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Over-hydration: too much water hurts

Drinking too much—water?

You hear about the importance of staying hydrated, especially in hot or dry conditions, and especially when you're exerting yourself. However, there IS such a thing as drinking too much—you put yourself at risk for a condition called hyponatremia.


It’s important to get enough water, especially when it’s hot. However, too much water can lead to a dangerous condition known as hyponatremia in which the sodium levels in your blood drop too low. It’s often caused by drinking too much water and is common among military personnel, athletes, and hikers. Significant weight gain (due to fluid retention) during exercise can occur, along with longer finish times for endurance activities. If you have a Body Mass Index (BMI) below 20, you are more likely to develop this condition. For more in-depth information, read HPRC’s InfoReveal.


You probably know you need to hydrate before, during, and after exercise, particularly when it’s hot. What you may not know is that drinking too much can do more harm than good, especially during physical exertion in the heat; it can lead to a condition known as “exertional hyponatremia.” This is a serious condition that can lead to swelling of the lungs and brain and can be fatal. It also can result from sweating so much that causes blood sodium levels drop or simply from not enough intake of sodium in your diet.

In 1997, Army training centers reported several cases of serious hyponatremia due to excessive water consumption during training in hot weather. Significantly, the cases occurred early in a training period and most commonly between June and October.  In 2003, the U.S. Army Research Institute of Environmental Medicine (USARIEM) revised its fluid replacement guidelines for military training in heat.

What We Know

While various factors can bring about hyponatremia, consuming too much fluid seems to be the most common. The U.S. Army Research Institute of Environmental Medicine (USARIEM) guidelines state that you should limit fluids to no more than 1.5 quarts an hour and 12 quarts a day, regardless of how hot it is or how hard you are working. Of course, your minimum fluid needs will vary depending on heat, work intensity, and your individual sweat rate. You can read more about staying hydrated here. Consuming sports drinks rather than plain water may also improve your electrolyte balance (which includes sodium) and delay or prevent the onset of hyponatremia. For more information, see HPRC’s monograph on sports drinks.

Women are at a higher risk than men for developing this condition. According to 12 years of data (1999-2011) from the U.S. Army, the rate of exertional hyponatremia is about 50% higher in women than in men. Since the number of women in the military has increased over the last decade, this could impact overall incident rates. The highest rates for both men and women were among those in the over-39 age group and the under-20 age group. Women with hyponatremia are also at higher risk for brain swelling and increased pressure inside the skull. Hyponatremia can be prevented if precautions are taken.


Symptoms of exertional hyponatremia are similar to those of heat illness and may include chills, vomiting, dizziness, and decreased coordination. Additional reported symptoms are disorientation and altered mental status, fatigue, and headache. Exertional hyponatremia can lead to swelling in the hands and feet, lungs, and brain, as well as seizures. Untreated it can be fatal.

Some studies have found a possible relationship between the use of medications called NSAIDs (non-steroidal anti-inflammatory drugs, which include ibuprofen) and the development of exertional hyponatremia. However, a definite link remains unproven and additional research is needed.

Besides exposure to hot weather conditions, additional risk factors for developing hyponatremia include a low sodium diet, medical conditions that affect water regulation in your body such as kidney disease or heart failure, and participation in intense physical activity such as triathlons and marathons.

Debrief (Military Relevance)

Exertional hyponatremia is a potentially deadly condition that can occur from drinking too much water; it occurs when blood sodium levels are too low. According to an updated report by the Armed Services Health Surveillance Center, in 2011 there were more cases of exertional hyponatremia than in any other year of their observation period (1999-2011)—in fact, three times as many cases in 2011 as in 1999.

Commanders, supervisors, and medical support staff should be familiar with the symptoms of hyponatremia, including dizziness, disorientation, fatigue, chills, and headache in order to be on the lookout for this condition. Enforcement of fluid consumption guidelines and work-rest cycles, as outlined in the 2003 USARIEM guidelines (TB-Med 507) is essential.