water and electrolyte balance during exercise

1. Water & Electrolyte
Balance During
Exercise
Dr. Usha (PT)
Assistant Professor

2. Rarely is water thought of as a nutrient because it has no caloric value. Yet its
importance in maintaining life is second only to oxygen’s.
Water constitutes about 60% of a typical young man’s and 50% of a typical
young woman’s total body weight; but this varies with body composition, since
the fat-free mass has a much higher water content (~73% water) than the fat
mass (~10% water).
It has been estimated that we can survive losses of up to 40% of our body
weight in fat, carbohydrate, and protein. But a water loss of only 9% to 12% of
body weight can be fatal.

3. Athletes commonly lose between 1% to 6% of
their body water during intense, prolonged
exercise.
However, a water loss in excess of 9% of a
person’s total body weight can lead to death.

4. Approximately two-thirds of the water in our bodies is contained in our
cells and is referred to as intracellular fluid. The remainder is outside
the cells, referred to as the extracellular fluid. Extracellular fluid
includes the interstitial fluid surrounding the cells, the blood plasma,
lymph, and other body fluids.
Water plays several critical roles in exercise. Among its most important
functions, water provides transportation between and delivery to the
body’s various tissues, regulates body temperature, and maintains BP
for proper cardiovascular function.

5. Water and Electrolyte Balance
For optimal performance, the body’s water and electrolyte
contents should remain relatively constant.
Unfortunately, this doesn’t always happen during exercise.

6. Water Balance At Rest
Under normal resting conditions, the body’s water content is relatively
constant: Water intake equals water output.
About 60% of our daily water intake is obtained from the fluids we drink
and about 30% is from the foods we consume. The remaining 10% is
produced in our cells during metabolism.
Metabolic water production varies from 150 to 250 ml per day,
depending on the rate of energy expenditure: Higher metabolic rates
produce more water.
The total daily water intake from all sources averages about 33 ml per kg
of body weight per day. For a 70 kg (154 lb) person, average intake is 2.3
L per day.

7. Water output, or water loss, occurs from four sources:
1. Evaporation from the skin
2. Evaporation from the respiratory tract
3. Excretion from the kidneys
4. Excretion from the large intestine

8. Human skin is permeable to water. Water diffuses to the skin’s
surface, where it evaporates into the environment.
In addition, the gases we breathe are constantly being
humidified by water as they pass through the respiratory tract.
These two types of water loss (from the skin and respiration)
occur without our sensing them.
Thus, they are termed insensible water losses. Under cool,
resting conditions, these losses account for about 30% of daily
water loss.

9. The majority of our daily water loss—60% at rest— occurs
from our kidneys, which excrete water and waste products as
urine. Under resting conditions, the kidneys excrete about 50
to 60 ml of water per hour.
Another 5% of the water is lost by sweating (although this is
often considered along with insensible water loss), and the
remaining 5% is excreted from the large intestine in the feces.

11. Water Balance During Exercise
Water loss accelerates during exercise, as seen in table 15.4.
The ability to lose the heat generated during exercise depends
primarily on the formation and evaporation of sweat.
As body temperature increases, sweating increases in an effort
to prevent overheating. But at the same time, more water is
produced during exercise because of increased oxidative
metabolism.
Unfortunately, the amount produced even during the most
intense effort has only a small impact on the dehydration, or
water loss, that results from heavy sweating.

13. In general, the amount of sweat produced during exercise is
determined by
• Environmental temperature, radiant heat load, humidity, and
air velocity;
• Body size; and
• Metabolic rate.
These factors influence the body’s heat storage and
temperature regulation. Heat is transferred from warmer areas
to cooler ones, so heat loss from the body is impaired by high
environmental temperatures, radiation, high humidity, and still
air.

14. Body size, specifically the ratio between surface area and mass,
is important because large individuals generally expend more
energy to do a given task, so they typically have higher
metabolic rates and produce more heat.
But they also have more surface area (skin), which allows more
sweat formation and evaporation.

15. As exercise intensity increases, so does the metabolic rate.
This increases body heat production, which in turn increases
sweating.
To conserve water during exercise, blood flow to the kidneys
decreases in an attempt to prevent dehydration; but like the
increase in metabolic water production, this too may be insufficient.
During high-intensity exercise under environmental heat stress,
sweating can cause losses of as much as 2 to 3 L of water per hour.

16. During an event such as the marathon, sweating may reduce
body water content by 6% or more.
In cold, dry environments or at altitude, water loss from
respiration contributes to the overall loss of body water as
well.

