Hyponatraemia at the End of an Ultramarathon

Detecting the change. Osmoreceptors in the hypothalamus respond to the osmolality of the blood. In the first runner, 6.0 L of water has been absorbed against 4.0 L of sweat lost, so plasma sodium and plasma osmolality have fallen below normal. Water therefore enters the osmoreceptor cells by osmosis and they reduce their firing rate.

The response. Less ADH is released from the posterior pituitary, so the collecting ducts of the kidney have fewer aquaporins in their membranes, become less permeable to water, and reabsorb less water. More water stays in the urine, which is therefore dilute and large in volume - matching the measured 85 mOsm kg-1. This is a negative feedback loop that opposes the initial fall in osmolality by conserving water loss.

The contrast. The second runner lost 3.0 L of sweat without replacement, so plasma osmolality rose. Osmoreceptors fired more, more ADH was released, collecting ducts became more permeable, and water was reabsorbed, producing a small volume of concentrated urine and a plasma sodium of 148 mmol dm-3.

The two runners show the same reflex operating in opposite directions, driven by the same variable - plasma osmolality.

Sodium loss. Sweat sodium lost = 4.0 L × 40 mmol dm-3 = 160 mmol.

Water balance. Volume in = 6.0 L; volume out as sweat = 4.0 L; net gain = +2.0 L.

Sweat is hypotonic to plasma (40 mmol dm-3 against about 140 mmol dm-3), so sweating alone would tend to concentrate the plasma. The fall in plasma sodium was therefore not caused by the salt lost but by drinking more water than was lost: 6.0 L of sodium-free fluid replaced a 4.0 L loss, diluting the extracellular fluid. The 160 mmol of sodium lost was not replaced at all, so the dilution was unopposed.

With the sports drink. Sodium taken in = 6.0 L × 30 mmol dm-3 = 180 mmol, which is slightly more than the 160 mmol lost in sweat. The sodium deficit would essentially be covered, so the dilution due to sodium loss would disappear. However, the +2.0 L positive water balance remains, and that excess water still dilutes the body fluids. The sports drink would therefore have reduced the fall in plasma sodium but would not have prevented it.

Conclusion. The decisive factor is total water balance, not the salt content of the drink; drinking 4.0 L, matching the sweat loss, would have kept plasma sodium close to normal whatever the drink.

Why the brain is at risk. A falling plasma sodium lowers plasma osmolality, so water moves by osmosis from the blood into cells, including brain cells. Because the skull is rigid, the swelling raises intracranial pressure and produces the confusion and nausea seen at the finish, and can progress to seizures, coma and death. The risk is greatest when plasma sodium falls rapidly, since the brain has little time to adjust its own solute content.

Why more fluid is the wrong treatment. Offering this runner more plain water would add to the positive water balance and lower sodium further, worsening the cerebral oedema. Her own kidneys are already excreting dilute urine as fast as they can, but they cannot excrete free water faster than it is being drunk.

Evaluating the advice. "Drink as much as you can" is unsafe because it ignores the body's own signal, thirst, and ignores sweat rate, which varies several-fold between runners and with temperature. Athletes who drink far beyond thirst can gain mass during a race, and weight gain during endurance exercise is a warning sign of dilutional hyponatraemia. Better guidance is to drink to thirst, to estimate losses from body mass change, and to use fluids containing some sodium in hot, long events. The second runner shows the opposite error - under-drinking - and the aim is to avoid both.