For the longest time, athletes and other exercise enthusiasts have been encourage to take in enough water to compensate for the water loss as sweat. Nobody would have ever expected that there can a problem with taking too much water.
Hyponatremia has gained media attention in the last few years, but it is important for athletes to realize that dehydration is much more common and 'overhydration' is a risk mostly associated with ultra-endurance sports and not sports events lasting less than 2 hours such as hockey, basketball and soccer, or shorter hikes/runs. Hyponatremia, a low concentration of sodium in the blood, has become more prevalent in athletes as more people are participating in endurance sports lasting more than 3 hours such as marathons. Such prolonged activity and excessive sweat production increases the risk of an athlete having too little sodium in their blood stream during training and competition. The hyponatremia or 'overhydration' associated with prolonged exercise arises primarily from fluid overload, under replacement of sodium losses, or both. When blood sodium concentration falls to abnormally low levels, a rapid and dangerous swelling of the brain occurs, that can result in seizures, coma, and death. Sourced from: https://sportmedbc.com/article/hyponatremia-or-overhydration%E2%80%94who-risk
The intake of too much water leads to a condition called hyponatremia/ overhydration that results from the low concentration of sodium ions in the body. The problem begins with having long periods of exercise that are characterized by large amounts of sweat and electrolyte loss.
Although rare, overhydration can occur during long bouts of exercise when electrolytes lost through sweat are not replaced, yet excessive amounts of water are consumed. Overhydration can lead to potentially dangerous imbalances of electrolytes, including hyponatremia, a serious condition in which the sodium level in the blood becomes too low. Hyponatremia can be a problem for athletes who experience excessive sodium loss through perspiration as part of prolonged exercise or heat exposure, such as running a marathon. The World Health Organization (WHO) has documented this condition in mountain climbers who have used melted snow to prepare their beverages, without supplementation with the necessary ions or electrolytes. Sourced from: https://www.beverageinstitute.org/article/hydration-for-athletes-special-considerations/
Too much sweating causes excessive sodium loss especially in athletes due to prolonged heat exposure and exercise. The working of the body comes down to the basic unit called a cell, which is surrounded by extracellular fluids made up of electrolytes.
Cells in the body are surrounded by extra-cellular fluid, which contains significant amounts of the electrolyte sodium, dissolved in the water. If that extra-cellular fluid becomes diluted with excess water, the water will migrate into the cells to keep the osmotic forces inside the cells balanced with those outside the cell. Otherwise, the cell walls could rupture and the cell would die. That ingress of water can cause tissues to swell and become puffy. The common example of that is seen in the hands, wrists and feet. While that is annoying, the real problem is that the swelling also happens to the brain (encephalopathy). That leads to a number of problems, which can result first in poor performance, but also could lead to DNF and possibly death if not addressed. Sourced from: http://www.succeedscaps.com/articles/over_hydration/ The extracellular fluid around cells has to be more concentrated that the fluid inside the cell to prevent intake of excess water in to cells. The following is a more comprehensive list of signs and symptoms of overhydration;
1.What can you expect to see with over-hydration? There are many diagnostic signs that a physician would look for, but most runners are not physicians and can only go by what they can easily recognize. Digestion is impaired. With the excess water comes stomach sloshing, poor absorption of food (because you need an adequate sodium concentration for absorption), and vomiting. Salty foods taste unusually good if sodium is simultaneously low. Thirst is low.
2.Neurological signs appear: dizziness, confusion, irritability, and possibly headache. If neurological signs appear, the athlete is heading toward a medical emergency and steps need to be taken immediately to prevent serious injury or death. Unfortunately, the mental confusion may impair judgment, and corrective actions may not be taken. If you are not feeling right late in an ultra, it is important to ask at an aid station if medical help is available. Medical people may be able to spot a developing problem and help before things get really serious.
3.Physical signs can be: weight is up 3 or more pounds, hands and wrists get tight and puffy, urination may be absent early in a run but appear later with a high volume of crystal clear urine, and there may be shivering in temperatures that would otherwise be warm enough for no shivering. If it is actually cold, and other signs of over-hydration are not present, shivering may just be due to poor thermoregulation.
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Sourced from: http://www.succeedscaps.com/articles/over_hydration/