In resting, thermoneutral individuals, whole-body insensible water loss is widely accepted to occur at about .03L/h
In most athletes exercising and sweating for 4-5 hours with a sweat sodium concentration of less than 50 mmol/L, the total sodium lost is less than 10% of total body stores (total stores are approximately 2,500 mmol, or 58 g for a 70-kg person) [REF].