Sweat Rate and Sweat Electrolyte Concentrations between Two Modes of Exercise in
Trained Triathletes
Literature review
Thermoregulation
Thermoregulation or temperature regulation is the body’s response to a thermal
load, by acting through an effector response, to maintain homeostasis 1,2. The effector
response is shivering, sweating, and vasomotor changes 2. For this literature review, only
sweating in response to a thermal load will be used for thermoregulation. Regulation of
core body temperature (Tc) is considered vital organs and head and trunk2. Measuring Tc
is achieved through various methods; oral, tympanic membrane, rectal, esophageal,
hypothalamic, or central blood temperature2.
An athlete during exercise will create large amounts of heat produced by
muscle contraction as a byproduct of metabolism1,3, especially true if an athlete is
performing a steady-state exercise. Core temperature during a steady-state will rise
linearly to metabolic rate and an increase in the level of exercise2,3. The rise in Tc will
first be recognized by the hypothalamus1-4, 6, 12. The heat produced by working
muscle will be carried away via the venous drainage system2,3.
Within the hypothalamus, there is a set core body temperature that the body uses a
reference point during exercise2,4. The setpoint is a concept that describes the control of
thermoregulation within the body4. When the hypothalamus recognizes an increase in core
temperature, one of the first mechanisms is sweating and increase in skin blood flow
(vasodilation/ vasoconstriction)4. The earliest response is vasoconstriction of the superficial
1
veins2. As the thermal load increases, sweating and vasodilation begin, and skin blood
flow to the limbs is the greatest to try to dissipate the heat2.
Vasodilation is an essential concept for cooling the core. When Tc rises above 37 C1,
the body will begin to sweat, as well as, vasodilate blood vessels8. Warmblood from the
core is distributed to the skin3. The warm blood at the skin surface will lose heat (being
cooled) through convection at the skin surface3. The blood that is cool, at the skin surface,
is carried to the core where Tc will fall3. Increase of skin blood flow will cause more
significant vasodilation of the vessels which, opens new skin capillaries; these new
capillaries will allow a more considerable amount of time for heat dissipation to the
ambient air through convective cooling3.
If the ambient air temperature is 28°C, increase skin blood flow will be the primary
method for cooling the body3. Therefore, vasodilation is limited to the surrounding ambient air
temperatures3. However, if the ambient air temperature is higher than Tc, then the body will store
heat3, causing a reversal in heat exchange, and heat is stored within the body, resulting in an
increased core body temperature3. Therefore, the body must try to cool itself off from sweating.
Activation of vasodilation and sweat glands is key to keep the body cool.
I. Sweat glands
Apoeccrine, apocrine, and eccrine glands are all different type of sweat glands1,5.
Apoeccrine and apocrine are limited to specific regions on the body (axillae region) and are not
responsible for thermoregulatory sweat and only become active at the start of puberty
1,5,6. Therefore, only the eccrine gland will be the focus of this literature review, and we
refer to as sweat glands.
2
Humans have approximately 1.6 – 4.0 million eccrine glands1,5,7, and the
distribution of the eccrine glands over the body is the greatest in the palms of hands
and feet, forehead, following upper limbs, and finally lower limbs and trunk5,6. The
size of the sweat glands varies both individually and regionally6. However, sweat gland
density decrease with body surface area1.
Sweat glands have a secretory coil and a duct1,5,6, tubular epithelium make up
the two structures1. The secretory coil is located in the lower dermis, while the duct
extends through the dermal layer and then opens to the skin surface5. The size of an
adult secretory coil ranges from 30 – 50 µm in diameter and 2 – 5 mm in length5, while
the size of the duct ranges 1 to 8 X 10 -3 mm3,5. A positive correlation has shown
between maximal sweat rate and glandular size5,6.
II. Heat balance theory
The heat balance theory is how the body maintains equalibrium1. The body
cools primarily from the evaporation of sweat from the skin surface. Factors that
impact body heat gain or loss are clothing or equipment, body composition, and body
size will directly modify sweat rate1,8.
