The short answer is that no one has yet provided good evidence that salt intake beyond the homeopathic amounts present in sports drinks is necessary to sustain performance during something like the Ironman. This does not mean that the case is conclusively proven; just that there is no definitive support for this practice at present. Interestingly our and other’s data suggests that the body probably has a reserve of sodium stored in an unionized form, perhaps in bone and skin, that can then be activated in the short term should a deficit in the blood sodium content develop. However this is still a controversial issue.
But more to the point is the absolutely clear evidence that subjects who drink to thirst will maintain or increase their serum sodium concentrations whether or not they ingest salt during exercise. Only in those who drink in excess of thirst and who either maintain or increase their weight during exercise is their some evidence that the extent to which the blood sodium concentration falls will be reduced (but not prevented) by the ingestion of sodium during exercise. (This fall can only be prevented by drinking less). Of course this finding has been seized upon by the sports drink industry as absolute evidence that sodium ingestion is essential during exercise. What they have failed to say is that if athletes just drank less during exercise (ie to thirst), they would not need to ingest salt in order to maintain their blood sodium concentrations. This we have known since blood sodium concentrations were first measured in runners in the 1960’s.
-Dr. Tim Noakes, exerpt from slowtwitch.com forum.