Hyponatremia, Hydration - Too Much of a Good Thing
Dehydration warnings and preventive measures tend to dominate endurance performance nutrition advice. It is well established that dehydration has a negative impact on health and performance.
Severe dehydration is life threatening although rare in standard endurance events. Although moderate dehydration may not be life threatening it can affect performance. In matters of performance, Dr. Barr of UBC in Vancouver published in1999, that just a 2% reduction in body fluid weight can result in a 1.8-2% negative performance result. A 2-3% fluid loss produces a 7% reduction in performance.
Since 1984, a growing number of athletes requiring emergency medical tent attention following endurance events including standard marathons, ironman triathlons and long distance bicycle races have been diagnosed with over hydration (also known as ‘water intoxication’ or the medical term ‘hyponatremia’).
D B Speedy from the University of Auckland in New Zealand published in 2001 that as little as 2% “over hydration” can cause life-threatening hyponatremia. Several deaths due to hyponatremia have been reported since 2000 at events like the Jacksonville marathon, the Boston Marathon, and Ironman events.
So it has now become clear that, you need to get your hydration needs just right. Hydration must be coupled with adequate sodium intake; sodium losses can be significant during prolonged exercise. Barr and associates observed an average loss of 12 g salt in six hours of cycling at 86 degrees F (30 degrees C) and 30% relative humidity (2 g more than the average daily intake of 10 g).
The actual incidence of hyponatremia in endurance events is not yet clear but Speedy reported in 1999 that 58 out 330 (18%) ultra-marathoners that completed the race, suffered at some level of hyponatremia. Hsieh and associates at the University of Pittsburgh reported 5.6% of runners requiring medical tent attention in the 2002 Pittsburgh marathon where diagnosed with hyponatremia. Dr. Timothy Noakes at the University of South Africa reported in 1984 that 29% of medical tent attendees suffered from low sodium in the blood.
Hyponatremia is a fluid / electrolyte imbalance that results in abnormally low blood sodium concentrations. A consistent reduction in blood sodium concentration reflects a disruption in osmotic pressure balance across the blood-brain barrier, resulting in a rapid influx of water into the brain. This causes brain swelling, headaches, and neurological responses like confusion, seizures, and even coma.
In prolonged severe cases, death is the result. It should be noted that although sodium deficiency is the identified culprit, a sodium deficiency is usually be associated with a deficiency of other electrolytes. Hyponatremia is a combination of too much water and not enough sodium.
Symptoms of Hyponatremia
- Weight gain
- Bloating and/or swelling
- Seizures (severe cases)
- Coma (severe cases)
Those at Risk
It appears that in some cases where advice and practice has lead to “consumption of as much water as tolerable” that the attempt to prevent dehydration has been taken too far.
Those at risk tend to be:
- Slower runners who take their time at drink stations to ensure prevention of dehydration and actually finish a race heavier than when they started.
- Longer duration events exceeding 3 hours and a significant rise in prevalence occurs when events exceed 8 hours.
- Those on NSAIDS (non steroid anti-inflammatory drugs) medications including “Aleve and Advil”.
- The more you sweat the more sodium (and other electrolytes) you lose. Sweat loss can vary from one pint (16 oz or 480 ml) to over 3 quarts (96 oz or 2.9 litres) during one hour of intense cycling or running, particularly in a hot environment.
- Cyclists are more prone to over-hydrating simply because the mechanics of taking a drink during weight supported exercise is easier than while running.
Guidelines to Responsible Hydration
Guidelines for responsible hydration have been published by:
- The American Dietetic Association,
- Dieticians of Canada,
- The American College of Sports Medicine and
- The National Athletic Training Association.
These guidelines can best be summarized as follows:
- Pre Exercise (1-2 hours prior to an event lasting more that 2 hours) – drink 14 - 20 Ounces (400 to 600ml) of fluid (some should be a high quality sports drink).
- During exercise drink 6 – 12 ounces (180-300ml or a cup to a cup and a half) every 15 to 20 minutes. At least half of this fluid should be a high quality sport drink to ensure adequate sodium and other electrolyte replacement. For events lasting more than 3 hours it is also important to have the sport drink include a source of energy to prevent glycogen depletion and consequently “bonking” before the race is completed.
You need to experiment so you match your body size and rate of sweat to your drinking program. Try weighing yourself before and after a long workout. If lose more than 2 pounds you have under-hydrated. If you gain weight you are over hydrating.
L. Lee Coyne