Hyponatremia and Marathons

I love it when some of the arcane nephrology knowledge makes headlines. When I heard NPR covering hyponatremia I almost cried. I am training for a half marathon in October and so I have been thinking about this topic.

Almond, Et al's study published in the NEJM looked at 488 blood samples from 766 runners recruited from the 2002 Boston Marathon. They found post-race:

• Average sodium 140±5 mmol/L
  
• 13% had a sodium <>
  • 22% of woman
  • 8% of men
    
• 3 runners had Na <>

When they looked at predictors of hyponatremia, univariate predictors included:

• Female gender (p<0.001)
• Lower BMI
• Fewer prior marathons (p=0.008)
• Slower training pace (p<0.001)
• Longer race duration (p<0.001)
• Hydration frequency (p<0.001)
• Hydration volume (p=0.01)
• Urination during the race (with more frequent voiding having a higher risk of hyponatremia) (p=0.047)
• Weight gain during the race (p<0.001)

Of note use of sport drinks compared to pure water made no difference. In the multivariate analysis, hyponatremia was associated with:

• weight gain
• longer racing time
• body-mass index of less than 20

Of note the female gender falls out in the multivariate analysis as it likely was accounted for both by the longer running time and lower BMI.

In the discussion the authors mention that most sport drinks have only 18 mmol/L of Na.