Hyponatremia – Can You Drink Too Much?

By Rick Morris

Can you drink too much? There’s a playground not far from the track at which I coach a weekly speed session. The most popular piece of playground equipment there is my personal childhood favorite – the swing set. It seems like there is always a large group of kids playing on the swings. I never fail to be fascinated by the physics of the swings. The kids are thrilled by the pendulum motion of the swing set. They soar high through the arc until the relentless force of gravity overcomes the momentum they have built up. Then their downward motion builds up its own momentum and pulls them through the arc in the other direction before starting the cycle over again.

Historically, hydration in runners has worked much in the same way. When I first became involved in running and athletics, the conventional hydration wisdom was to avoid drinking when exercising. Coaches in those days believed that you would be more likely to suffer from muscle cramps and your performance would decline if you drank during your training or event. That represented one extreme end of the hydration pendulum. Later, as the dangers of dehydration, heat exhaustion and heat stroke become better known, the recommendations changed. In order to avoid heat illness and the performance declines associated with dehydration, we were told to drink early and often. It was thought that your hydration levels lagged behind your feelings of thirst. The new recommended hydration guidelines were to drink before you were thirsty. It was thought that there was no such thing as drinking too much fluid.

The new recommendations did their job. The incidence of dehydration and other heat illnesses plummeted. But, did the pendulum swing too far? It appears that it did. Hydration recommendations traveled from the extreme of under hydration to the opposite extreme of over hydration. The momentum gained in the effort to eradicate dehydration pushed the recommendations too far in the other direction. Now, instead of drinking too little, we are drinking too much.

Can we drink too much fluid? In recent years we have been told how important it is too stay fully hydrated. It has been suggested that we should drink even if we are not thirsty. Experts have said that we should drink copious amounts of fluids in the days leading up to a marathon and take every opportunity to drink during the marathon. As a result we are now seeing a problem that is relatively new in the world of running. The new problem is on the opposing side of the existing dehydration issue. It is hyponatremia.

Hyponatremia is not a new disorder. It has been a known medical issue for many, many years, but it is fairly new to athletics. Hyponatremia is an imbalance in the fluid-electrolyte ratio in your body. More simply put it means that your blood sodium concentration falls below normal levels. Because athletes in the past tended to drink less fluid, they were usually in a dehydrated or normal hydration state. That kept their blood sodium concentration levels either normal or a bit on the high side. The recent recommendations of high fluid intake caused some athletes to be hyper hydrated. This caused the sodium concentrations in their blood to become diluted, which can cause hyponatremia.

Normal blood sodium levels are between 136-142 mmol/liter. If your level drops below 135 mmol/liter you are considered to be in a state of hyponatremia. Between 125-135 mmol/liter there is usually either no or mild symptoms such as: nausea, bloating or gastrointestinal upsets. When levels drop below 125 mmol/liter symptoms can become more severe and serious with a throbbing headache, swollen extremities, wheezing, confusion and extreme fatigue. Sodium levels below 120 are extremely serious and can result in seizure, coma and death.

Runners and other athletes are most often affected by a form of hyponatremia known as “dilutional hyponatremia”. This type is caused by more water than normal for the amount of sodium and other minerals in the blood plasma. There are other types of hyponatremia including “isotonic” (normal water to mineral ration) and “hypertonic” (less water to mineral ratio), but these are far less common among athletes.

Who Is at Risk

All runners are at risk of developing hyponatremia but the development of this disorder has been more strongly associated with certain risk factors. A study published in the New England Journal of Medicine[1] recruited the services of 488 runners in the 2002 Boston Marathon.  Thirteen percent of the participants finished the race in a state of hyponatremia. Slightly less than 1 percent were suffering from critical hyponatremia (less than 120 mmol/liter). The researchers found that the subjects who were hyponatremic tended to have the following characteristics.

