Re: Why does ice cream make you thirsty?

John,
   This was a tough question, partly because thirsty can mean a lot of 
things.  I think you mean "thirsty-thirsty" as opposed to "cottonmouth 
thirsty," but I will address both.  :)  The answer for your 4 year old is 
beautifully simple.
  
"Salt is one of the ingredients of most ice creams.  Salt makes you 
thirsty."

  This is overly simplified, I know, but in all honesty that is pretty 
much what happens.  Ice cream makes you thirsty on the same principle.  
For those with more biology, it all boils down to osmosis.
   Osmosis is the tendency for water to travel across a semi-permeable 
membrane from a place of low concentration of solutes to a place of high 
concentration of solutes.  
  When you eat ice cream, you blast your body with all sorts of solutes.  
Ice cream has salts, but also sugars fats, amino acids, and more for your 
body to absorb into the blood stream. When your blood becomes laden with 
these chemicals, (mostly sugars) your blood becomes more "concentrated," 
giving your brain (hypothalamus) the signal of dehydration.  When water in 
the hypothalamus leaves to the blood through osmosis, then the blood 
concentration is greater than that of the hypothalamus.  This triggers the 
thirst response in the body and brain.
  If you were dehydrated, there would be a high concentration of solutes 
in your blood, so you would become thirsty.  Eating ice cream, in a sense, 
makes your body think it is dehydrated, and in a sense, I suppose it is.
  This is one of the reasons that diabetics are thirsty all the time.  
Diabetes causes high blood glucose, and that increased concentration of 
blood solutes makes the diabetic feel thirsty.
  I hope this answers your question.  I thought of another possibility and 
that refers to the saliva of the mouth.   The saliva and mucus of the 
mouth makes the mouth feel wet.  Have you ever noticed that sometimes you 
can drink a lot of lukewarm water and feel that dry cottonmouth feeling?  
By drinking the water, you have swallowed the saliva and your mouth feels 
dry, even though it really isn't.  If you are referring to 
the "cottonmouth-thirsty," then the ice cream (depending on the type and 
person) may aid in the swallowing of the mucus and saliva.

  Hope that helps!

   Mike Gasink

PS- sorry for the delay!   I had a busy week.  I think supervisors think 
that stress is a giffen good ;-)