The championship marathon in the heat: fluid, sodium and a 36-hour arrival
A 34-year-old club runner (mass 70 kg, marathon PB 3:05) is entered in a championship marathon forecast at 31°C and 65% relative humidity (WBGT approximately 28°C). She flies in 36 hours before the race. In a 90-minute training run in similar conditions she weighed 70.0 kg before and 68.6 kg after, drinking 800 mL. She plans to drink 1.2 L/h on race day 'to be safe', and has cramped in her last two hot races. Her only medication is an SSRI. She asks whether she should take salt tablets, and whether 36 hours is enough to acclimatise.
1. Calculate her sweat rate from the training run and critique her planned race-day fluid intake. What would you advise her to do instead?
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Sweat rate. Mass loss 70.0 − 68.6 = 1.4 kg, plus 0.8 L ingested = 2.2 L over 1.5 h ≈ 1.47 L/h. High, but plausible in heat and humidity.
What that means. 1.4 kg is exactly 2% of body mass, the usual upper limit of a tolerable acute deficit, and she will be running for over 3 hours on race day.
Critique of 1.2 L/h. That is roughly 80% replacement. The number itself is not the problem; a fixed target is. At 3.5 h she would take ~4.2 L, and if she runs slower than expected or drinks ahead of thirst she will finish heavier than she started - the classic set-up for exercise-associated hyponatraemia. SSRIs are associated with impaired free-water clearance, which raises her risk further.
Advice.
- Drink to thirst rather than to a fixed rate; a realistic range is 400–800 mL/h, adjusted for conditions.
- Cap intake so she never gains weight; weigh before and after long training runs.
- Use a sodium-containing drink (20–30 mmol/L) rather than plain water, and practise the exact product in training.
- Salt tablets are not required and add gastrointestinal risk; cramp is multifactorial (pacing, fatigue, previous cramp), not simply sodium depletion.
2. She arrives 36 hours before the race. What is your advice on heat acclimatisation, and what can still be salvaged in the time available?
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36 hours does not acclimatise her. Meaningful heat acclimation needs repeated exposure - typically 60–90 minutes per day for 5–14 days - and produces expanded plasma volume, an earlier and more dilute sweat onset, and lower core temperature at a given workload. None of that happens in a day and a half.
Timing trap. Days 2–5 after arrival are the worst window: partial acclimation plus travel fatigue, dehydration and disrupted sleep. Advise either arriving 10–14 days early or as late as possible (under 24 hours), not 2–5 days out.
What is still achievable.
- Pre-travel heat sessions at home in the final 2 weeks: 6–10 post-exercise hot baths (40°C for 30–40 minutes) or sauna sessions are the best available substitute when travel is fixed.
- Heat acclimation is not the only lever: pacing, pre-cooling (ice slurry ~30 minutes before the start, cold towel, iced hat), and choosing shade and aid-station ice.
- Realistic goal-setting: in WBGT ~28°C expect 4–8% slower running; a personal best is unlikely, and a finish without collapse is the sensible target.
Explain the physiology plainly so she can see why a 36-hour arrival is a plan for survival, not performance.
3. During the race she becomes confused at 32 km and finishes heavier than she started. What is your differential and immediate management?
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Differential, in priority order:
- Exercise-associated hyponatraemia (EAH) - weight gain during the race, confusion, womens' greater risk, SSRI, prolonged slow running, high fluid availability. This is the leading diagnosis.
- Exertional heat stroke - but she is not hyperthermic in the classic sense and she has gained weight.
- Hypoglycaemia, seizure, intracranial event (unlikely but must be excluded).
Immediate management.
- ABC, remove from the course, measure rectal temperature and point-of-care sodium and glucose. Confusion plus weight gain plus a low sodium confirms EAH.
- If she is symptomatic (confused, vomiting, seizure, reduced consciousness): hypertonic 3% saline 100 mL IV bolus, repeated up to three times until symptoms improve. Do not rely on fluid restriction alone in symptomatic EAH, and do not give hypotonic fluid.
- If asymptomatic and sodium mildly reduced: restrict oral fluid and observe.
- Treat seizures, and monitor for cerebral oedema; arrange hospital transfer.
- Debrief with the race medical team: pre-race messaging on drinking to thirst prevents more cases than any treatment protocol.