have you read or studied anything that says or suggests this? so far I've found this just to be an internet rumor
I did find this tho
Originally Posted by Reptile Basic Inc's PVC Rack FAQ
Now to the OP I used to use belly heat when I used newspaper as substrate because it was easy, but now that I use cypress I find it harder to control the temps through multiple cages, different amounts of substrate change the temps and the snakes like to move it around. Its not a big deal, but i find back heat just alot easier.
back heat is alot easier with substrate in my experience, substrate won't change the temps, I am currently working on switching all my racks to back heat now. The racks that are back heat, I have not seen any difference in their digestion. They still eat, digest, and crap just like normal lol.