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Validity of Hydration Non-Invasive Indices during the
Weightcutting and Official Weigh-In for Olympic Combat
Sports
Valentı
´n E. Ferna
´ndez-Elı
´as
1
, Alberto Martı
´nez-Abella
´n
2
, Jose
´Marı
´aLo
´pez-Gullo
´n
2
, Ricardo Mora
´n-
Navarro
2
, Jesu
´s G. Pallare
´s
1,2
, Ernesto De la Cruz-Sa
´nchez
2
, Ricardo Mora-Rodriguez
1
*
1Exercise Physiology Laboratory, University of Castilla-La Mancha, Toledo, Spain, 2Department of Physical Activity and Sport, University of Murcia, Murcia, Spain
Abstract
Background:
In Olympic combat sports, weight cutting is a common practice aimed to take advantage of competing in
weight divisions below the athlete’s normal weight. Fluid and food restriction in combination with dehydration (sauna and/
or exercise induced profuse sweating) are common weight cut methods. However, the resultant hypohydration could
adversely affect health and performance outcomes.
Purpose:
The aim of this study is to determine which of the routinely used non-invasive measures of dehydration best track
urine osmolality, the gold standard non-invasive test.
Method:
Immediately prior to the official weigh-in of three National Championships, the hydration status of 345 athletes of
Olympic combat sports (i.e., taekwondo, boxing and wrestling) was determined using five separate techniques: i) urine
osmolality (U
OSM
), ii) urine specific gravity (U
SG
), iii) urine color (U
COL
), iv) bioelectrical impedance analysis (BIA), and v) thirst
perception scale (TPS). All techniques were correlated with U
OSM
divided into three groups: euhydrated (G
1
;U
OSM
250–
700 mOsm?kg H
2
O
21
), dehydrated (G
2
;U
OSM
701–1080 mOsm?kg H
2
O
21
), and severely dehydrated (G
3
;U
OSM
1081–
1500 mOsm?kg H
2
O
21
).
Results:
We found a positive high correlation between the U
OSM
and U
SG
(r = 0.89: p = 0.000), although this relationship lost
strength as dehydration increased (G
1
r = 0.92; G
2
r = 0.73; and G
3
r = 0.65; p = 0.000). U
COL
showed a moderate although
significant correlation when considering the whole sample (r = 0.743: p = 0.000) and G
1
(r = 0.702: p = 0.000) but low
correlation for the two dehydrated groups (r =0.498–0.398). TPS and BIA showed very low correlation sizes for all groups
assessed.
Conclusion:
In a wide range of pre-competitive hydration status (U
OSM
250–1500 mOsm?kg H
2
O
21
), U
SG
is highly associated
with U
OSM
while being a more affordable and easy to use technique. U
COL
is a suitable tool when U
SG
is not available.
However, BIA or TPS are not sensitive enough to detect hypohydration at official weight-in before an Olympic combat
championship.
Citation: Ferna
´ndez-Elı
´as VE, Martı
´nez-Abella
´nA,Lo
´pez-Gullo
´n JM, Mora
´n-Navarro R, Pallare
´s JG, et al. (2014) Validity of Hydration Non-Invasive Indices during
the Weightcutting and Official Weigh-In for Olympic Combat Sports. PLoS ONE 9(4): e95336. doi:10.1371/journal.pone.0095336
Editor: Reury F.P. Bacurau, University of Sao Paulo, Brazil
Received December 11, 2013; Accepted March 26, 2014; Published April 16, 2014
Copyright: ß2014 Ferna
´ndez-Elı
´as et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study was supported by grants from the High-Performance Sports Center Infanta Cristina (General Directorate of Sports, Government of Murcia).