17. Dehydration and Exercise
Performance
Even minimal changes in the body’s water content can impair
endurance performance.
Without adequate fluid replacement, an athlete’s exercise
tolerance shows a pronounced decrease during long-term
activity because of water loss through sweating.
The impact of dehydration on the cardiovascular and
thermoregulatory systems is quite predictable.

18. Fluid loss decreases plasma volume - decreases blood
pressure- reduces blood flow to the muscles and skin.
In an effort to overcome this, heart rate increases.
Because less blood reaches the skin, heat dissipation is
hindered, and the body retains more heat.
Thus, when a person is dehydrated by 2% of body
weight or more, both heart rate and body temperature
are elevated during exercise above values observed
when normally hydrated.

19. As one might expect, these physiological changes will decrease
exercise performance.
Figure 15.14 illustrates the effects of an approximate 2%
decrease in body weight attributable to dehydration from the
use of a diuretic on distance runners’ performance in 1,500 m,
5,000 m, and 10,000 m time trials on an outdoor track.
The dehydration condition resulted in plasma volume
decreases between 10% and 12%.

21. Although the average VO2max did not differ between the
normally hydrated and dehydrated trials, mean running
velocity decreased by 3% in the 1,500 m run and by more than
6% in the 5,000m and 10,000 m runs.
The greater the duration of the performance, the greater is the
expected decline in performance for the same degree of
dehydration.

22. These trials were conducted in relatively cool weather. The
higher the temperature, humidity, and radiation, the greater
the expected decrement in performance for the same degree of
dehydration.
The decrement in performance would be progressively greater
with greater levels of dehydration.

23. In one of the best-controlled studies, researchers at Penn State
University reported that 2% dehydration resulted in
significant deterioration of basketball skills in 12- to 15-year-
old boys who were skilled basketball players.
Wrestlers and other weight-category athletes commonly
dehydrate to get a weight advantage during the weigh-in for a
competition.
Most rehydrate after the weigh-in before the competition and
experience only small decrements in performance.

24. Summary of Water Balance
Water balance depends on electrolyte balance, and vice versa.
At rest, water intake equals water output. Water intake includes water ingested from
foods and fluids and produced as a metabolic by-product. The majority of water output
at rest occurs from the kidneys, but water also is lost from the skin, from respiratory
tract, and in feces.
During exercise, metabolic water production increases as metabolic rate increases.
Water loss during exercise increases because as heat in the body increases, more water
is lost with increased sweating. Sweat (90%) becomes the primary avenue for water
loss during exercise. In fact, the kidneys decrease urine production in an effort to
prevent dehydration.
When dehydration reaches 2% of body weight, aerobic endurance performance, and
even skilled performance in sports such as shooting free throws in basketball, is notably
impaired. Heart rate and body temperature increase in response to dehydration.

25. Electrolyte Balance During
Exercise
Normal body function depends on a balance between water
and electrolytes.
The effects of the other component of this delicate balance:
electrolytes.
When large amounts of water are lost from the body, as during
exercise, the balance between water and electrolytes can be
disrupted quickly.
The two major routes for electrolyte loss: sweating and urine
production.

26. Electrolyte Loss in Sweat
Human sweat is a filtrate of blood plasma, so it contains many substances
found there, including sodium (Na+), chloride (Cl–), potassium (K+),
magnesium (Mg2+), and calcium (Ca2+).
Although sweat tastes salty, it contains far fewer minerals than the
plasma and other body fluids. In fact, sweat is 99% water.

27. Sodium and chloride are the predominant ions in sweat and blood.
The concentrations of sodium and chloride in sweat are about one-third
those found in plasma and five times those found in muscle.
Each of these three fluids’ osmolarity, which is the ratio of solutes (such
as electrolytes) to fluid, is also shown.
Sweat’s electrolyte concentration can vary considerably between
individuals. It is strongly influenced by genetics, the rate of sweating, the
state of training, and the state of heat acclimatization.

29. At the elevated rates of sweating reported during endurance
events, sweat contains large amounts of sodium and chloride
but little potassium, calcium, and magnesium.
Based on estimates of the athlete’s total body electrolyte
content, such losses would lower the body’s sodium and
chloride content by only about 5% to 7%.
Total body levels of potassium and magnesium, two ions
principally confined to the insides of cells, would decrease by
about 1%. These losses probably have no measurable effect on
an athlete’s performance.

30. As electrolytes are lost in sweat, the remaining ions are
redistributed among the body tissues. Consider potassium. It
diffuses from active muscle fibers as they contract, entering
the extracellular fluid.
The increase this causes in extracellular potassium levels does
not equal the amount of potassium that is released from active
muscles, because potassium is taken up by inactive muscles
and other tissues while the active muscles are losing it. During
recovery, intracellular potassium levels normalize quickly.

31. Thank you