As thousands tackle the long distances of the summer racing season, from city marathons to mountain ultras, one of the most serious medical risks they face has nothing to do with running out of fluid. Exercise-associated hyponatremia, defined as a serum sodium concentration below 135 millimoles per litre that develops during or within 24 hours of exercise, has been documented in endurance events for decades, yet it remains widely misunderstood. The condition is driven not by dehydration but, in most cases, by its opposite: drinking more fluid than the body can excrete.
The mechanism is a combination of behaviour and physiology. When runners take on large volumes of water or sports drink over many hours, particularly at a pace that outstrips fluid losses, the excess dilutes the sodium in the blood. This is frequently compounded by the inappropriate secretion of antidiuretic hormone during prolonged exertion, which limits the kidneys' ability to shed the surplus. A 2025 review of the pathophysiology and treatment of the condition reinforced that the most dangerous cases stem from fluid overload rather than salt depletion, a distinction with significant implications for how it is prevented and treated.
Field studies bear out how common, and how silent, the problem can be. Research conducted at the Olympus Marathon, a 44-kilometre mountain race rising to 2,780 metres, found that 8 per cent of participants finished with asymptomatic hyponatremia, with lower dietary sodium intake among those affected. A separate investigation into sodium ingestion and climate during ultramarathon running reported that 6.3 per cent of runners developed hyponatremia while 17.2 per cent swung the other way into hypernatremia, illustrating just how variable individual fluid and electrolyte balance can be over a long day on the trails.
The role of salt supplementation, a popular insurance policy among endurance athletes, is less clear-cut than many assume. A 2025 study that followed runners through a seven-stage, seven-day ultramarathon found no association between reported sodium supplement consumption and plasma sodium concentration, and observed an overall decline in plasma sodium across the event among the female participants. The findings echo a recurring theme in the literature: swallowing salt tablets does not reliably protect against dilutional hyponatremia if a runner is simultaneously over-drinking, because the underlying problem is one of fluid balance rather than a straightforward sodium deficit.
The practical guidance that emerges is reassuringly simple. Sports medicine consensus has long favoured drinking to thirst rather than to a fixed schedule or a self-imposed target, allowing the body's own signals to govern intake over the course of a race. Runners should be wary of the instinct to pre-empt dehydration by drinking at every opportunity, especially in cooler conditions or slower finishing times when sweat losses are modest. Hyponatremia is a sensitive subject because, in its severe forms, it can be life-threatening; anyone planning a marathon or ultra, and particularly those with a history of fluid problems, is best advised to discuss a personal hydration strategy with a qualified professional.