Dehydration
Medical Author: Benjamin C. Wedro, MD, FAAEM
Medical Editor: Melissa Conrad Stöppler, MD
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Exercise-Associated Hyponatremia: Who's at Risk?
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor:
William C. Shiel Jr., MD, FACP, FACR
 We've all learned that fluid replacement is critical to
replace fluids lost when exercising, but drinking pure water exclusively isn't the safest choice for
those who participate in very strenuous or long-duration exercise.
When significant amounts of fluid are lost through
high-intensity exercise, replacement with water alone can lead to a chemical
imbalance in the body and deficiencies in electrolytes, which are nutrients
critical for organ
functioning. The electrolytes in our body include sodium, potassium, chloride,
calcium and phosphate, but sodium
is the substance of most concern when
replacing fluids lost through exercising.
Hyponatremia is a condition in which the body's stores of sodium are too low,
and this condition can result from drinking extreme amounts of water.
Hyponatremia can lead to confusion, lethargy, agitation, seizures, and in
extreme cases, even death. Early symptoms are nonspecific and subtle and may
include disorientation, nausea, or muscle
cramps. The symptoms of hyponatremia may also mimic those of dehydration, so athletes experiencing these symptoms may
be given more water to drink, further worsening the condition.
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What is dehydration?
The majority of the body is made up of water, with up to 75% of the body's
weight due to H2O. Most of the water is found within the cells of the body
(intracellular space). The rest is found in what is referred to as the extracellular space,
which consists of the blood vessels (intravascular space) and the spaces between
cells (interstitial space).
Total body water = intracellular space + intravascular space + interstitial
space
Dehydration occurs when the amount of water leaving the body is greater than
the amount being taken in. The body is very dynamic and always changing. This is
especially true with water in the body. We lose water routinely when:
- we breathe and humidified air leaves the body;
- we sweat to cool the body; and
- we
urinate or have a bowel movement to rid the body of waste products.
In a normal
day, a person has to drink a significant amount of water to replace this routine
loss.
If intravascular (within the blood vessels) water is lost, the body can
compensate somewhat by shifting water from cells into the blood vessels, but
this is a very short-term solution. Signs and symptoms of dehydration will occur
quickly if the water is not replenished.
The body is able to monitor the amount of fluid it needs to function. The
thirst mechanism signals the body to drink water when the body is dry. As well,
hormones like anti-diuretic hormone (ADH) work with the kidney to limit the
amount of water lost in the urine when the body needs to conserve water.
What causes dehydration?
Dehydration occurs because there is too much water lost, not enough water
taken in, or most often a combination of the two.
- Diarrhea: Diarrhea is the most common reason a person loses excess water. A significant
amount of water can be lost with each bowel movement. Worldwide, more than four
million children die each year because of dehydration from diarrhea.
- Vomiting: Vomiting can also be a cause of fluid loss
and it is difficult for a person to
replace water by drinking it if they are unable to tolerate liquids.
- Sweat: The body can lose significant amounts of water when it tries to cool itself
by sweating. Whether the body is hot because of the environment (for example,
working in a warm environment), intense exercising in a hot environment, or because a
fever is present due to an
infection; the body uses a significant amount of water in the form of sweat to
cool itself. Depending upon weather conditions, a brisk walk will generate up to
16 ounces of sweat (a pound of water).
- Diabetes: In people with
diabetes, elevated blood sugar levels cause sugar to spill
into the urine and water then follows, which can cause significant dehydration. For
this reason, frequent urination and excessive thirst are among the symptoms of
diabetes.
- Burns: Burn victims become dehydrated because water seeps into the damaged skin.
Other inflammatory diseases of the skin are also associated with fluid loss.
- Inability to drink fluids: The inability to drink adequately is the other potential cause of
dehydration. Whether it is the lack of availability of water or the lack of
strength to drink adequate amounts, this, coupled with routine or extraordinary
water losses can compound the degree of dehydration.
Next: What are the signs and symptoms of dehydration? »
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