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: Ricardo.Mora@uclm.es
Introduction
Severe dehydration has physiological consequences negatively
affecting health and athletic performance. Body water losses
exceeding 2% of body weight reduce physical work capacity and
exercise performance [1–3] and higher dehydration levels (i.e..4–
5%) has been reported to increase heat-stroke risk [1,4]. These
adverse effects include impaired glycogen use [9], increases in core
temperature inducing central nervous system fatigue [10,11],
cardiovascular strain [12,13] and loss of efficacy of the metabolic
acid buffer system [14]. All these effects could compromise health
and physical performance in military personnel, firemen, athletes
training and competing in hot environments, or those involved in
Olympic weight-class sports (e.g. wrestling, boxing, judo, taek-
wondo and weightlifting). In these sports weight loss throughout
dehydration is a very common strategy prior to competition [15].
Weight loss by dehydration has been shown to affect boxing and
wrestling performance [5,6]. If that weight loss is quickly
recovered the effects on performance are not evident [7,8]. Many
techniques are available to assess body water deficit, however it is
not clear which it is best to use in a pre-competition setting.
Ideally, this should be a non-invasive index, as well as being fast,
accurate, inexpensive and easy-to-use.
Out of the available techniques to measure hydration status,
blood osmolality is the gold standard [16–18]. However, the
measurement of blood osmolality requires an invasive technique,
PLOS ONE | www.plosone.org 1 April 2014 | Volume 9 | Issue 4 | e95336
costly measurement apparatus and qualified personnel to handle
blood. All these conditions are rarely available to scientists and
coaches at the field. Urine analysis of hydration status has been
recommended as an alternative measurement because it involves a
noninvasive evaluation of body fluid [19]. The main criticism of
the use urine as an index of dehydration is that urine does not
respond as fast or as accurately as blood to body fluid deficit [16].
However, we have recently found that urine readily tracks blood
responses during progressive dehydration induced by exercise
[20,21]. Urine can be analyzed for color, density, osmolality or its
constituents resulting in a wide range of hydration indexes.
Nonetheless, not all indexes are adequate, accurate or practical,
and some are costly and require technical expertise [22].
A non-invasive surrogate of blood osmolality is urine osmolality
(U
OSM
) considered the most valid measurement of hydration status
through urine [16,23]. However, similarly to blood osmolality it
requires expensive biochemical analysis. Urine specific gravity
(U
SG
) assessment requires a simpler apparatus (i.e., refractrom-
eter). Some authors have found that U
SG
[20,21,24]and urine
color (U
COL
) [22,25] are highly correlated to urine osmolality
(U
OSM
). Armstrong and co-workers, found acceptable validity of
U
SG
and color analysis in different populations at moderate
dehydration levels [17]. However, the agreement between these
urine indexes after severe dehydration in weight class sports [15],
has not been reported.
Finally, there are non-invasive indexes that do not entail urine
collection and analysis. Bioelectrical impedance analysis (BIA;
[26–29]) and thirst perception scale (TPS; [30–33]) have been
proposed as simpler indexes of body fluid deficit. Despite all these
studies, to our knowledge, there is insufficient evidence to decide
about the suitability of these indexes to readily detect whole body
dehydration. Furthermore, these indexes have not been evaluated
in a large population of athletes undergoing different degrees of
dehydration. We believe that a good test for BIA and TPS will be
to assess its agreement with U
OSM
during the weight cutting in
Olympic combat sports.
Therefore, the purpose of this study was to compare several
non-invasive indexes of hydration in a large number of Olympic
combat sport athletes undergoing different degrees of weight loss
by dehydration before a real competition. Our intention is to
obtain a wide range of hypohydration levels to fully evaluate the
detection power of all indexes in comparison to U
OSM
.We
hypothesized that techniques involving urine analysis may have
high levels of agreement while other estimations (i.e. BIA and TPS)
will not.