The following equation is evaporative heat loss (Ereq)
formula Ereq = M – W (R + C + K)
M refers to metabolic energy expenditure. Metabolic expenditure is the total rate of energy
used during oxygen consumption1. Metabolic heat expenditure is the most significant contributor to
a rise in core temperature during exercise1,3. W is external work. Work is the rate at which external
work (force X distance) is being perfromed1. R is radiant heat exchange. Radiation can either give
the body heat or cause heat loss occurring through the
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skin or clothing and the surrounding surfaces1. C is convective heat exchange. Convective
heat exchange is the movement of air past the body1. Convection will have the most effect
on cooling the body during exercise. K is conductive heat exchange. Conduction is the
transfer of heat to the surface by direct contact with the body or clothing1. Radiation,
conduction, and convection rely on environmental conditions.
III. Hydration
Hydration plays a critical role in thermoregulation. An individual’s total body water
counts as 60% of their weight9. Therefore, everyone has a day-to-day variation in their total
body weight, hypohydration (less water in their body), or hyperhydration (more water in their
body) that exceeds normal euhydration of approximately ± 0.2 – 0.5% of total body mass9. Daily
water loss occurs through urinary/ fecal, respiration, and sweating, known as dehydration9.
Dehydration is the process of losing water10. Dehydration through exercise occurs primarily
through sweat. For example, an individual can start an exercise bout already below their normal
euhydrated state (hypohydrated), because their total body water is low from the previous
exercise, they will become more dehydrated with exercise9. Starting an exercise bout already
hypohydrated can have serious effects.
The state of being hypohydrated has found to increase body heat storage while at the
same time, decrease an individual’s tolerance to exercise in the heat9. Since water stores heat,
and when the core temperature is above 37 C, in a hypohydrated individual, it increases the
core temperature quicker and higher because there is less fluid in the body to store the heat,
putting them at risk of a heat illness10. An increase is core temperature reduces the blood flow
to the skin, thus leading to a hypovolemic state9,11.
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Hypovolemia during exercise commonly occurs during exercise through sweating and
also through the movement of blood or fluid to the interstitial space surrounding the
muscle3,11. Nadel et al.11 found that an extensive exercise, there is a relationship between
forearm skin blood flow and core temperature. The researchers found that when compared to a
hyperhydrated and control (euhydrated) individual, hypohydration has a decrease in forearm
skin blood flow and a higher core temperature11. The decrease in skin blood flow to the
forearm will affect how the body cools during convective heat loss.
Sweating
As the body losses water via sweating, numerous minerals are lost. Researchers have
determined that sodium (Na+), potassium (K+), chloride (Cl-) are most commonly studied. A rise in
body temperature initiates thermal sweating5,12. As stated previously, the sweating response
correlates with the central thermoregulator within the hypothalamus1-3,12. Sweat initiation begins
when the hypothalamus detects a Tc of 37°C 1,5. Studying the neural tracks in humans is hard to
understand because it is not entirely understood, but evidence in animal studies suggest that
nerves surrounding the eccrine gland are nonmyelinated class C sympathetic postganglionic
fibers1,5. Acetylcholine is the primary neurotransmitter that stimulates the C fibers1,5,7,12. In
addition to sweat stimulated through a rise in Tc, the eccrine sweat gland has shown to be
stimulated by non-thermal sudomotor responses mediated by a feed-forward response regulated
by mechanoreceptors, osmoreceptors, and baroreceptors1.
When the activated eccrine sweat gland becomes stimulated, a release of
acetylcholine that diffuses into the presynaptic terminal, which enters the gland via
intercellular canaliculi12, the ramification from the release of acetylcholine causes an influx of
calcium ions enters and stimulates the pumping of sodium ions into the cell12 and
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increases the permeability of potassium and chloride channels5. Water that is resting in the
interstitial space follows the sodium, due to osmosis12. Sodium potassium pumps, located in the
luminal membrane, are activated when they reach their threshold potential12. The pumps actively
transport sodium, along with chloride and water, to the skin surface12.
