As activity level and heat rise, rationing is less and less effective; many marathon runners have found this out hte hard way. A lot of hiking in S. Arizona in warm weather convinced me that rationing didn't work - at least when you had to move during the heat of the day, as we did on more than one search effort.<br><br>I just finished a bike tour where I was pedaling in 100 degree heat for 65 - 100 + miles a day. I found it imperative to start early when temperatures were lower, sit out the midday heat and then ride into the dusk. I vividly experienced the correlation between riding efficiency, adequate hydration, and relatively cool temperatures.