Methods
Participants
Two hundred and forty-four male (age 22.864.1 yr, body mass
74.1615.1 kg, height 176.166.7 cm) and one hundred one female
(age 22.764.5 yr, body mass 57.168.9 kg, height 164.967.2 cm)
high performance athletes of three different Olympic combat
sports volunteered to participate in this study: wrestling (n = 157),
taekwondo (n = 152) and boxing (n = 36). All participants had at
least 4years of training and competition experience, and all of
them made the weight in the official weigh-in of their respective
national championship during the experimental phase of this
study. The subjects and coaches were informed in detail about the
experimental procedures and the possible risks and benefits of the
project. The study, which complied with the Declaration of
Helsinki, was approved by the Bioethics Commission of the
University of Murcia, and written informed consent was obtained
from athletes prior to participation.
Study design and experimental protocol
Athletes’ hydration status was evaluated through 5 different
techniques (i.e., U
OSMO
,U
SG
,U
COL
, TPS and BIA) between 60
and 5 minutes before the official weigh-in of their respective
National Championship. No instructions were given to athletes or
their coaches about their weight control management. Participants
filled out a nutritional questionnaire and twelve of them were
excluded from the study for being ingesting vitamins, nutritional
supplements or prescription drugs prone to alter urine color,
amount or composition [25]. Women were tested out of the
proliferative phase of their menstruation.
At arrival to the official weigh-in facilities, a 10 ml mid flow
urine sample was obtained from each athlete. After the recipient
with the urine sample was handed over and codified, subjects filled
out the thirst perception scale, and their body impedance was
determined using a Bio-impedance analyzer. Urine specimens
were immediately analyzed in duplicate for urine osmolality
(U
OSM
), urine specific gravity (U
SG
), and urine color (U
COL
) by the
same experienced investigator. The final value for each assessment
was the average of the two trials.
Urine osmolality. U
OSM
is the measure of the total urine
solute content. As has been repeatedly reported [16,23], we
considered this assessment as our gold standard measurement to
determine the athletes’ hydration status. Athletes urine specimens
(20 mL) were immediately analyzed in duplicate by freezing point
depression osmometry (Model 3250, Advanced Instruments,
USA).
Urine specific gravity. U
SG
is the analysis of urine density
compared to double distilled water (density = 1.000). After
apparatus calibration and thorough mixing of the urine specimen,
a few drops were placed on the refractometer (URC-NE, Atago,
Japan) visor and U
SG
was determined.
Urine color. U
COL
is determined by the amount of
urochrome present in the urine specimen. When large volumes
of urine are excreted, the urine is dilute and pale. Conversely,
when small volumes of urine are excreted, the urine is
concentrated and dark [23]. U
COL
was determined as described
by Arsmstrong et al., [17,18,22,23,25]. Briefly, an 8 number scale
ranging from very pale yellow (number 1) to brownish green
(number 8), was used. U
COL
was determined in duplicate by
holding each specimen container next to a validated color scale in
a well-lit room.
Bioelectrical impedance analysis. BIA has the potential to
assess changes in hydration status and has been previously used
and validated in combat sports athletes [29]. Athletes BIA was
determined using an 8-contact electrode segmental and mono-
frequency body composition analyzer (Tanita BC-418, Tanita
Corp., Tokyo, Japan) while they were barefoot, wearing shorts and
a sports-top for females.
Thirst perception scale (TPS). Thirst perception is physi-
ologically related to the hydration status of an individual since it is
mediated by fluid-regulating hormones urging the ‘‘need to drink’’
[31]. Thirst perception was assessed using a Liker scale[32,34] that
ranged perceived thirst from 1 (‘‘not thirsty at all’’) to 9 (‘‘very,
very thirsty’’).
Statistical analysis
Descriptive values were provided for all the outcome variables.
Engagement scores were non-normally distributed for all mea-
sures, as assessed by Shapiro-Wilk’s test (p,0.05). A Spearman’s
rank-order correlation was run to assess the relationship between
U
OSM
and the rest of the hydration status markers (U
SG
,U
COL
,
TPS and BIA). The size of the correlation was evaluated as
follows; r,0.7 low; 0.7#r,0.9 moderate; and $0.9 high [35].