Sweat is primarily isotonic with blood/ interstitial fluids1,5,12, but eccrine glands can
actively reabsorb ions1,12. Reabsorption occurs in the duct12. As the sweat is making its way to
the surface of the skin, sodium passively reabsorbed via epithelial sodium channels on the
luminal membrane1,13. Sodium is also actively reabsorbed through sodium/ potassium-
ATPase transporters on the basolateral membrane14. The cystic fibrosis transmembrane
conductance regulator passively reabsorbs chloride1. Aldosterone is the primary hormone
that is responsible for the activity of the reabsorption transporter1,15. Reabsorption is also
related to sweat rate 1,5. The higher the sweat rate, the less reabsorption occurs, resulting in a
more significant surface sweat electrolyte concentration. Vice versa, the less significant the
sweat rate, the higher the reabsorption rate is resulting in a lower surface sweat electrolyte
concentration.
I. Heat Acclimation
Exercising in the heat can affect the physiology of the eccrine sweat gland 1,16. Sweat
rate, as well as, sweat composition was the first to be studied and reported on in the study of
acclimation4. Heat acclimation develops through repeated exposure to a thermal environment,
which will reduce the likelihood of the adverse effect of heat stress4. Heat acclimation will
cause an increase in sweat rate as well as decrease sweat chloride and sodium
concentrations4,16. Exercising in the heat shifts the onset of sweating, which can occur at a
lower TC4,17,18- this shift in an adaptive response to central and peripheral
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mechanism4. Meaning, exercising in a thermal environment will decrease core body
temperature (central), as well as an increase in the onset of sweating and increase skin blood
flow (peripheral)4. The peripheral adaptations occur at the sweat gland level. Adaptations that
occur can improve cholinergic sensitivity and increase the size and efficiency of the eccrine
sweat gland4,6. Since there is the onset of an increase of sweating at a lower core temperature,
the sweat glands can adapt against hidromeiosis4,19. Hidromeiosis is a saturated epidermis
(stratum corneum), which suppresses sweating20.
As mentioned previously, sweat electrolyte concentration decreases with repeated
thermal training4, due to the sweat gland being able to reabsorb Na+ and Cl- more efficiently,
resulting in a more dilute sweat4,19,21,22. For example, an unacclimated individual may have a
sweat sodium concentration of 70 mmol/l. With proper training in the heat, the sweat gland
will adapt and be able to reabsorb more sodium giving this individual new sweat sodium of 15
mmol/l. Changing the sweat electrolyte concentration of sweat will lower the water vapor
pressure and will allow for the sweat to be evaporated easier because of the gradient between
the air and the sweat23.
Within the research on heat acclimation, repeated training in the heat will affect sweat rate
and sweat sodium (SwtNa+). Buono et al.22 studied the effect of heat acclimation on sodium ions of
healthy individuals. The conclusion of this study leads the researchers to report that heat
acclimation increased the sodium reabsorption rate within the sweat duct22. They hypothesized
that when the body acclimated to a hot environment, it produced a more diluted sweat22. Once
acclimated, the body will secrete fewer mineral solutes and thought the body would store these
solutes in the extracellular space22. The extra pressure within the extracellular space (increase in
osmotic pressure) will push the solutes into the cell22. Being
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able to have a reduced SwtNa+ secreting may lead to an individual being able to have a
more maintainable thermoregulatory sweat rate (SwR) and skin blood flow process
while exercising in the heat22.
II. Sweat rate measurement
Sweat testing has been extensively studied and tested regarding their
methodology and reliability1. There are two ways to measure sweat rate, local sweat
rate (SwRL) and whole-body sweat rate (SwRWB).
There are two ways to measure the SwRL. The first method to measure SwRL is
hygrometry. Hygrometry or ventilated sweat capsules pump dry air into the capsule
with a known temperature, and measures the change in temperature of the water
vapor that is pumped out1. Limitations of this method do arise. This method creates a
microclimate1. The microclimate can overestimate SwRL because the ventilation
creates dry skin, which stimulates sweating1.
Gravimetric is the second method of measuring SwRL 1,24. Gravimetric involves
collecting sweat on the skin surface using absorbent patches24 filter paper, Parafilm-M
pouches, cotton gloves/socks, or plastic sweat collector1. With those methods, the
difference in mass of the saturated sweat patch between pre-exercise and post-exercise
weight determines SwRL1. The patch is removed when is appears to be soaked in
sweat1,8 or after the completion of exercise session24-26.
