An important warning given the usual admonition to drink lots!
Exercise-Associated Hyponatremia incidence can be high in 161-km ultramarathons in northern California. In this environment, EAH is more common with dehydration than overhydration and is more common in hotter ambient temperature conditions. Because weight loss >3% does not appear to have an adverse effect on performance, excessive sodium supplementation and aggressive fluid ingestion beyond the dictates of thirst are ill-advised
It reminds me of elements of this advice given to those preparing for the West Highland Way ultramarathon