Post-Race Confusion in a Well-Hydrated Marathoner
A 34-year-old recreational runner (68 kg) completes her third marathon on a warm, humid day (26 °C, roughly 70% humidity) in 4:35. She followed a plan she found online: about 1 L in the two hours before the start, 500 mL at each of the 16 aid stations, and two salt tablets at mile 18. At the finish she is bloated, nauseated, and confused. She has a headache and says she feels 'puffy.' Race medical staff weigh her and find she is 2.1 kg heavier than at check-in. She insists she is dehydrated and asks for more water and electrolytes. She takes no medications and has no history of kidney, liver, or heart disease, and she has never had a heat illness before.
1. What is the most likely explanation for her symptoms, and why would the treatment she is asking for be dangerous? What is your first action?
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Most likely: exercise-associated hyponatremia from fluid overload, not dehydration.
Reasoning from the data:
- Body weight is the discriminator. A 2.1 kg gain across the race means intake exceeded losses. Genuine dehydration always shows as weight loss.
- About 9 L in 4.5 hours (roughly 1 L pre-load plus 8 L on course) is close to 2 L/hour, well above typical sweat rates at her pace and body size.
- Bloating, nausea, headache, puffiness, and confusion fit dilutional low blood sodium. Confusion is a red flag for cerebral edema.
Why her request is dangerous: additional water or other dilute fluid worsens the dilution. Salt tablets are absorbed slowly, can aggravate nausea and vomiting, and will not correct an acute dilutional state. You cannot confirm the diagnosis without a blood sodium measurement, and that is exactly why this is a medical emergency rather than a coaching problem.
First action: stop her from drinking, keep her with someone, and involve race medical or EMS immediately. Do not send her home or let her 'walk it off.' If she deteriorates, this becomes an emergency transport.
2. She recovers and wants to race again next season. Explain how you would build an individualized fluid plan, including the measurements you need and the limits you would set.
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Step 1 – Measure sweat rate under race-like conditions. Weigh her before and after a 60-minute run at goal pace in similar heat and humidity. Record fluid consumed and estimate urine output. Sweat rate (L/hour) = (pre-weight − post-weight in kg) + fluid consumed (L) − urine (L). Repeat on 2–3 occasions, because rate changes with pace, clothing, and weather.
Step 2 – Convert to a race plan. Replace roughly 1–1.5 L per kg of body mass lost across the race, adjusted for stomach comfort. For many athletes, drinking to thirst with a defined upper limit works better than a rigid schedule.
Step 3 – Set safety limits rather than a target volume. The key rule is never finish heavier than you started. A loss under about 2% of body mass is normal and acceptable. Provide sodium through a sports drink or food rather than tablets.
Step 4 – Rehearse and educate. Practice the plan in long training runs. Teach her that thirst, urine color, and race-day weight checks are feedback tools, and that bloating, nausea, or confusion means stop drinking and seek medical help. Also flag that her online plan was the actual cause of the problem, not bad luck.