Limitations to gravimetric techniques also arise because of the adhesive film collects
the sweat. The adhesive film creates a microenvironment and ultimately traps the sweat
inside1. Trapping the sweat on the surface will increase the rate of hidromeiosis1,24.
8
Regional variations do exhibit but are similar for males and females27. A study by
Havenith et al.27 found that the sweat rate of the extremities (arm) was significantly less
when compared to the sweat rate of the torso (chest, sides, and back).
Finding a reliable body placement to measure SwRL to estimate SwRWB is an issue.
Each local site across the body produces different SwRL24. Mid-back, chest, and forehead then
to overestimate SwRL, while the forearm is the most accurate to estimate for SwRWB1,24 due to
the vast differences in sweat gland density. A particular region of the body will have a higher
density of sweat glands when compared to another region24. The eccrine glands’ ability to
secrete sweat remains the same, but there is a more significant density (number) of the sweat
gland. The more glands in an area, there will be an increase in SwRL1. Baker1 found that SwRL
could be used to are an economical and reliable method to determine SwRWB. However, her
conclusion is only reliable after 30 minutes of exercise1.
The most practical and method of obtaining SwRWB is by subtracting the subject’s body
mass before to exercise to their body mass after exercise1,8,24,25,28,29. Nude body mass is
necessary if clothing is soaked or if conducting a field study. Weighing with clothing or other
equipment worn will drastically overestimate calculations by 10%1. Urine output, fluid intake
and exercise time is also accounted for SwRWB calculation1,8,24,25,28,29.
III. Sweat Electrolytes collection
Whole-body washdown (WBW) is considered to be the most accurate method of sweat
electrolyte collection. A study done by Shirreffs and Maughan28 collected sweat through the WBW
method. The researchers strongly recommended using this method to collect sweat samples
because sweat is not absorbed into clothing, dripped off the body, and does not interfere with
natural evaporative process 28. The researchers constructed a sweat
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collection apparatus made out of polyethylene28. This apparatus collected all sweat that ran off
the subjects but also allowed for a natural evaporative process to occur28. Before entering the
sweat collection apparatus, each subject was washed with 4 liters of deionized water and then
dried with a towel28. The subjects would only exercise until they lost 2% body mass, which
lasted from 55 – 135 minutes28. Upon completion of the exercise bout, each subject washed
with 4 liters of deionized water and took off all clothing and left the clothing at the bottom of
the apparatus28. The researchers took sweat samples from the bottom of the bag28. This
mixture would give an accurate representation of whole-body sweat loss28. A limitation of
using this method of sweat collection is that it can be used only in the laboratory setting, and
subjects can only use a cycle ergometer1,14,28.
Since the WBW method was limited to the laboratory setting, other methods of sweat
electrolyte concentration analysis need to occur in a field study. Patterson et al.24 wanted to
determine which anatomical sites are most accurate when compared to the WBW method. As
state previously, the body’s distribution of sweat glad density differs across the body24, which
will affect sweat electrolyte concentration. Patterson et al.24 tested ten anatomical sites.
Forehead, chest, scapula, abdomen, lower back, forearm, hand, thigh, calf, and foot. Their results
showed that three anatomical sites that could provide an accurate representation of whole-
body sweat electrolyte loss, forearm, thigh, and calf1,24. Researchers usually remove the sweat
patches once they have become saturated or at the end of practice1. However, research has
found that there was no significant difference if the absorbent patches were removed 30
minutes into the exercise bout or 70 minutes1.
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IV. Individual considerations
Both SwR and sweat electrolytes vary among individuals. Both males and females
of various ages have very individual differences in sweat electrolytes30. Research has
shown that males have a higher incidence of onset of sweating1,8,31, thus potentially
having a higher secretion rate of Na+ 30. Meyer et al.30 reported that prepubescent males
and females have lower amounts of sweat Na+ and Cl- but have higher K+ levels when
compared to adults. They concluded that this was due to the maturation of the eccrine
sweat gland and its ability to reabsorb sweat electrolytes and the transportations
properties of the sweat gland30. Aldosterone levels within the duct were lower in the
prepubescent population when compared to the adult population30.
