|MadSci Network: General Biology|
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 ;-)
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