Are Female Athletes More Susceptible To Hyponatremia?

Female endurance athletes may be more susceptible to complications from low-sodium levels, but all athletes can drink too much water

“The last thing I remember is feeling slightly dizzy in the final mile...The next thing I remember is waking up from a coma four days later in the intensive care unit at Duke University Medical Center, with no idea how I got there,” writes Carol Krucoff in a 2005 article in the LA Times.

She reports that she and her friend were walking a 26.2-mile marathon course when she collapsed at the finish line. On contemplating the aftermath (which included multiple seizures and being hooked up to a ventilator), she remarked that all this ensued “because I drank too much water.” Just three years either, two healthy, young female runners died of hyponatremia while participating in marathons.

These incidents underscore that exercise-associated hyponatremia (EAH), defined as a low blood sodium concentration, isn’t a rare condition limited to competitive, ultra-endurance marathoners or triathletes.

While it remains true that ultra-endurance athletes competing in hot and humid conditions are the most likely to be hyponatremic, there are cases of low sodium in a wide range of activities. From football players engaged in regular practice to a policeman engaging in a 19 km bike to a yoga class participant, hyponatremia is becoming a greater concern.

Not only do we have a better understanding of the range of conditions that can induce hyponatremia, but we’re also learning about who is most susceptible. Interestingly, it seems to affect women disproportionately to men.

Female athletes appear to be at greater risk of hyponatremia

Statistically, it appears that female athletes are more susceptible to low sodium levels (usually defined as [Na⁺] below135 mmol/L).

• In a report on hyponatremia following the San Diego Marathon, 23 of the 26 cases of hyponatremia were females. (Mitchel et al.)

• An analysis of hikers suffering from heat exhaustion after hiking the Grand Canyon found that out of the 7 with hyponatremia, 6 were women. (Backer et al.)

• Hyponatremia was three times more common in women than men in the 1997 New Zealand Ironman triathlon. (Mitchel et al.) (Speedy et al.)

• A 2000 study reported that five of seven marathon runners who collapsed after a marathon were hyponatremic females. (Ayus et al.)

• Researchers studying hyponatremia in the Boston marathon concluded that “Female runners remain a readily identifiable risk group.” (Almond et al.)

To be clear, anyone can become hyponatremic, especially those who are undertrained, not acclimated to the heat, or misinformed about hydration (the conventional wisdom that you can’t drink too much water while exercising is, obviously, incorrect).

However, being female does appear to play a role in susceptibility. Female endurance athletes who lessened their training load during the winter could be more likely to become hyponatremic as they increase intensity and duration of exercise in the spring.

Why are female athletes experiencing low sodium levels?

There are a few hypotheses that might explain why women experience hyponatremia at greater levels than men.

Estrogen inhibits the movement of potassium

Hyponatremia causes swelling, and the body’s response is to transport potassium out of the cell to help maintain consistent pressure inside and outside the cell.

However, if the sodium-potassium ATPase enzyme is inhibited by estrogen, the clinical outcome of hyponatremia may be more grave. According to Ayus et al., young women with relatively high levels of estrogen are 25 times more likely to die or have permanent brain damage as a result of hyponatremia, as opposed to men or postmenopausal women. (Adrogué et al.) (Ayus et al.)

Women have less fluid in their bodies than men

Researchers hypothesize that salty sweaters, or those who lose more sodium while exercising, are at greater risk of hyponatremia. The lower the sodium levels, the less water it takes to induce hyponatremia. (Interestingly, these researchers found that high sweat sodium losses can result in hyponatremia during prolonged exercise, even in the absence of overdrinking.)

Following that logic and their calculations, smaller athletes are at greater risk of hyponatremia because they have less extracellular fluid (body fluid) to dilute.

Actually, a male and female with the same body mass will have different amounts of extracellular fluid. For example, a 160 lb male contains about 42 liters of water, or 60% his body weight. A 160 lb female contains only 55% of her body weight in water. This can largely be explained by women’s higher percentage of adipose tissue.

Perhaps women are more likely to become hyponatremic because it takes a smaller change in sodium to create a difference in osmotic pressure. (Montain et al.) (Brandis)

Women are getting bad hydration advice

Many athletes, and even some coaches and trainers, believe that you can’t drink too much water. Perhaps because of estrogen’s effect on potassium paired with a lower extracellular fluid level, that bad advice can be upgraded to dangerous advice for female athletes. It’s also possible that coaches, trainers, and friends are basing their advice on hydration requirements for standard males rather than standard females.

When adjusted for variables, female athletes might not be more susceptible than male athletes

A large study of 488 runners at the Boston Marathon found that sex was not a factor of hyponatremia when adjusted for variables. In this study, 13% of runners finished with a serum sodium concentration of 135 mmol per liter or less.

When researchers considered one variable at a time (univariate), they listed the following as factors leading to hyponatremia:

• weight gain during the race
• consumption of more than 3 liters of fluids during the race
• consumption of fluids every mile
• a racing time of >4:00 hours
• low body-mass index
• female sex

Researchers noted that the female runners were on average younger than male runners, had a lower prerace weight, a lower body-mass index, a slower training pace, less marathon experience, and longer racing time. When adjusted for those variables, sex was not a factor.

Instead, hyponatremia was associated with the following:

• weight gain (suggesting the athlete consumed more water than they lost)
• longer racing time
• a body-mass index of less than 20. (Almond, et al.)

Clearly, more research is needed to further explore hyponatremia in relation to female athletes. However, what is clear is that small athletes competing in long endurance events need to be aware of how much water they consume.

How can I hydrate optimally?

Dehydration impairs performance and can cause illness. That’s well established in the research. Certainly, the way to prevent hyponatremia isn’t to avoid fluids during exercise.

It’s also well-known that there is a wide variance in sweat between athletes. Some sweat a lot while others, not so much. Some sweat a lot of sodium, while others retain it. That means that there is no generic, hydration recipe for all athletes.

But there are a few things to keep in mind when considering your hydration strategy.

• Be skeptical of any “rules of thumb” advice

If someone tells you to drink “a bottle an hour” or even “as much as possible during exercise,” be skeptical. Especially if drinking that amount makes you feel sick or uncomfortable. The more we understand about hydration needs, the clear it becomes that there is no one-size-fits-all approach.

• Record your weight before and after exercise

One thing researchers agree is that it is dangerous to gain (water) weight during exercise. Try weighing yourself before an event and then immediately after. Ideally, you’ll be the same weight or slightly less.

• Consider drinking to thirst

If you are at risk of hyponatremia, you might consider only drinking to thirst. While it’s been shown that this method won’t produce optimal hydration, it’s also a clear way to prevent hyponatremia. If you have a history of hyponatremia, are a less experienced athlete, or often gain weight during exercise, you might decide that you’d rather accept small losses in performance from mild dehydration rather than risk hyponatremia.

• Supplement sodium

The best way to prevent hyponatremia is to not over-hydrate. However, sodium supplementation can help. Multiple studies have shown that sodium supplementation can significantly attenuate sodium levels during exercise. (Vrijens, et al.) (Anastasiou, et al.) Twerenbold, et al.)

You can safely supplement sodium and other electrolytes with ElectroElite. On hot or long days of exercise, add 1-2 tabs to your hydration bottle and continue supplementing as needed. Ingest sodium before and after exercise.

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References:

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