Exercise And Hydration - More Is Not Better

 

Hyponatremia  is an electrolyte disturbance in which the sodium ion concentration in the plasma is lower than normal. Sodium is the dominant extracellular cation (positive ion) and cannot freely cross from the interstitial space into the cell, because charged sodium ions attract up to 25 water molecules around them, creating a large polar structure that is too large to pass through the cell membrane. Its homeostasis (stability of concentration) inside the cell is vital to the normal function of any cell. Normal serum sodium levels are between approximately 135 and 145 mEq/L (135 - 145 mmol/L). Hyponatremia is generally defined as a serum level of less than 135 mEq/L and is considered severe when the serum level is below 125 mEq/L.

What that means to you and me is that when you drink too much water all at once, it flushes out your electrolyte levels and brings your sodium levels to a dangerous low. Symptoms of hyponatremia include nausea and vomiting, headache, confusion, lethargy, fatigue, loss of appetite, restlessness and irritability, muscle weakness, spasms or cramps, seizures, and decreased consciousness or coma. These symptoms are often mistaken for heat injury. 

My experience with sodium and exercise has spanned over two decades. From my beginnings in fitness and martial arts training, back in the early 1980’s, the information taught about hydration, electrolytes, sodium and exercise was only partially accurate. Information and protocols that I was taught during the 1980’s to late 1990’s tended to lean towards the consumption of large amounts of fluids with sodium supplementation in extreme heat (Kratz, A., Siegel, A., Verbalis, J., Adner, M., Shirey, T., Lee-Lewandrowski, E., Lewandrowski, K., 2005).

Over the twenty years that I spent in the U.S. Army, I witnessed an evolution of procedures and protocols regarding hydration, sodium and exercise. It was always taught that you had to replace the water and sodium that you lost during exercise, especially in high temperature environments. The issue was primarily with the frequency and amounts of fluids ingested. 

The military used a forced hydration policy for many years where the concept of “more is better” prevailed. This led directly to a surge in cases of hyponatremia. There was a lack of knowledge and understanding about hyponatremia and many early cases of hyponatremia were mistakenly treated as heat injuries. As military medical doctrine improved, based upon newer science, hydration and sodium intake guidelines were updated as well as education and first responder treatment procedures for hyponatremia (Venuto, M., Brosch, L., Tchjanda, J., Cropper, T., 2011). 

Based on the literature I have reviewed for this topic and on my knowledge of heat, hydration and heat related illness; I have changed my recommendations on hydration in the heat to a policy of consistent, moderate hydration while avoiding the extremes of too little and too much water intake. Studies indicate that a consistent maintenance of fluid and sodium levels before, during and after exercise, are optimal and prevent most heat injuries and lower the risk of hyponatremia (Moreno, I., Vanderlei, L., Pastre, C., Vanderlei, F., Abreu, L., Ferreira, C., 2013).

Generally, the recommended fluid intake for men is 125-130 oz/day (approx. 16 cups) and 91-95 oz/day (approx. 12 cups) for women. One gallon of water equals 128 ounces. To keep it simple, I recommend one gallon a day as a guide for active people. More or less depending on activity level and climate.

References:

Venuto, M., Brosch, L., Tchjanda, J., Cropper, T. (2011). Retrospective Case Series of Five Nontraumatic Deaths Among U.S. Air Force Basic Military Trainees (1997-2007). Military Medicine, 176, 8:938.

Kratz, A., Siegel, A., Verbalis, J., Adner, M., Shirey, T., Lee-Lewandrowski, E., Lewandrowski, K. (2005). Sodium Status of Collapsed Marathon Runners. Archives of Pathology & Laboratory Medicine.

Moreno, I., Vanderlei. L., Pastre, C., Vanderlei, F., Abreu, L., Ferreira, C. (2013). Cardiorespiratory effects of water ingestion during and after exercise. International Archives of Medicine, 6:35.

Penney, S., Hydration for Health and Performance. 

http://blog.nasm.org/nutrition/hydration-health-performance/

Eric Dempsey
NASM Certified Personal Trainer & Weight Loss Specialist
Dempsey’s Resolution Fitness