Too Much Water An Underestimated Exercise Risk

Tim Noakes, from the University of Cape Town (South Africa) submits that heat-induced dehydration rarely causes athletes to collapse during workouts or competition.  In most cases, the culprit is exercise-associated postural hypotension (low blood pressure when standing), in which case primary treatment consists of recovery in a head-down position.  Noting that healthy athletes face "barely any risk" of dehydration during competition in an endurance event, the study author states that serious health risks -- including inhibition of voluntary motor activity and paralysis -- occur only when total body water decreases by 15% or more, which would require 48 hours in the desert with no water.  Instead, the author writes that a 2% increase in total body water leads to generalized edema that can impair physical and mental performance. Further increases in overhydration can cause hyponatremic encephalopathy, leading to confusion, seizures, coma, and -- if not reversed -- death by respiratory arrest.  "Over the past 40 years humans have been misled ... to believe that they need to drink to stay 'ahead of thirst' to be optimally hydrated," urges the author, continuing that: "In fact, relatively small increases in total body water can be fatal."