during the different conditions (p-value = 0.179) (see Table
1). On average, subjects were euhydrated at the beginning of the
session in all conditions; there were only two individual clear cases
of dehydration (USG = 1.030).
Total volume of urine eliminated.
Figure 1 shows average volumes of urine eliminated in the different
conditions. Since no statistically significant differences were
observed between men and women in the total volume of urine eliminated
in any of the conditions (p> 0.05), data is presented together. This
graph shows that there is no significant difference (p-value = 0.44)
among conditions (EuA, EuB and EuExer).
Exercise. Table 2 shows body weight
of participants before starting the 45 minutes of rest or exercise and
the end of those periods between the conditions of 0% BM (with and
without exercise).
![imagen001](https://www.revistas.una.ac.cr/index.php/mhsalud/article/download/21/version/1/10975/28996/imagen001.JPG)
![imagen002](https://www.revistas.una.ac.cr/index.php/mhsalud/article/download/21/version/1/10975/28997/imagen002.JPG)
![imagen003](https://www.revistas.una.ac.cr/index.php/mhsalud/article/download/21/version/1/10975/28998/imagen003.JPG)
Time of urine collection. In order to
test whether there were any differences in the volume of urine
eliminated over time, volumes eliminated from minute 0 to minute 180
were analyzed. Figure 2 shows the differences in the volume of urine
eliminated by time in each condition (EuA, EuB, and EuEXER).
Reliability. To determine reliability
a paired t-test was used with the urine volume in the two resting
conditions where no exercise was performed (EuA and EuB): no difference
was observed between conditions (t = -0.58, p-value = 0.95). In
addition, an intraclass correlation coefficient was calculated, which
yielded a high association between tests (r = 0.849, p-value =
0.001). A 95% confidence interval was obtained for the intraclass
correlation coefficient (0.496 to 0.961). Figure 3 shows the
relationship between conditions.
![imagen004](https://www.revistas.una.ac.cr/index.php/mhsalud/article/download/21/version/1/10975/28999/imagen004.JPG)
![imagen005](https://www.revistas.una.ac.cr/index.php/mhsalud/article/download/21/version/1/10975/29000/imagen005.JPG)
Statistical power. A statistical
power test was used to rule out the possibility that no differences
were found between resting conditions due to the sample size. It was
determined that the power of the test is 95% with a sample equal to
N=10 to detect a 200 ml difference in the total volume of urine
eliminated.
DISCUSSION
The objective of this study was to
determine the reliability of a simple method to assess
euhydration. With a high correlation between EuA and EuB,
measurements are consistent in this method, which makes it reliable.
The study also aimed to determine
whether exercise has any effect on urine production of euhydrated
participants in response to a standardized amount of water. No
differences were found for either total urine volumes eliminated or
partial urine measurements between resting conditions and exercise (EuA
and EuB vs. EuExer), considering that participants started under the
same conditions each time.
During physical activity body
temperature rises and the body attempts to dissipate heat by sweating.
Fluid loss through sweating causes a decrease in total body
water. To avoid unnecessary water loss during exercise the body
increases the secretion of antidiuretic hormone or vasopressin, which
is responsible for reducing the production of urine during exercise
(Wilmore & Costill, 2004).
A change in plasma osmolality is the
primary regulator of vasopressin secretion. Therefore, if osmolality is
below the threshold (280 mOsm/kg) secretion will be practically nil
(Ulate, 2007), which means that urine output is not altered. On the
other hand, if this value increases (e.g. due to dehydration), the
secretion of vasopressin also increases, thereby reducing urine
production.
Therefore, as stated in the theory,
the body eliminates through urine any excess fluid ingested (Wilmore
& Costill, 2004). This means that when the body is euhydrated and
is given a volume of water, it would become extra fluid in the body and
be eliminated by the organism, thus increasing urination. Consequently,
if a balance is maintained during exercise between what is lost through
sweating and what is ingested, the mechanisms that stimulate the
decrease of urine volume, such as the secretion of the antidiuretic
hormone, should remain unchanged. The volumes of urine eliminated, both
in resting conditions (EuA and EuB) and exercise were very similar,
indicating that the production of urine during euhydration was not
related to whether or not exercise was performed. This suggests that
urine output is more dependent on the hydration status than on having
exercised or not.
When urine volumes were analyzed by
time, no differences were found between resting conditions (EuA and
EuB) at 60 minutes into the urine collection. The analysis was
performed with the urine eliminated after 60 minutes since this is a
reasonable time to determine differences (Capitán &
Aragón, 2009b). This reinforces the reliability test because not
only are there no differences in total urine volume, but also the
partial urine volumes are the same.
Although this study was able to find
that the difference between conditions is due to the water volume
ingested and not to exercise, there is still a problem to be solved:
even though the method is sensitive and reliable, it loses a lot of its
practicality since after 60 minutes of urine collection most
individuals would still have in their bodies between 67% and 13%
(Capitán & Aragón, 2009b) of the water ingested.
Therefore, it is necessary to determine the amount of water that would
allow detecting differences, but at the same time be a smaller volume
of extra water remaining in the body at the end of 60 minutes. If this
is achieved, this method will be sensitive, reliable, practical, and
suitable to evaluate the acute stage of hydration, with the advantage
of being inexpensive, easy to use and applicable to many individuals or
an entire team simultaneously.
In conclusion, this study found that
Capitán and Aragón´s method is reliable and also
showed how exercise, under the conditions of this experiment, did not
cause a decrease in urine output when compared with urine produced at
rest.
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Reception date: January 26, 2010.
Correction date: June 11, 2010.
Acceptance date: June 11, 2010.
Publication date: December 31, 2010.
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