<<Disclaimer: Verify this information before applying it to your situation.>>
 
Loose unformed stools contain more water than "formed" stools.   (I don't
know, but I suspect that what makes "formed" stools "formed" is that most
of the water has been removed and recycled.)   In addition (or rather
because of this)  the volume is increased, even in some "treated"*
celiacs.   More water would be lost in the stools, therefore intake would
need to be greater or heat intolerance would result from dehydration.   In
addition, if there is any degree of malabsorption, there might be a
deficiency of electrolytes which would also tend to cause heat intolerance.
 
Solution:  drink more water and fruit juices (to help with electrolytes)
 
(Note:  I am not a physician, but this all seems to be just common sense.)
 
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*   In my opinion,  because of the huge number and variety of hidden gulten
"traps" in processed foods, waxed vegetables, etc. etc. etc.  most celiacs
on a "gluten free"  diet are really only on a gluten reduced diet and
therefore probably have some degree of (silent) malabsorption.