Hyponatremia
(Low Blood Sodium)
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel,
Jr., MD, FACP, FACR
Dehydration & Exercise = Hyponatremia (Low Blood Sodium)
Author: Richard Weil, MEd, CDE
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
Viewer Question: My trainer is always telling me to stay hydrated. How much water should I drink when exercising? What will happen if I drink too much?
Fitness Expert's Response: The National Athletic Trainers' Association recommends the following hydration guidelines for exercise:
- Two to three hours pre-exercise: 17 to 20 fluid ounces of water or sports drink.
- Ten to 20 minutes pre-exercise: 7 to 10 ounces of water or sports drink.
- During exercise: Fluid replacement should approximate sweat and urine losses and at least maintain hydration at less than 2% body weight reduction. This generally requires 7 to 10 ounces of water or sports drink every 10 to 20 minutes. Include carbohydrates in the beverage if the exercise is intense or lasts more then 45-50 minutes. Water alone will suffice, and save calories, if the exercise is moderate or less than 45-50 minutes.
- Post-exercise: Athletes should weigh themselves nude before and after workouts to learn how much weight is lost from sweat (water and salt) and then ingest fluid equal to 150% of the weight loss, ideally within two hours, and no more than four to six hours after the event. Including sodium in the drink allows fluid volume to be better conserved and increases the drive to drink, and carbohydrate in the drink will improve the rate of intestinal absorption of the fluid as well as replenish glycogen stores in the muscles and liver.
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What is hyponatremia (low blood sodium)?
Hyponatremia refers to a lower-than-normal level of sodium in the blood.
Sodium is essential for many body functions including the maintenance of fluid
balance, regulation of blood pressure, and normal function of the nervous
system. Hyponatremia has sometimes been referred to as "water intoxication,"
especially when it is due to the consumption of excess water, for example during
strenuous exercise, without adequate replacement of sodium.
Sodium is the major positively charged ion (cation) in the fluid outside of
cells of the body. The chemical notation for sodium is Na. When combined with
chloride (Cl), the
resulting substance is table salt (NaCl).
The normal blood sodium level is 135 - 145 milliEquivalents/liter (mEq/L), or
in international units, 135 - 145 millimoles/liter (mmol/L). Results may vary
slightly among different laboratories.
What causes hyponatremia (low blood sodium)?
A low sodium level in the blood may result from excess
water or fluid in the body, diluting the normal amount of sodium so that the
concentration appears low. This type of hyponatremia can be the result of
chronic conditions such as kidney failure (when excess fluid cannot be
efficiently excreted) and congestive heart failure, in which excess fluid
accumulates in the body. SIADH (syndrome of inappropriate anti-diuretic hormone)
is a disease whereby the body produces too much anti-diuretic hormone (ADH), resulting in retention of water in the body.
Consuming excess water, for example during strenuous exercise, without adequate
replacement of sodium, can also result in hyponatremia.
Hyponatremia can also result when sodium is lost from the body or when both
sodium and fluid are lost from the body, for example, during prolonged
sweating and severe
vomiting or diarrhea.
Medical conditions that can sometimes be associated with
hyponatremia are adrenal insufficiency, hypothyroidism, and
cirrhosis of the
liver.
Finally, a number of medications can lower blood sodium levels. Examples
of these include diuretics, vasopressin, and the sulfonylurea drugs.
Next: What are the symptoms of hyponatremia (low blood sodium)? »
- Electrolytes - Read about blood electrolytes (sodium, potassium, chloride, and bicorbonate) and the effects of electrolyte imbalances like kidney failure, low blood pressure, hypokalemia, and hyperkalemia.
- Kidney Failure - Learn about kidney failure, in which the body has fluid retention, risen blood pressure, toxin build up and lack of red blood cells. Symptoms include fatigue, nausea, and apetite loss.
- Urinalysis - Learn about the urinalysis test. Diseases and conditions such as urinary tract or kidney infection, kidney stones, kidney failure, diabetes, or high blood pressure can be diagnosed or monitored.
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