This may have been answered already, but I keep getting error messages when I try to search the forums, so here goes.
I find that even when I eat properly and drink huge amounts of water/gatorade and so on I get serious problems on hot weather rides -- my head pounds, my body sags and I lose all energy and will to go on. The last time I felt almost physically sick, and it took 15 minutes resting in the shade, followed by an ice cream before I felt half human enough to complete the ride, which was only 50-60 km to start with. I drank lots of water the night before, more in the morning before I started riding and at least 4 bottles during the ride, so it wasn't just straight dehydration. But it was scary. Any tips to avoid problems in really hot weather riding?