I did one of my first long workouts in the heat on Friday. I headed out around 1:15pm for a 50 mile bike followed by what was supposed to be a 8-10 mile run. It was 87° F without a cloud in sight. I weighed myself before and after the workout so that I determine my sweat rate.

Weight before my ride/run = 169.8
Weight after = 165.0
Weight lost = 4.9 lbs!

Fluid intake during workout = 64 oz on the bike, 10 oz in transition, 14 oz on the run = 88 oz total (5.5 lb)

Time – 2:30 (Bike), 1:04 (Run) = 3:34 total

So I take the amount of weight I lost (4.9 lbs) and multiply it by 16 to convert it to ounces. This comes out to be 78.4 oz. See how that Engineering degree paid off right there?!
I then add my water intake (88 oz) to that number and get 166.4 oz….this is the total amout of fluid that I lost through sweat. Crazy.
I then divide this number by the length of the workout (in hours), which was approx. 3.5 to get how many ounces of water I need to take in per hour during exercise.

166.4 oz / 3.4hr = 47.5 oz/hr

That actually seems pretty reasonable to me. I’ll do this test again next time I workout in the heat and have this dialed in before Ironman.
It also explains why I bonked about 2.5 miles into what ended up being a 7 mile run. I only took in 14 oz during the hour that I was running. I need to find a way to carry 33.5 more ounces of water per hour on my runs. Actually, the 64 oz that I took in on the bike wasn’t enough either. Although I felt good on the bike and even on the start of the run, the effects of dehydration had already started to set in.
Hard to believe the heat makes this much of a difference in my sweat rate…but the proof was there. I was struggling to hold 10 minute miles at the end of my run. 
I drank over 120 oz of water in the first few hours after my workout and didn’t have the urge to pee until right before I went to bed. I even had a bit of a headache when I woke up Saturday (another sign of dehydration). After eating breakfast and drinking lots of water on Saturday morning I weighed myself again and I was back up to my normal 174 lb.
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