Aerobically trained versus untrained population was looked into by Hamouti et
al25. In their study, they conducted a cycle-test. Their research found that trained
individuals tended to secrete more Na+ than untrained at a higher exercise intensity25.
When there was a spike in exercise intensity, the untrained individual had higher
sweat sodium levels than trained individuals25.
If an individual is secreting large amounts of sodium when sweating, their body fluid
balance may be difficult to maintain8. Supplementing with salt may aid in the prevention low
blood sodium and plasma volume8. An introduction of a high sodium diet may aid in reversing
the expected fall in sweat sodium when sweating, particularly in the heat8.
For sports medicine professionals working with and around athletes, it is particularly
important to be aware of how much sweat sodium an athlete is losing as well as how much the
athlete is replacing the lost Na+ through dietary supplementation. More tremendous athletes may
be consuming a large amount of sodium due to them consuming more calories
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than smaller athletes, but that is not always the case8. An increase in Na+ desire may be
correlated with hypohydration and deprivation8.After intense exercise, these athletes may
increase their desire for salt and fluids8. A severe loss of Na+ during exercise can be described
as exercise-associated hyponatremia (EAH) or hyponatremia of exercise8, 32.
Exercise associated hyponatremia is a clinical condition in an athletic population or
general population where blood sodium concentrations drop below 135 mmol/l 8, 32.
Athletes who have a high sweat rate might think they need to replace their loss with more
fluid. However, these athletes are putting their wellbeing in danger. High rates of fluid
consumption, consuming hypotonic fluids, are hazardous in individuals who consume to
much over a single exercise bout32. Due to the sudden increase in fluid in the extracellular
space relative to the sodium content32, it will dilute the blood Na+.
Some health care professionals believe that sports drinks that advertise as an electrolyte
replacement source are the answer for sodium replacement8. However, these sodium
replacement beverages do not contain enough sodium to restore the body to normal levels
effectively8. The body has defense mechanisms that can protect against rapid sodium depletion
during exercise33. The body will immediately release sodium stores from the internal body stores,
as well as matching extracellular fluid that equals 140 mmol/l of sodium33. However, there is
limited research regarding information on the sodium stores within the body34. This process is
individualized, and some individuals cannot adequately utilize their stores, which will lead them
closer to a hyponatremic state34.
Body characteristics were looked into by Havenith et al.35 In their study, their subjects
cycle on an ergometer. Their research found that there is no relationship present when
comparing sex and adipose tissue when looking at sweat loss35. However, when
12
Havenith et al.35 compared body surface area (BSA) to sweat loss, they found a
significant correlation.
An individual with a high BSA will be able to dissipate heat not as well when compared
to an individual with a low BSA31. Square root of height (cm) times weight (kg) over 3600 is
the formula for body surface area. Godek et al.31 looked into American football athletes and
cross-country runners. These two groups of athletes have different demographics31. The
American football players have a bigger body size than the cross-country runners. The
researchers conclude that American football athletes have a higher sweat rate when compared
to the cross-country runners due to American football players having a greater BSA; this is due
to a more significant number of sweat glands31.
Due to the research available, the sweat rate is dependent on body size (BSA)1, 8, 31.
Males would have a higher sweat rate when compared to females. Since females will have a
smaller BSA when compared to males. Males are have a more considerable amount of
metabolic heat production when exercising36, 37. Males and females have the same number
of eccrine glands36, but females have a lower sweat rate, due to lower BSA27. In a study by
Avelline et al.27 the researchers found that when males and females exercised, males have
a more significant metabolic workload and aerobic capacities, and males are more waste
full sweaters8,31. Males would have a higher sweat rate, because the sweat would drip off
the body; this is waste full sweating because of the sweat is not evaporating off the body
for a cooling mechanism8,31.
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V. Comparison
Although no articles compare sweat rate or sweat concentration within the
same individual performing two different modes of exercise, there are a few studies
that studied sweat electrolyte concentrations comparing groups of people.
Godek et al.8 studied sweat rates and sweat sodium concentrations in three groups
of American football players. The authors looked at backs and receivers, linebackers and
quarterbacks, and linemen8. Sweat rate variation’s between positions were due to BSA1, 8,
31. There was no difference in sweat sodium, but value ranges from 15 – 99 mEq/L8. Since
sweat rates were different within groups, sweat sodium loss was the greatest for the
linemen8. The authors have shown that sweat sodium is highly individualized, and heavy,
salty sweat needs an increase of sodium in their diet8.