Validity of Hydration Status Markers
PLOS ONE | www.plosone.org 2 April 2014 | Volume 9 | Issue 4 | e95336
Subjects were stratified according to their hydration status using
U
OSMO
values. A value of 700 mOsm?kg H
2
O
21
marks the limits
between a correct hydration status and dehydration [1]. Thus,
three intervals of equal amplitude (according to measurement
units) were established according to the following cutoffs: from 250
to 700 mOsm?kg H
2
O
21
(euhydrated - G
1
), from 701 to
1.080 mOsm?kg H
2
O
21
(dehydrated - G
2
) and from 1.081 to
1.500 mOsm?kg H
2
O
21
(severely dehydrated - G
3
). Also, a
Kruskal-Wallis test was performed between groups. Pairwise
comparisons were performed using Dunn’s [36] procedure with
a Bonferroni correction for multiple comparisons.
Results
Hydration status indexes were not different between males and
females (U-Mann Whitney Wilcoxon test; p.0.05) or between
sports (wrestling, taekwondo and boxing; Kruskal-Wallis test;
p.0.05) and thus results are reported with all athletes as a group.
A high linear and positive correlation was detected between U
SG
and U
OSMO
in the whole sample (r = 0.89; p = 0.000; n = 345).
However, the correlation became lower as the dehydration status
increased (G
1
r = 0.92; p = 0.000; G
2
r = 0.73; p = 0.000 and G
3
r = 0.65; p = 0.000; Figure 1A).
The relationship between the U
OSMO
and other hydration
status markers was weak. U
COL
showed a moderate although
significant correlation when considering the whole sample
(r = 0.743; p = 0.000) or the euhydrated group (G1: r = 0.702;
p = 0.000). However, the correlation was low for the two
dehydrated groups (G2: r = 0.498; p = 0.002; G3: r = 0.398;
p = 0.004) (Figure 1B). TPS showed a significant but low
correlation with the U
OSM
in the whole sample and for G3 group
(r,0.315 and r = 0.298, respectively; p,0.05) (Figure 1C). No
significant correlation (p.0.05) was detected between the BIA
assessments and U
OSM
in any group (Figure 1D).
Finally, a complementary Kruskal-Wallis analysis according to
the athletes’ dehydration status (euhydrated – G1, dehydrated –
G2; and severely dehydrated –G3) reveals significant differences
(p,0.05) between the 3 groups for the U
SG
and U
COL
methods.
Nevertheless, the TPS cannot differ (p,0.05) between the first two
groups (G1 and G2), and BIA do not distinguish (p,0.05) between
any of the 3 groups (G1, G2 and G3) (Figure 2).
Figure 1. Correlation between U
OSM
and U
SG
(A), Urine color (B), Thirst perception scale (C) and Bioelectrical impedance analysis (D)
in the whole sample and in each group. G1: U
OSM
250–700 mOsm?kg H
2
O
21
; G2: U
OSM
701–1.080 mOsm?kg H
2
O
21
; G3. U
OSM
1.081–
1.500 mOsm?kg H
2
O
21
.
doi:10.1371/journal.pone.0095336.g001
Validity of Hydration Status Markers
PLOS ONE | www.plosone.org 3 April 2014 | Volume 9 | Issue 4 | e95336
Validity of Hydration Status Markers
PLOS ONE | www.plosone.org 4 April 2014 | Volume 9 | Issue 4 | e95336
Discussion
The current study compares different indexes of hydration
status to urine osmolality (U
OSM
) as the gold standard non-invasive
index [16,23]. This comparison took place in a large sample of
Olympic combat sports athletes (i.e. 345 athletes) during the
official weigh-in of a real competition. While rapid reduction in
body weight before competition is the easier non-invasive index of
weight cutting thru dehydration it requires knowing what the
‘‘normal’’ weight of the athlete is. Referees and medical personnel
at the competition arena do not have this information and thus
require another index that is accurate, fast and non-invasive. Our
aim was to determine which of the available non-invasive indexes
(U
SG
;U
COL
; BIA and TPS) showed the better combination of
sensibility to detect hypohydration, with affordability and simplic-
ity in its use. This may prove useful to sport’s governing bodies
which are interested in preventing rapid weight loss during
competition. Coaches and trainers can benefit too from easily
assessing the degree of hypohydration in their combat athletes. We
believe that fast and accurate identification of hypohydration is the
first step into the prevention of weight cutting unhealthy practices.
