A 35-year-old marathon runner collapses at mile 24 with nausea and confusion. Serum Na is 124 mEq/L. She had been drinking large volumes of plain water throughout the race. What physiological failure led to her hyponatremia?
- A Sweat-induced sodium losses exceed free water losses, creating hyponatremia
- B Exercise-associated non-osmotic ADH secretion combined with excessive hypotonic fluid intake overwhelms renal free water excretion ✓
- C Lactic acidosis causes transcellular shift of sodium into cells
- D Renal cortical ischemia during exercise impairs diluting segment function
Correct answer: B. Exercise-associated non-osmotic ADH secretion combined with excessive hypotonic fluid intake overwhelms renal free water excretion
Explanation
Exercise-associated hyponatremia (EAH) results from the combination of excessive hypotonic fluid (free water) intake and non-osmotic ADH release (triggered by pain, nausea, exercise intensity, cytokine release). ADH prevents the kidneys from excreting the water load despite falling osmolality, causing dilutional hyponatremia. Sweat is actually hypotonic relative to plasma, so exercise alone would tend to raise sodium; it is the over-replacement with free water that drives the dilution. This is distinct from sodium depletion hyponatremia.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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