by Lance Tarr, PT, DPT, MSCH, CSCS
Regardless of whether one is an athlete or not, we are barraged with the mantra "Drink plenty of water." Any endeavor requiring long duration and high intensity will suffer if dehydration becomes excessive. However, it is possible to drink too much water and arrive at a life threatening state called hyponatremia. Before this aspect is discussed, reviewing hydration and the body's fluid balance will aid in understanding this topic.
Our bodies are made up mostly of water. Approximately 60+% of the human body is water. To put that in perspective, a 154 lb. individual is made up of about 94 lbs. of water. To accentuate the importance of proper hydration, realize that just a 1-2% loss of water weight during activity results in about a 6% decrease in performance.
We lose about one pound of body weight per every 15 minutes of vigorous training. At about a two pound weight loss during activity, heart rate increases eight beats/minute, cardiac output decreases one liter/minute, and core temperature rises about three tenths of a degree centigrade. Core temperature can rise to 105 degrees Fahrenheit within 15 minutes--much over that and dire consequences begin. The primary physiological barrier to an excellent performance by a healthy, well-trained, well-nourished athlete is dehydration. Not only does it rob performance potential, it is also a precursor to heat cramps, heat exhaustion, and heat stroke.
Indeed, a study by Sawka and Pandolf in 1990 showed that runners with a 2% body weight loss from activity ran at a race speed of 240 meters/minute vs. 260 m/minute when well hydrated.
Therefore, in order to stay hydrated, we need to drink as much as we lose. We lose about 500-1000ml per hour (500ml is about 16 oz.) in strenuous conditions. To stave off significant dehydration, we need to drink about four to eight oz. of fluid every 15 minutes during an event provided we are not already dehydrated prior to the start.
In order to pre-hydrate, the following guidelines are sensible:
1. Drink 16 oz. of fluid before going to bed the night before the event and another 16 oz. when you get up.
2. Drink 16-32 oz. one hour before the event, and about 8-16 oz. 20 minutes pre-event.
Electrolyte drinks are a good idea as they add carbohydrate and sodium chloride which aid in maintaining thirst and decreasing urine output. With the mention of "electrolyte drinks," we can now move on to hyponatremia.
Believe it or not, one can drink too much water. Simply put, if one drinks too much water, it is possible to dilute the electrolyte balance maintained in our blood volume. This can disrupt the osmotic balance between the blood-brain barrier and cause a rapid influx of H2O into the brain. This influx causes the brain to swell leading to nausea, disorientation, vomiting, and possibly seizures, coma, and death. These symptoms are also similar to heat-related illness, so treatment is dependent upon differential diagnosis.
Though it is possible for anyone tojust sit and drink too much water, it's very unlikely. The scenario becomes more common when an athlete is sweating buckets and sodium loss is high. Those who participate for significantly longer durations (four hours plus) are also more likely to deplete sodium and "over-drink." Plain water does not contain sodium; however, electrolyte drinks do not replace the total amount of sodium lost during the event either.
Physiologically, there are actually three different mechanisms which can account for hyponatremia during exercise. In all likelihood, combinations of these mechanisms probably occur.
1. Simplistically put, water and sodium are lost in sweat during exercise and are replaced unequally where replacement of the former is far greater than the latter.
2. Another potential mechanism is the body's inappropriate release of ADH (anti-diuretic hormone also known as vasopressin). Normally, if sodium levels drop even marginally, ADH secretion also drops, causing one to urinate more. Consequently, sodium concentration increases. However, if both blood volume and sodium levels drop due to prolonged/profuse sweating, confusion occurs. Low sodium would cause the body to decrease ADH output, but the low blood volume would make it want to increase. Although blood volume and sodium levels are both critical, blood pressure takes a priority here, so ADH release continues. If ADH is released in excess, then a syndrome known as SIADH (Syndrome of Inappropriate ADH release) occurs with hyponatremia resulting. Exercise can be a trigger for this syndrome in some individuals, even exercise of a relatively short duration.
3. Finally, a condition known as "third space effect" may result in which the excessive ingestion of fluids, particularly those with high carbohydrate concentration (>10%), cause sodium in the blood volume to be redistributed to the intestinal tract causing hyponatremia.
Hyponatremia is far rarer than dehydration and is not a likely compromise to suffer if one is well trained, well hydrated, and cognizant of their body's physiology. Prevention of both dehydration and/or hyponatremia is really common sense. If we drink when we are thirsty while keeping in mind the loss/replacement volumes alluded to earlier, utilize an electrolyte replacement drink, and acclimate to environmental/physiological stresses, then dehydration and hyponatremia will be kept in check.
About the Author: A 2nd year VIP member, Dr. Tarr has participated in endurance sport for over 30 years. He is a licensed physical therapist, an exercise physiologist, and is a NSCA certified strength and conditioning specialist. He can be reached at: firstname.lastname@example.org.