Over-Hydration - Too Much of a Good Thing?

As published in the March 2007 issue of UltraRunning Magazine. 

For many years, the focus for hydration in endurance running has been on drinking enough because of to become dehydrated in the heat because common sweat rates of 1 to 3 liters/hour are greater than the rate at which water can be replaced by drinking. Also, the stress in endurance running can lead to a situation where runners are not well aware of their fluid needs. It is not unusual for a runner to neglect drinking late in a run.

Serious performance problems are found with increasing dehydration because of reduction in blood plasma volume. Blood flow to the skin is reduced, and that reduces cooling, leading to higher body temperature. Blood flow to the stomach is reduced, leading to poor digestion and poor transport of nutrients. Reduced blood flow also reduces cardiac output, and mental function. These effects become noticeable at 2% or greater dehydration ( 3 pounds for a 150 pound body ).

Thus the old adage: drink before thirst. It has been pounded into us for a long time, so it’s not surprising that runners can over-do their hydration.

Little has been made of this problem in the past, but now we know that over-hydration can cause very serious problems. Witness the woman in California who died after a contest to see who could drink the most water without urination, and the death of a female runner at the 2002 Boston marathon due to over-hydration.


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’s 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.

Over-hydration is more likely in the back-of-the-pack runner who is moving slower ( sweating less ) and has time to drink to excess. Time multiplies all consistent errors in hydration. Poor hydration practice may be no big deal in a 10K, a moderate problem in a 50K, but could be devastating at 50 miles or more. Significant, un-corrected hydration errors in a 100 mile run will usually result in a DNF.


What can you expect to see with over-hydration? There are many diagnostic signs that a physician would look for, but most runners aren’t 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.

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.

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.


A runner’s first goal in this balance is to keep weight in the right range. Weight gain is water gain ( the weight of food and electrolytes are small ), so if your weight is up a lot, drink only to keep your mouth wet until the weight returns to the expected range. Similarly, weight loss is a sign of dehydration. If you don’t have a scale, look at your skin: is it puffy ( too much water ) or dry and loose ( too little water )?

Get to know your scale during training. Weigh yourself so you gain an understanding of your hydration needs. Don’t worry about being up or down a pound or two. If more than that, consider why, and adjust your hydration practice. You can calculate your average sweat rate for a run, and your percent dehydration after a run. Just weigh yourself before and after the run, and keep track of how many ounces of fluid you drank during the run, and the length of time you ran. Weights are assumed to be in pounds, and fluid drunk is in ounces.

Sweat rate ( Liters/hour ) = ( weight before -weight after)*0.4535 + 0.03* ounces drunk / Hours run

Percent dehydration = 100%* ( weight before -weight after )/weight before

Example: Weight before = 160 pounds, weight after = 155 pounds, drank 80 ounces in 4 hours of running.

Sweat rate = ( 160 -155 )*0.4535 + 0.03*80 / 4 = 4.6675/4 = 1.17 liters/hour

Percent dehydration = 100* ( 160 -155 )/160 = 3.125 percent

The runner’s sweat rate is not unusual, but the percent dehydration is a little high.

How much should you drink? That depends a lot on personal characteristics and the environmental conditions. Running generates heat, and sweat carries away excess heat, so a slow, cold run will require a low drinking rate, while a fast run in desert heat will require a much higher drinking rate. If conditions are not extreme, a drinking rate of 12 to 24 ounces per hour should be sufficient.


The electrolyte sodium is very important in hydration and electrolyte status. The Latin word for sodium is natrium, thus the term hyponatremia for the condition where the sodium concentration in body fluids is dangerously low.

The sodium concentration in extra-cellular fluid is the ratio of weight of sodium ions to weight of water. So, if I have too much water, I can just add more sodium, right? Well, it depends on where you are with respect to normal sodium content in the body. If you have too little sodium, then adding more will help you return to normal. An example many runners have experienced would be low sodium with adequate water, leading to puffiness in the hands and wrists. Taking in more sodium will correct the situation and the puffiness will go down.

But if you have the right amount of sodium, adding a lot more is not good. Excess sodium can increase thirst and prompt more drinking, which is bad if you already have too much water on board ( excess weight ).

Thus, the safest course is to drink to maintain body weight ( or be a little down ), and take sodium supplementation conservatively. A deficiency of water or sodium can be corrected within minutes, but correcting excesses of either one can take hours.


In a nutshell, watch your weight, and consult your stomach and your hands/wrists. If your weight is up, cut your drinking a lot until the weight comes down. Don’t worry about dehydration if your weight is up -you can’t be dehydrated under those conditions. In general, correct your water content first, and then see if you need more or less sodium.

The stress of endurance running can lead to a buildup of Anti Diuretic Hormone that will prevent urination. Walking or just sitting may relieve some of the stress, allowing ADH to fall. When that happens, urination will remove excess water. ADH is affected by stress, high serum concentration, or low plasma volume. While it is an important variable, it is not observable by the runner during a run, and primarily of academic or medical interest.

If your stomach is not happy, or wrists and hands are getting puffy, increase your sodium intake. If that does not relieve the puffiness or stomach problems, then cut back on the fluid intake.

Also, if your weight is up substantially, a one-time dose of electrolytes, along with sitting or walking may prompt urination that will dump excess water.

If your weight is down, take water ( or sports drink with electrolytes ) and sodium ( salt or electrolyte caps ) until your weight returns to the expected range.

Beware of extreme conditions, especially when traveling to distant places where the weather conditions are not what you are used to: altitude, very hot, very cold, very dry, or very humid conditions can require different drinking patterns than what you have used in your normal training. Dry conditions can easily fool one into dehydration as the runner thinks that sweat rate is low simply because the sweat is evaporating too fast to be seen.

What if you are on a course where water is scarce? It might be better to drink more than the optimum amount in case you can’t find water later. If the course has many water stops, there’s no need to drink any surplus as you can correct a deficiency a little further down the course.

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