By Roberta H. Anding, M.S., Baylor College of Medicine and Texas Children’s Hospital

Edited by Kate Findley, The Great Courses Daily

In our quest to stay hydrated—particularly while working out—we can sometimes go overboard. Professor Anding explains what happens when you over-hydrate and how to avoid it.

What Is Overhydration?

Although dehydration is a serious health issue, overhydration can also cause issues. Overhydration means that you drink more water than you could possibly need. You drink so much fluid that you’re actually diluting the amount of sodium in your blood. 

Sodium and potassium are essential electrolytes, and your body works hard to keep them in balance. Water has very low sodium, and when you drink too much water, you dilute the sodium level in your blood to the point where you can have cerebral edema, or swelling of the brain, which leads to death.

There have been instances on college campuses where fraternities could not haze with alcohol, so they hazed their new inductees with water instead. Some fraternities have been featured in the news for injuries that occurred while promoting this water hazing, at least one of which has resulted in death.

Overhydration and Exercise

Overhydration is also a danger for people who are physically active, particularly for new marathon runners. They’ve heard from trainers and nutritionists that dehydration impacts their performance. 

Rather than measuring precisely how much fluid they need by calculating their sweat rate or checking their urine color, they may decide to simply drink based on how thirsty they are. They may even drink far beyond satisfying their thirst, becoming overzealous in their attempt to avoid dehydration.

For example, suppose that prior to running a marathon you weigh 120 pounds, and when you finish the marathon, you’ve gained two pounds. This means you drank more fluid than you lost in sweat, and you’re more likely to become hyponatremic, which is the medical term for inadequate amounts of sodium in the blood. 

Differentiating from Dehydration

If a runner finishes the race a little confused and lightheaded, initially the medical team may think the runner is dehydrated, rather than overhydrated, since these are also signs of dehydration. 

“In the Houston marathon, we weigh people at the beginning of the marathon, and we weigh them again at the end, and record their weight in the timing chip that goes in their shoe,” Professor Anding said. “If you’re not familiar, that’s one of the ways that they pace during a marathon, is they have a timing chip that they can tie into their shoe.”

The reason they record the runners’ starting weight is because if a runner is confused after the race, either from dehydration or overhydration, they might not be able to remember their normal weight. Anding and her team compare the starting weight to the ending weight in order to make sure that they’re giving the runners the appropriate treatment.

Someone who’s just run a marathon should have lost a little bit of weight. They just ran 26.2 miles. For every mile you run, you burn 100 calories, so after a marathon, you’ve burned about 2,600 calories. 

Thus, weighing someone before and after a marathon—or any intense workout—is an effective way to monitor levels of dehydration and overhydration. An individual who dies during or after a marathon is more likely to die from dehydration, but overhydration can be an issue as well. It is a medical emergency when someone is hyponatremic, because, again, the brain is going to swell.

If you engage in lots of physical activity and want to stay adequately hydrated while avoiding the extremes of over or under-hydration, then, it is crucial to use precise methods. Do not just rely on instinct or general guidelines such as drinking eight glasses of water a day.

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.