  • Weight gain – Runners who over drink during the race will gain water weight. This dilutes the blood plasma and results in hyponatremia. In contrast, runners who do not over hydrate will lose weight over the course of the race keeping the blood sodium ratios stable. Their sodium balance remained stable despite weight loss because they lost sodium through sweating.
  • Drinking more than 3 liters of fluids during the race – Consuming large amounts of fluids will lead to weight gain and blood plasma dilution.
  • Consumption of fluids each mile – Most major marathons have aid stations at every mile marker. This practice encourages runners to drink at each mile which can lead to over hydration.
  • Finishing time of over 4 hours – Since slower runners spend more time on the course, they have more opportunities to drink. Again, this can result in over hydration.
  • Small body size – Smaller bodies require less fluid to dilute the blood plasma. That makes it much easier to over hydrate.
  • Female runner – Hyponatremia can strike anyone but historically females have been shown to be 3 times more likely than males to suffer from this disorder. This is most likely partly because of smaller body size and also because of learned behavior.

Avoiding Hyponatremia

It is a well established fact that dehydration presents both a health hazard and threat to your running performance. You must drink to replace lost fluids. But, how much is enough? The previous recommendations suggested that one good strategy was to drink enough fluid to replace your lost body weight. For example, if you weighed 150 lbs at the start of a marathon and weighed 145 lbs at the end of the race, you should have consumed an additional 5 lbs of fluids to maintain proper hydration.

In theory, that practice seems sound. But recent data[2] suggest that may not be the case. Data indicates that runners who lose less than 2% of their bodyweight during a long distance race experience a decrease in serum sodium. Athletes that gain weight are usually in state of hyponatremia. Runners that lose between 2% and 4% of their bodyweight tend to keep their sodium levels in balance and avoid hyponatremia. Those that lose more than 4% of their bodyweight actually increase their levels of serum sodium.

Studies conducted in a laboratory confirmed the accumulated data. The studies showed that replacing 100% of the bodyweight lost during a race resulted in decreased serum sodium levels and put the athletes at risk of developing hyponatremia.

So, what should you do to avoid hyponatremia and dehydration at the same time? The most important thing to keep in mind is no two runners are exactly alike. Each athlete sweats at different rates. Every runners produces sweat with different levels of “saltiness”. Each athlete runs at different speeds and has a unique level of fitness. “One size fits all” does not apply when dealing with the fluid recommendations for runners. Here are some basic recommendations for your fluid replacement.

  • Drink according to your thirst level. Thirst is your body’s method of telling you it needs fluid. Drink when you are thirsty and drink only enough to quench your thirst. Do not avoid fluids early in your race. If you become dehydrated your performance level will drop and you will be at risk of developing heat illness. Do not ignore physical thirst.
  • Use generalized fluid recommendations as a baseline, but remember that you must adjust those recommendations according to your individual needs. The current recommendation from The American College of Sports Medicine (ACSM) is to drink 600 – 1200 ml per hour. The International Marathon Medical Directors Association (IMMDA) lowered the recommendations to 400 – 800 ml/hour to protect smaller and/or female runners.
  • If you are female, have a small frame or are a slower runner, use the low end of the recommendations as a baseline.
  • Start well, but not over hydrated. Drink 400 – 700 ml of fluid 2 to 3 hours before your race or exercise. Remember that hyperhydration provides no performance benefit so don’t over drink.
  • Do not rely on plain water. You need to replace sodium and other electrolytes lost through sweat. Drink a sports drink containing sodium. You also need the performance boost of carbohydrates contained in sports drinks.
  • Take advantage of aid stations with available food and drink to replace lost sodium and carbohydrates. Let your individual needs and tolerances dictate how much to consume.
  • If you are exercising in a high heat environment you will need to drink ahead of thirst to avoid dehydration.

[1] Hyponatremia Among Runners in the Boston Marathon, N Engl J Med 2005:352:1500-6

[2] Updated Fluid Recommendations: Position Statement From the International Marathon Medical Directors Association, Tamara Hew-Butler, DPM., Joseph G. Verbalis, MD, Timothy D. Noakes, MBChB, MD, DSc, Clin J Sport Med 2006;16:283-292