While a similar question has been addressed in previous studies
[14,17,18,23], to our knowledge, this is the first study identifying
the best non-invasive index using a large sample with a wide range
of hydration statuses under a real competition situation. As a
consequence of the real situation, we detected a large number of
competitors with severe dehydration (176 samples with U
OSM
above 701 mOsm?kg H
2
O
21
and 122 samples with U
OSM
above
1.080 mOsm?kg H
2
O
21
) beyond what has been previously
reported [19,37,38]. Also, we observed that independent of sport
discipline and gender a similar distribution of athletes were
dehydrated or extremely dehydrated suggesting, as previously
reported [37–41], that weight cutting is a broadly extended
practice in Olympic combat sports.
Our results indicate that U
SG
is the hydration index that better
correlates with U
OSM
(r = 0.89; p = 0.000; Figure 1) the assessment
of U
SG
being easier, cheaper and faster than that of U
OSM
. These
results are consistent with the finding of Popowski et al [16] who
compared the validity of U
SG
and U
OSM
to plasma osmolality, and
concluded that both, U
SG
and U
OSM
correlate and are good
measurements of hydration status. This data is also in agreement
with results from our laboratory [21] reporting that U
SG
is as
sensitive as serum osmolality to detect 2 to 3% hypohydration.
Based on the present results using an important sample size of elite
athletes in a wide range of hydration statuses, we can substantiate
that U
SG
is a highly recommended index to assess hypohydration.
Nevertheless, when dehydration increases U
SG
presents lower
correlation values (G2: r = 0.75; G3: r = 0.66; both p = 0.000;
Figure 1). This validity decline, as body water loss increase, has
been previously observed by Oppliger et al [24]. Nevertheless,
dehydration is usually assessed based on a threshold value that is
much below the values where U
SG
starts to deviate from U
OSM
.
Thus, a lowering in this correlation will rarely affect the
classification of an individual as dehydrated or euhydrated.
Previous studies agreed that U
COL
presents lower precision and
accuracy values to determine the hydration status in humans
compared to U
OSM
and U
SG
[17,18,25]. Nevertheless, different
researchers consider that U
COL
would be helpful in athletic, army
or industrial settings where high precision assessment of body fluid
deficit is not required [17,22,25]. Likewise, our data coincides in
that U
COL
is effective at discriminating different levels of
dehydration (Figure 2) despite its lack of preciseness (G2:
r = 0.498; p = 0.002, G3: r = 0.398; p = 0.004; Figure 1). As in
previous studies, we can recommend U
COL
analysis as an index to
estimate hydration status of combat sports athletes; especially
when water loss is not extreme. The low precision level of U
COL
could be offset by its simplicity and low cost to assess hydration
status on the field.
Some studies propose that BIA is a valid tool to assess hydration
status in different populations [26,27,29]. However, our data
suggests that BIA is not a good instrument to assess hydration level
in combat sports athletes (Figure 1). In agreement with our results,
other investigations argued that BIA may be a non-adequate
instrument to evaluate exercise induced dehydration [28,42–44].
