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Hi, thanks for all the responses.  While it's true that grapes do cause a
laxative effect in lots of people, it obvious that it does so more severely in
some people than others.  Someone mentioned that it might be due to the tannic
acid.  Tannic acid has some good properties, but it also can inhibit gastric
enzymes.   Red grapes are definitely high in that, and there are some people
that have reactions to sources of red grapes, but not green/white (i.e. red
wine vs. white wine).

However, other fruit also contains tannic acid, some in even higher amounts
than grapes, and I'm now using juices and jams that contain such fruit, and I
don't have any gastrointestinal reaction to them.  So I did a little more
research in grapes, and discovered that grapes are also high in tartaric acid,
which can be an irritant, and supposedly has a known laxative effect.  One web
page says that it "stimulates the intestines".  Tartaric acid is also found in
high amounts in only just a few fruit.  The other common fruit where it's
found, is pineapple (although not as high as grapes), and coincidentally,
someone just emailed me saying they had a very similar response to pineapple
as they do to grapes.  (I rare eat pineapple myself, so I can't say if it's a
problem for me).

Whatever the reason, I sure wish I figured this out months ago. I probably
would have, if not for the fact that being celiac, I was actually looking for
sources of gluten and dairy, or other health problems, rather than simple
fruits and juices.  -  Mark

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