Henkin et al.26 looked at sweat sodium concentrations and sweat rate in 3 groups as
well, athletes, swimmers, and nonathletes. In order to match groups, the runners and
swimmers would be exercising in their respective sport for 2 hours a day for 5 days a week,
while the nonathletes could only weight train for 3 days a week26. The authors concluded that
the runners had the highest sweat rate at 1.5 ± 0.2 L per hour, while the nonathletes had the
lowest sweat rate at 0.6 ± 0.2 L per hour. The swimmers and nonathletes shared similar sweat
electrolyte concentrations (65.4 ± 5.5 and 67.3 ± 8.5, respectively)26. The authors concluded
that since runners were already acclimated to running, they exhibited a higher swat rate and
lower sweat electrolyte concentration; conversely, the aquatic environment might have limited
body heat transfer, which resulted in limited sweating26.
14
VI. Disorders of Sweat glands
Hyperhidrosis is in individuals who experience abnormally large amounts of sweat
from their palms and soles or their feet38. This condition occurs when someone is
experiencing social and or occupational distress38. Hyperhidrosis does not occur during
sleep or sedentary activity38. Interestingly enough, hyperhidrosis of the axillae can occur
to38. However, sweat from these glands occurs through apoeccrine38. Hyperhidrosis of the
axillae has led researchers to believe that large amounts of sweat from this region do not
have an increase in odor38. They are suggesting that excessive sweating washes away the
odor-producing properties of sweat38.
On the other end of the spectrum, there is a condition called hypohidrosis or
anhidrosis. Anhidrosis is a clinical condition where an individual cannot produce or deliver
sweat to the surface of the skin38. Individuals who have localized or full-body, anhidrosis could
put the body at risk for the inability to cool themselves due to a lack of sweating2, 3. Anhidrosis
can occur from surgery or trauma, inflammation of the skin, scar formation, or infections38. It
remains to be researched if these individuals with anhidrosis occur due portal occlusion or if
sweat is not secreted to the skin surface38. Sweat transported to the surface can irritate the
already infected skin in individuals with psoriasis or atopical dermatitis38. These individuals
experience a burning sensation of the skin on a hot and humid day38- suggesting that sweating
does occur under the skin lesion38.
Cystic fibrosis (CF) is an autosomal recessive disorder that alters the way the sweat gland
can reabsorption of sweat chloride as well as sweat sodium39. Cystic fibrosis genetically alters the
cystic fibrosis transmembrane conductance regulator, which makes the sweat duct impermeable
to chloride39. For instance, an individual with CF will be unable to
15
reabsorb sweat chloride because of the occlusion of the cystic fibrosis transmembrane
conductance regulator. The absence of a non-functioning cystic fibrosis transmembrane
conductance regulator will limit the amount of sodium reabsorbed as well because the
epithelial sodium channel is activated together with the cystic fibrosis transmembrane
conductance regulator39. This condition will cause an increase of 3-5 times the normal
limit of surface sweat sodium and chloride39.
Conclusion and Future Research
Thermoregulation, as well as sweat testing, are frequently researched. The production of
sweat and the end electrolyte concentrations are individualized1,4,6,7,8-10,25,27,30,36,37. Each of these
studies all examined sweat electrolytes, sweat rate, and sweat collection.
Various methods of sweat methodology are discussed in this literature review. The use of
local absorbent patches is the most realistic and practical method to used to collect sweat in a field
study1. Sweat rate and sweat electrolyte concentration is very individualized, based on individual
considerations and heat acclimation1,4,6,7,8-10,25,27,30,36,37.
There is a gap in literature and research that does not compare sweat electrolyte
concentration and sweat rate when performing two different modes of exercise in the same
individual. It is understood that there will be a biomechanical difference in comparing two
different forms of exercise, but exercise intensity was controlled. For instance, oxygen
consumption increases with exercise intensity12. Therefore, sweat rate increases with core body
temperature, while sweat sodium concentrations increase linearly with sweat rate12,40.
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