Moderation in all things is a simple, consistent guideline to most things in life including hydrating in summer endurance events. Get your fluid and electrolyte balance right  and you can underwrite a performance, get it wrong and drink water to excess and you can potentially be facing up to catastrophic ill health. In the worst case scenario,  you can overwhelm your sodium levels in your plasma with excess water and  be battling hyponatremia.

Hyponatremia is a disturbance in the fluid-electrolyte balance, causing an abnormally low plasma sodium concentration (less than 135 millimoles per liter). The normal range of plasma sodium concentration is 136 to 142 millimoles per liter. An ongoing decrease in plasma sodium concentration upsets the osmotic balance across the blood-brain barrier and can trigger a rapid entry of water into the brain.

A reduction in plasma sodium concentration to 130 to 134 millimoles per liter generally causes no apparent symptoms. Early signs and symptoms of hyponatremia tend to develop when the plasma sodium concentration falls below 130 millimoles per liter and include bloating, “puffiness,” nausea, vomiting, and headache.

There are many potential causes of hyponatremia, but most often it is simple over-hydration. Exercise-associated hyponatremia is primarily caused by drinking an amount of fluid that exceeds sweat and urinary water losses. The most likely explanation for exercise-associated hyponatremia is that excessive drinking reduces the plasma sodium concentration. During exercise, urine output decreases and sets the stage for hyponatremia if too much fluid is ingested or
retained.

In general, an athlete who drinks too much during exercise lasting more than four hours in hot, humid climates is at risk of developing hyponatremia. Smaller and slower athletes who go over the top with fluid intake are at high risk. While large athletes are not ‘immune’ to hyponatremia, the fact is small athletes require less fluid to dilute their extracellular fluid. Slower-paced endurance athletes have more time and more chance to consume too much fluid.

In a recent study of hyponatremia among 488 runners in the 2002 Boston Marathon, Almond and colleagues found that 13% had hyponatremia (a serum sodium concentration of 135 millimoles per liter or less) and 0.6% had critical hyponatremia (120 millimoles per liter or less).7 Substantial weight gain was the strongest single predictor of hyponatremia and correlated with excessive fluid intake. Longer racing times and low body mass index (BMI) extremes were also
associated with hyponatremia.

Dr. Ian Rogers, suggests that between 500-750 milliliters/hr (about 17-25 fluid ounces/hr) will satisfy most athletes’ hydration requirements under most conditions.

Athletes should increase dietary sodium intake in the days prior to an event in hot conditions. They should also avoid overdrinking in the days or hours prior to exercising to prevent lowering blood sodium levels before the event. A high degree of self-discipline and common sense is required, as added sodium may stimulate thirst. Consuming a sodium-containing electrolytic sports drink helps to maintain plasma sodium levels and reduce the risk of hyponatremia.

References

Hydration and Hyponatremia in Athletes
By Ellen Coleman, MA, MPH, RD
Today’s Dietitian
Vol. 7 No. 10 P. 14