Background
Multiple studies have tested the effects of dehydration on human body
while exercising. Experiments conducted particularly emphasized on the limiting
effects of dehydration on hot/humid environments (Montain et al. 1992, Cheuvront
et al. 2005, Casa et al. 2010). Results from those studies have presented that
dehydration has a significant impact on performance especially when exercise is
prolonged and under warm/hot conditions. For this reason guidelines (Convertino
et al. 1996) have been established to encourage and help athletes and other
population groups who perform under such conditions to minimize the effects of
dehydration during physical activity.
Body water loss results in fluid losses from both the extracellular and
the intracellular fluid compartments. However those losses can cause a wide
span of effects on the body water pools that remain, depending on the type of
water loss. Hypertonic fluid losses, cause reductions in body fluid tonicity.
Hypotonic fluid losses, which can occur through sweating, results in an
increase in body fluid tonicity, and isotonic loss causes a standard fluid
loss, but no increase or decrease in the body fluid tonicity.
Scientists agree that dehydration equal or exceeding 2% can have
negative impacts on performance (Montain et al. 1992, Cheuvront et al. 2005).
It is not uncommon for athletes in training or competition to finish with -2%
body mass or more even when fluid is available. The primary reason for this
“voluntary” hypohydration is because the athlete fails to keep pace with the
rate of which water is lost through sweat, especially in sports or tasks that
there is not enough time for resting.
Method
The articles selected for the review had to be peer-reviewed with
maximum one review article. The studies had to be focused on dehydration and
its effect on performance. Articles were searched in the PubMed database in the
English language. Another criterion was that articles should be after 1990 so
they are relative modern, and could be accessed for free.
Database
|
Keywords
|
# of hits
|
Reviewed
|
Selection #1
|
Selection #2
|
PubMed#1
|
fluid balance, human performance
|
597
|
21
|
0
|
0
|
PubMed#2
|
dehydration, human performance
|
450
|
23
|
5
|
3
|
PubMed#3
|
dehydration, human performance AND temperature
|
300
|
14
|
4
|
1
|
PubMed#4
|
dehydration, temperature, human performance AND
Heart Rate
|
75
|
5
|
3
|
2
|
Results
Influence of graded dehydration on
hyperthermia and cardiovascular drift during exercise
On 1992, Montain et al. published an article that examined the effect of
different rates of dehydration, induced by drinking different amount of fluid
during continuous exercise, on hyperthermia, stroke volume (SV) and heart rate
(HR). On four different situations, eight trained cyclists, cycled at a power
output equal to 62-67% of their Vo2max for 2 hours in a warm environment (33° C).
During exercise they randomly ingested no fluid (NF) a small (SF), moderate
(MF) or large (LF) volume of fluid ingested, that replaced 20±1, 48±1 and 81±2%
of the fluid lost during exercise.
The different volumes of fluid
ingestion during the four trials resulted in a progressively greater magnitude
of dehydration. In the NF group, cardiac output (CO) progressively declined
during the 10 to 110 min exercise. Fluid replacement attenuated the decline in CO
with the magnitude of decline in CO in SF, MF, and LF groups, graded in
proportion the magnitude of dehydration accrued after 2 hours of exercise. At
110 minutes CO was significantly higher in LF than during NF, SF, and MF.
Similarly during NF, HR increased progressively during the 10 to 110 minute
period of exercise. Fluid replacement attenuated the increase in HR observed
during NF, with the magnitude of attenuation graded in proportion to the amount
of fluid ingested during exercise. The increase of HR during the 10 to 110
minute period of exercise was linear to the percent of weight loss after 120
minute of exercise. Stroke volume (SV) in NF declined 27% during the 10 to 110
minute period of exercise, and in the other groups, fluid replacement
attenuated the decline in SV in proportion to the volume of fluid ingested
during exercise. The decline in SV during the 10 to 110 minute period of
exercise was inversely related to the percent of body weight loss after 2 hours
of exercise. In NF temperature in skin and in esophagus increased progressively
during 2 hours of exercise, and fluid ingestion reduced the magnitude of
hyperthermia.
In conclusion the results of that
study show that the
magnitude of increase in core temperature in HR and the decline in SV were
directly related to the body weight loss (and this dehydration accrued) during
exercise. In other words a linear
relationship between the
magnitude of hypohydration
and hyperthermia was found. A major consequence of dehydration
is an increase in core temperature during physical activity, with core
temperature rising an additional 0.15 to 0.20°C for every 1% of body weight
lost during the activity. The authors state that dehydration reduces heat
dissipation by reducing skin blood flow during exercise, usually resulting in
an increased body core temperature. The results also showed that dehydration of
more than 2% to 3% of body mass increases the rating of perceived exertion and
impairs mental functioning.