Furthermore, our results show that during dehydration and severe
dehydration (G2 and G3) BIA agreement with U
OSM
worsens
compared to euhydration (G1) (Figure 1). This is in accordance
with the investigation of Asselin et al [45] which indicated that
with dehydration levels of 2–3% of body mass, BIA standard
equations failed to predict changes in total body water. As a
limitation we used segmental BIA but mono-frequency analysis
since, in our experience, this are the technical characteristics of the
BIA equipment commonly found in combat sports clubs and high
performance sports centers. Novel systems of BIA employ multi-
frequency to determine the characteristics of the body fluids and
tissues. Although they have shown even lower validity to estimate
body composition [46], it has been recently reported that they are
sensitive to evaluate acute dehydration in wrestlers [29]. It is
unclear if the use of multi-frequency BIA could have increased its
association with U
OSM
in our data set.
Engell et al. [30] showed a high correlation between the
perceived thirst and hypohydration before and after exercise in the
heat. Young et al [33] agreed with this statement and added that
using the 9 point scale (1 = not at all thirsty; 9 = very, very thirsty)
a score between 3 and 5 could be a good indication that an
individual is mildly dehydrated. Our results suggest that this
perception scale is a valid indicator of hydration status, but only
discriminating between euhydration and extreme dehydration.
However, it does not distinguish low levels of dehydration from
correct hydration (Figure 2). Maresh group [31,32] provided data
showing a thigh correlation between hypohydration and thirst
perception when subjects are moderately dehydrated (i.e., ,4%).
It is known that numerous factors, apart from body water deficit,
may alter the perception of thirst such us fluid palatability, time
allowed for fluid consumption, time since last fluid ingestion,
gastric distention, older age, gender, and heat acclimatization
status[17]. Thus, while thirst perception may serve as an indicator
of extreme dehydration, our data suggest that TPS is not accurate
enough to correctly evaluate low and moderate levels of
hypohydration during weight cutting in Olympic combat athletes.
Athletes involved in combat sports (e.g., wrestling, taekwondo
and boxing) habitually weight-cut (i.e. weight loss through
dehydration) to be included in a lower category at the official
weigh-in before competition. Our study compares four different
non-invasive hydration indexes (U
SG
,U
COL
, TPS, and BIA) to
U
OSM
as our gold standard non-invasive measure in a wide sample
of competitive Olympic combat sports athletes. The aim is to find
an alternative measure that, unlike U
OSM
, does not involve costly
Figure 2. Descriptive values (whisker and box plot) and group differences according to U
OSMO
status classification. Also, differences
according to the Kruskal-Wallis test and Dunn’s pairwise comparisons (Bonferroni correction for multiple comparisons).G1: U
OSM
250–700 mOsm?kg
H
2
O
21
; G2: U
OSM
701–1.080 mOsm?kg H
2
O
21
; G3.U
OSM
1.081–1.500 mOsm?kg H
2
O
21
.
doi:10.1371/journal.pone.0095336.g002
Validity of Hydration Status Markers
PLOS ONE | www.plosone.org 5 April 2014 | Volume 9 | Issue 4 | e95336
biochemical analysis and that can be readily used by sports
medicine doctors, coaches and trainers on the combat arena. Our
data suggests that U
SG
is a good alternative to U
OSM
since it highly
correlates with U
OSM
, in conditions of low and severe dehydration
(i.e. G2 and G3). However, U
COL
can be an alternative and
adequate tool to evaluate dehydration, especially if the dehydra-
tion level is not extreme. In contrast, our data discourages the use
of TPS and BIA to measure hydration status after weigh-cut in
combat sports athletes.
Author Contributions
Conceived and designed the experiments: VEFE AMA JGP RMR.
Performed the experiments: VEFE AMA RMN JMLG JGP. Analyzed the
data: EDCS JGP RMR. Contributed reagents/materials/analysis tools:
VEFE AMA RMN JMLG RMR. Wrote the paper: VEFE AMA RMN
JMLG EDCS JGP RMR.
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PLOS ONE | www.plosone.org 6 April 2014 | Volume 9 | Issue 4 | e95336
