Marathoner With Weight Gain and Confusion in Heat
A 42-year-old recreational marathoner (68.0 kg, 178 cm) is preparing for a summer race expected to take 4 hours in 28°C and 75% humidity. In a 4-hour simulation he drank 750 mL of plain water per hour and ate two gels. He started at 68.0 kg and finished at 69.5 kg; he urinated about 0.4 L during the run. He reports nausea, headache, bloating, and mild confusion in the last 30 minutes. His sweat is visibly salty and stains his cap. He has no history of kidney or heart disease and takes ibuprofen before long runs for knee pain. He asks whether he should drink more to stay ahead of dehydration and whether salt tablets will prevent cramps. You are the sports nutrition professional reviewing his data.
1. Calculate his sweat rate from the simulation and interpret the weight change. Why are his symptoms not typical dehydration?
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Sweat loss = starting body mass + fluid intake − finishing body mass − urine output = 68.0 + 3.0 − 69.5 − 0.4 = 1.1 L over 4 h, or about 275 mL/h. He drank 3.0 L but lost only about 1.1 L, so he finished 1.5 kg heavier. That is not dehydration; it is fluid retention. His nausea, headache, bloating, and confusion after a long race with weight gain point to dilutional hyponatremia, especially with high sweat sodium and NSAID use. I would not advise more plain water. This is a medical red flag; if symptoms are active, he needs urgent assessment and should not be forced to drink more.
2. Design a race-day fluid and electrolyte plan for him using the simulation data. State what you would specifically avoid.
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Use the simulation to set a starting range, not a fixed prescription. At 275 mL/h sweat rate, a reasonable race intake is about 150–250 mL every 20–30 minutes, adjusted to thirst and to race conditions, with a ceiling near 400–500 mL/h unless measured losses are higher. Because he is salty, choose a sports drink or electrolyte beverage providing roughly 300–600 mg sodium/L, and use food or gels as practical. Salt tablets are not a stand-alone fix and can worsen problems if he overdrinks. Weigh before and after key runs; goal race change is roughly 0 to −2% body mass, not gain. Avoid NSAIDs before long runs and trial all fluids in training.
3. He messages you after the run saying he feels confused and has a bad headache. What is your professional response and scope limit?
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My scope is to educate, screen, and refer, not to diagnose or treat hyponatremia. Confusion, severe headache, vomiting, seizure, or collapse after endurance exercise require emergency medical care. I would document the weight gain, intake, symptoms, and medications, advise him not to drive or continue drinking, and contact the race medical team or emergency services. After recovery, collaborate with a physician or registered dietitian on a written hydration plan and review NSAID use. I would also flag that a race-day plan should be practiced and that salt tablets are not a routine requirement.