Stroke volume during exercise:
interaction of environment and hydration
Gonzalez-Alonso et al. (2000) published an article that also examined
the effect of dehydration on stroke volume. Euhydrated (well-hydrated) and
dehydrated athletes exercised in a hot and a cold environment to discover the
factors that reduce SV.
Eight trained cyclists, cycled for 30 minutes in the heat (35° C) or
cold (8° C) when euhydrated or dehydrated by 1.5, 3.0, or 4.2% of their body
weight. The reason this particular study was selected, was because the methods
were closely related with the previous study and it would provide additional
information on how dehydration understanding progressed 8 years later.
The results of the study show that when euhydrated, CO displayed a
significant increase in the heat compared with the cold. In the heat, CO was
lowered in response to the degree of dehydration, and it was lower significantly
than euhydrated. In contrast, in the cold, CO was not significantly reduced by
dehydration. At all levels of hydration, HR was significantly higher by 11–14
beats/min during exercising in the heat compared with the cold, moreover, in
the heat, HR elevation as an effect of increasing dehydration was higher
slightly than in the cold. In the heat, systolic blood pressure tended to
decline with increasing dehydration, but in the cold it was not altered. At any
level of hydration, the rating of perceived exertion in heat was higher significantly
than the cold. Continuous dehydration increased the rating of perceived
exertion only in the heat.
An important finding of the study concerning hemodynamics, as mentioned
by the authors, was that the reductions in SV of dehydrated subjects exercising
in the heat were attenuated by ½ in the cold, allowing for the prevention of
the significant reductions of CO in the cold compared with dehydration in the
heat. The authors also associate the bigger reduction in SV in the heat with
higher elevations in core temperature and HR. Moreover, the results present that
SV reduction in heat stress while exercising moderately paralleled the increase
in the core temperature.
Hypohydration impairs endurance
exercise performance in temperate but not cold air
Progressing into the environmental temperature effects on dehydration,
Cheuvront et al. (2005) published an article comparing the effects of
hypohydration on the performance of endurance exercise in temperate and cold
air environments.
On four trials, two women and six men were exposed to 3 hours of passive
heat stress (45°C) in the early morning with or without fluid ingestion. Later,
subjects sat in a cold (2°C) or temperate (20°C) environment with minimal
clothing for 1 hour before cycling 30 minute on an ergometer at 50% Vo2max
followed immediately by a 30 minute performance time test. Rectal and mean skin
temperatures, ratings of perceived exertion and HR measurements were regularly
tested. Performance was measured by the total work (kJ) done in the 30 minute
time test.
The fluid deficit that was measured before the beginning of each HYP
trial was -2.9 ± 0.7 and
-3.0 ± 0.8% of body mass
for cold and temperate, respectively. Differences were significant between
hydration levels (hypohydrated vs. euhydrated) but not between environments
(cold vs. temperate). The results of the study showed that all eight subjects
performed worse when hypohydrated in temperate air, whereas five of eight
experienced the same from hypohydrated in the cold. HR for temperate
hypohydrated at 30 min was higher than for temperate euhydrated and cold hypohydrated.
Rating of perceived exertion increased over time with no differences among
trials.
In conclusion the data demonstrate that hypohydration impairs endurance
exercise performance in temperate but not cold air. The corresponding change in
performance was greater for temperate (-8%) than for cold (-3%).
Influence of hydration on
physiological function and performance during trail running in the heat.
Casa et al. (2010) in their study had 17 distance runners completing 12
km runs in the heat in 4 conditions (hydrated race (HYR), dehydrated race
(DYR), hydrated submaximal race (HYS) and a dehydrated submaximal race (DYS)).
Heart rate, intestinal temperature and body mass loss were measured.
Dehydrated participants (DYR and DYS) had reduced body mass, completing
the trial with an average loss of 2.8 kg, whereas those in the HYR had lost an
average of 1.37 kg. Hydration status before all trials did not influence core
body temperature. Post-run core body temperature in DYR was greater (39.49 ± 0.37°C) than in HYR (39.18 ± 0.47°C). Similar difference was
observed for DYS, as core body temperature was greater than for HYS. Body core
temperature increased 0.22°C for every additional 1% of body mass loss The HR
for DYR was greater than that for HYR at 10 minutes post-run (132 ± 18 beats/min versus 118 ± 10 beats/min). The HR for DYS
was greater than that for HYS post-run (179 ± 11 beats/min versus 164 ± 10 beats/min). HR was 10 beats/min higher at 10
minutes post-exercise for every additional 1% of body mass loss. Performance
was similar in HYS and DYS trials. Rating of perceived exertion was greater in
DYS versus HYS.
Concluding, in submaximal running, when dehydration increased, post-run
body core temperature and HR were approximately 0.56C and 15 beats/min higher, differences in hydration status were just 2.5%.
