Emily asks: I just ran my second marathon. I felt good up until about 32 kilometres when I suddenly became nauseous. After that, I had to walk intermittently the remainder of the race. My fingers were swollen and my stomach was sloshing. I had taken water at each aid station because it was hot so I don’t think it was dehydration. Fortunately, I still made my time goal, but I’m not sure what happened.

It sounds to me like you may have been experiencing overhydration rather than dehydration. As Runner’s World Ask the Sports Doc columnist William Roberts, explains, a combination of high water intake and low renal output (urinating) can cause abnormally low blood-sodium levels, leading to a dangerous condition known as hyponatremia.

Sodium is an electrolyte that plays a key role in regulating many important functions in the body, like regulating the body’s fluid balance and supporting the central nervous system.

In an effort to be well-hydrated on race day, runners often drink too much water in the last few days leading up to the race, or they drink too much during and after the run, making themselves susceptible. It often occurs after running for four or more hours – the idea being the longer you’re out on the course, the more fluids you may be taking in.

Hyponatremia can cause many health issues that range from mild to life-threatening. The brain is especially sensitive to changes in the sodium levels, which is why often the first symptom of hyponatremia is confusion. Other symptoms include dizziness, headaches, fatigue, irritability, muscle weakness, cramping, nausea, and vomiting. Runners may even become disoriented, unconscious, or experience seizures.

Oddly enough, the symptoms of hyponatremia and heat illness are very similar and often can only be differentiated by a blood test. It’s very important to distinguish between the two conditions because while symptoms are similar, the treatments are different. Get to the medical tent immediately if you experience any of these symptoms again.

Here are some tips for avoiding hyponatremia:

• Drink small amounts of fluids if you’re thirsty, but don’t force yourself to drink.
• Add electrolytes to your pre-race fluid consumption, in capsule form and/or include some sports drink to your intake.
• Alternate taking water and sports drink during the race; water at one aid station and sports drink at the next one.
• When aid stations are frequent, like every kilometre, take fluid at every other station.
• Try salty snacks like pretzels or crackers for alternative run nutrition.
• Take electrolytes during training runs and/or races.
• Avoid taking any anti-inflammatories before or during the race if at all possible.
• Eat a good pre-race breakfast, one that you are familiar with like toast with peanut butter and a sports drink.
• Stop drinking if you hear or feel your stomach sloshing.
• If you don’t feel well, get to the medical tent quickly.

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