One of the gurus of the medical aspects of endurance exercise, Dr. T.D. Noakes, of Cape Town has published a detailed study of 2,135 endurance event participants and gives us slow-marathon and slow- ultramarathon runners some authoritative insight ( Only an abstract seems available so far)into marathoner's hyponatremia. First of all some folks simply drink too much water.These people actually gain weight during an ultra event.Most runners loose weight, which, it turns out, is safer.The second reason is "inadequate suppression of ADH". The third reason he offers is failure to mobilize osmotically inactive sodium from bone. Looking at his data, 19% of the 2,135 runners were hyponatremic in the range of a sodium from 129-135 and another 11% had values less than 129. Weight gain during the race was the tip-off to low serum sodium values but most of those gaining weight (70%) did not have low sodium values. In a separate,earlier publication, Noakes et al studied 6 Iron man participants with a history of hyponatremia and 6 control Iron Men with a water loading experiment and was unable to discern any characteristic pathophysiological features to explain why some do and other do not develop low serum values. Hyponatremia can occur in some elite runners although data from several marathons indicate that the current apparent increase in marathon hyponatremia more typically occurs in slower runners who tend to overhydrate,are inexperienced and tend to be thin and female. The older advice to drink at each hydration station and do not wait for thirst has changed and some race directors are actually recommending fewer aid stations.The official hand book of the New York marathon recommends only 8 oz. every 20 minutes as a maximum. Mild hyponatremia, according Noakes' chapter in "Endurance in Sport" by Shepard and Astrand, is usually mild and self limiting but there are dramatic exceptions including fatal cases.
Hopefully,as inexperienced marathoners are disabused of the notion of excessive water intake during a marathon we will be seeing fewer cases.