However finishing times were the same, although dehydrated runners (2.53% additional
body mass loss at the end of the run) experienced higher physiologic strain finishing
the trial.
In the maximal running, finishing body core temperature was lower in the
hydrated runners, although they were exercising at a faster rate. As indicated
by similar HR, higher rating of perceived exertion and body core temperatures,
the dehydrated athletes were performing in maximal effort. However, their performance
was lower and the perceptual environmental symptoms connected with maximal effort
increased.
Result
Tables
Article
|
Title
|
Abstract
|
Introduction
|
Method
|
Results
|
Discussion
|
References
|
Montain, S.J. and Coyle,
E.F. (1992). Influence of graded dehydration on hyperthermia and
cardiovascular drift during exercise. Journal
of Applied Physiology, 73, pp. 1340–1350.
|
Contains 11 words.
Reflects the content of the study well.
|
Big abstract with many
detailed information about subject’s characteristics and results.
|
Short but aimed, defines
the problem.
|
Cross-sectional study.
Detailed description of the methodology, the statistical analysis and the
tools-equations used in the research. Ethically approved.
|
Short but informative. Clear presentation of results with tables and
figures.
|
Separated in paragraphs
concluding the findings. Enriched with 3D graphs.
|
Contains old studies. Few
references but adequate.
|
Article
|
Title
|
Abstract
|
Introduction
|
Method
|
Results
|
Discussion
|
References
|
Gonzalez-Alonso,
J., Mora-Rodriguez, R. and Coyle, E.F. (2000). Stroke volume during exercise:
interaction of environment and hydration. American Journal of Physiology,
278, pp. H321-H330.
|
Contains 9
words. Does not define exactly the word “environment”. “Temperature” maybe
more appropriate word.
|
Short and
informative.
|
Short but
aimed, defines the problem. Reviews previous studies around the problem.
|
Cross-sectional
study. Detailed description of the methodology and statistical analysis.
Ethically approved.
|
Clear
presentation of results and correlations between different variables. Tables
and figures were comprehensive
|
Separated in
paragraphs that compare the findings with other studies. Comprehensive and
legible.
|
Some of the
studies date back to 1940’s. Few references but adequate.
|
Article
|
Title
|
Abstract
|
Introduction
|
Method
|
Results
|
Discussion
|
References
|
Cheuvront
S.N., Carter R. III, Castellani J.W., Sawka M.N. (2005) Hypohydration impairs
endurance exercise performance in temperate but not cold air. J Appl
Physiol., 99(5) pp. 1972–1976.
|
Contains 11
words. Describes the study well.
|
Informative
and legible.
|
Short but
describing the problem.
|
Cross-sectional
study. Short description. Ethically approved.
|
Short although
descriptive. Tables easily understandable.
|
Short and not
separated into paragraphs which can be difficult to read.
|
Some of the
studies date back to 1940’s. Few references but adequate.
|
Article
|
Title
|
Abstract
|
Introduction
|
Method
|
Results
|
Discussion
|
References
|
Casa D.J.,
Stearns R.L., Lopez R.M., Ganio M.S., McDermott B.P., Walker Yeargin S.,
Yamamoto L.M., Mazerolle S.M., Roti M.W., Armstrong L.E., and Maresh C.M.
(2010) Influence of hydration on physiological function and performance
during trail running in the heat. J Athl Train , 45, pp. 147 - 156.
|
Contains 14
words. Describes the study well.
|
Very well
written, separated in paragraphs and informative.
|
Informative
about previous studies.
|
Cross-sectional
study. Very well written methodology separated into different paragraphs.
Ethically approved.
|
Easy to
understand both the results text and the graphs. Legible and comprehensive.
|
Separated and
informative about how the present results correlate with other studies’
foundings.
|
Mostly recent
studies. Adequately referenced
|
Article
|
Title
|
Summary
|
Introduction
|
Main Review
|
Conclusion
|
References
|
Convertino, V., Armstrong, L., Coyle, E., Mack, G., Sawka, M., Senay,
L. and Sherman, W. (1996). American College of Sports Medicine position
stand: exercise and fluid replacement. Medicine and Science in Sports and
Exercise, 28, i–vii.
|
Contains 11 words. Describes the review.
|
Long but very informative about the current knowledge.
|
Short but aimed.
|
Very informative review. Separated into sections that in turn include
paragraphs. Provides useful and wide knowledge in prevention of dehydration.
|
Short but summarizes the review well.
|
Mostly recent studies. Adequately referenced
|
Discussion
From the studies we can summarize that 2% dehydration, a common phenomenon
that can occur in every athlete, can be a serious limitation in performance and
in some cases even a health risk. Studies showed that when dehydrated,
subjects’ CO declined progressively due to reductions in SV (Montain et al 1992;
Gonzalez-Alonso et al. 2000), HR increased compared to euhydrated for a given
exercise intensity (Cheuvront et al. 2005; Casa et al. 2010), body core
temperature and skin temperature were significantly higher than euhydrated
(Montain et al 1992; Casa et al. 2010), rating of perceived exertion was higher
compared to euhydrated for a given exercise intensity (Gonzalez-Alonso et al.
