A Marathon Runner in a Maltese Summer: Fluid Balance Judgement
During a half-marathon in July, 70 kg Kevin runs for 2 hours at 32 °C and loses 1.6 L of sweat per hour. He drinks 1.5 L of plain water per hour because he is worried about dehydration. He finishes with a 1.2 kg increase in body mass, feels confused and nauseated, and has a plasma sodium of 126 mmol/L (normal 135–145). His urine osmolality is 620 mOsm kg−1 despite the water load, and his plasma ADH is high. He is not on any medication and has no endocrine history. The race doctor must decide whether to give intravenous 0.9% saline, restrict fluids, or give a diuretic. A colleague argues that his high ADH is appropriate because he has been sweating.
1. Explain how ADH and aldosterone normally respond to sweat loss, and why Kevin's ADH remains high despite water overload.
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Sweat loss lowers plasma volume and raises plasma osmolality. Hypothalamic osmoreceptors trigger ADH release; ADH inserts aquaporin-2 channels in the collecting duct, so water is reabsorbed and urine becomes concentrated. A fall in blood pressure also activates renin–angiotensin–aldosterone, increasing sodium reabsorption.
In Kevin, however, he drinks 1.5 L h−1, close to his 1.6 L h−1 sweat loss, and gains 1.2 kg. His plasma sodium is 126 mmol/L, so his ADH should be suppressed. The high ADH (urine 620 mOsm kg−1) is likely non-osmotic: exercise, nausea, pain and stress stimulate ADH, so he retains water and dilutes body sodium. This is dilutional hyponatraemia, not appropriate ADH action for sweat loss.
2. Calculate his sweat loss and explain why his body mass rose, and decide what the doctor should do.
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Sweat loss = 1.6 L h−1 × 2 h = 3.2 L. He drank 1.5 × 2 = 3.0 L, so he replaced most of the water but not the sodium; metabolic water and continued ADH action add to retention, explaining the 1.2 kg gain.
The doctor should restrict further fluid, monitor sodium and neurological signs, and if symptoms are severe give hypertonic (3%) saline under supervision. Normal 0.9% saline is not the best choice for symptomatic dilutional hyponatraemia because it is still relatively hypotonic to the concentrated urine and may not correct sodium quickly; a diuretic is inappropriate. The colleague is wrong: high ADH with water overload is maladaptive.
3. Why is drinking plain water alone risky, and how should hydration be planned?
Read the worked answer
Plain water replaces volume but not the 40–60 mmol L−1 sodium lost in sweat, so it dilutes extracellular sodium. Drinking to thirst, using an electrolyte-containing drink during long hot events, and weighing before and after exercise reduce risk. Over-drinking to a fixed schedule is the main danger. Acclimatisation and pacing also lower sweat sodium loss. The plan should be individualised because sweat rates and sodium concentrations vary widely.