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Question:
Team,
  I have always given the rule of thumb in class that material we eat
is with us for about 24 hours before exiting the body.  The question
arises about the time value of liquids.  Getting a big coke prior to a
3 hour drive generally means that there will be a stop along the way.
Is there a generalization made about liquids in the body similar to
the one for solid food?
Don Mancosh

Answer:
   A physician would give a better answer, but I hazard this: the only
liquids which people consume (deliberately) in significant quantities are
water, ethyl alcohol and various oils.  Water and alcohol are absorbed on a
time scale of seconds to minutes through the mouth, stomach and digestive
tract.  The oils are huge molecules, so I'd guess like any other greasy
food they get absorbed in the upper digestive tract.  Some of them, perhaps
the longest and most nonpolar, are not absorbed at all --- cf. the old-time
remedy of mineral oil for constipation --- so there should be some average
time-before-what's-left-is-excreted such as you're looking for, and my
(wild) guess is that it would not differ substantially from that for food.
   You can define an average lifetime in the body for alcohol, since the
natural level is zero.  Rough guidelines are widespread in the context of
drunk driving laws.  But this is not really possible for water.  One's body
is normally full up to the brim with water, and there's no way for the body
to distinguish between water molecules recently absorbed and molecules
that've been moping around since the Beatles split up.  Thus the water
entering the toilet bowl after the pit stop is not in general the same
water as was in the big coke.
   If you were to consider for water just the average time between drinking
and peeing, it would seem to depend strongly on how well hydrated the body
was before the drink, and how much was drunk.  During sustained heavy
exertion in the sun and dry air one can easily drink a pint of water an
hour without peeing at all.  On the other hand, if one is willing to drink
enough water fast enough, so as to establish a high excess of body water
one can pee 8 ounces 15 minutes or less after drinking 8 ounces.
Christopher Grayce