2000; Casa et al. 2010) and there is a significant drop in aerobic performance
(Cheuvront et al. 2005).
However trainers and coaches can prevent dehydration effectively in
athletes. Many guidelines have been published that provide adequate information
on the prevention of dehydration. It is recommended that athletes consume a
balanced diet and drink an adequate amount of fluid in the 24 hour period
before the event, and especially in the period of the meal before exercising,
to reach a proper hydration level before exercising or competing (Convertino et
al. 1996). Another recommendation is that the athletes should drink about 500
ml of fluids 2 hours before exercising to reach a proper hydration level and to
allow the removal of excess ingested fluid. During the exercise, individuals
should start drinking from the beginning and often to match and replace the
fluid loss through sweating, or to consume the maximum amount the athlete can
tolerate without limiting his ability to perform.
Guidelines also mention that complete restoration of body fluids cannot
happen without electrolyte replacement through food or electrolyte fluids, which
contain primarily sodium chloride and potassium, which are lost by sweating
during exercise (Convertino et al. 1996). A sodium deficit, combined with a
large amount of fluids with a small of no percentage of electrolytes can led to
low plasma sodium levels, a phenomenon known as hyponatremia. Thus, during
prolonged exercise that lasts longer than 1 hour, electrolytes (mostly NaCl)
should be added in the drink to reduce the probability of hyponatremia.
Moreover, carbohydrates should also be contained in the fluid replacement
solution to restore the blood glucose concentration and attenuate the onset of
fatigue. Carbohydrate requirements can be met by consuming 600 to 1200 ml ⋅ h –1 of fluids that contain 4 – 8 % of carbohydrates.
A limitation of the current review is that the articles reviewed focus
around the effects of dehydration on aerobic performance and not in power or
strength. The reason behind this selection was that the effects on dehydration
can be clearly determined in aerobic exercise, but in strength and power
exercises many studies have conflicting results (Judelson et al. 2007). Many
studies showed improve in performance in jumps, sprints and weight exercises
when subjects were dehydrated (Judelson et al. 2007). One potential reason for
that could be the reduced body weight of the athlete, an effect of dehydration,
which enables the athlete to move faster with the same amount of force.
However, from the previous studies that were reviewed in the current study, we
can say with certainty that this principle does not apply in endurance performance,
and that any benefits of dehydration in endurance exercise are far less than
the disadvantages.
Conclusion
Dehydration could impose a great limitation to human performance and a
health risk, especially during prolonged exercise in the heat. Fluid
replacement should always attempt to equal fluid loss. The trainers should pay
close attention to the loss of body weight of the athletes during exercise, and
make sure they consume an adequate amount of fluid to match weight loss.
References
Casa D.J., Stearns R.L., Lopez R.M., Ganio M.S., McDermott B.P., Walker
Yeargin S., Yamamoto L.M., Mazerolle S.M., Roti M.W., Armstrong L.E., and
Maresh C.M. (2010) Influence of hydration on physiological function and
performance during trail running in the heat. J Athl Train, 45, pp. 147 - 156.
Cheuvront S.N., Carter R. III, Castellani J.W., Sawka M.N. (2005) Hypohydration impairs endurance exercise performance in temperate but not cold air. J Appl Physiol, 99(5) pp. 1972–1976.
Convertino, V., Armstrong, L., Coyle, E., Mack, G., Sawka, M., Senay, L.
and Sherman, W. (1996). American College of Sports Medicine position stand:
exercise and fluid replacement. Medicine
and Science in Sports and Exercise, 28, i–vii.
Gonzalez-Alonso, J., Mora-Rodriguez, R. and Coyle, E.F. (2000). Stroke volume during exercise: interaction of environment and hydration. American Journal of Physiology, 278, pp. H321-H330.
Judelson D.A., Maresh C.M., Anderson J.M., Armstrong L.E., Casa D.J., Kraemer W.J., Volek J.S. (2007). Hydration and Muscular Performance; Does Fluid Balance Affect Strength, Power and High-Intensity Endurance? Sports Med 2007, 37 (10), pp. 907-921
Montain, S.J. and Coyle, E.F. (1992). Influence of graded dehydration on
hyperthermia and cardiovascular drift during exercise. Journal of Applied Physiology, 73, pp. 1340–1350.
