>Could someone please explain why adults who were apparently lactose
tolerant all their lives suddenly become intolerant, especially in their
thirties.<

People who have the gene for lactose intolerance will at some point in
their lives begin to lose the ability to manufacture lactase, the enzyme
that digests lactose.  It can happen at any age past weaning, although the
vast majority of people in this country do not report problems with dairy
products until they are adults.  That's why LI is sometimes referred to as
adult-onset LI.  

>Subsequently, a gastroenterologist told me that people do not normally
become lactose intolerant so suddenly, and that therefore, lactose couldn't
be my problem.<

Your symptoms do not start when all lactase disappears, assuming it ever
does, but when the amount of lactase you have left is insufficient to
digest what you once considered a "normal" amount of milk.  That's why they
appear to come on suddenly.

The gastroenterologists I know personally aren't this stupid, but there are
always a few in every bunch.

>I've found that it is not easy to be an intolerant minority member in a
tolerant majority world (I live in New York City).<

Are you kidding?<g>  With the number of kosher restaurants, delis, and
stores in NYC, it is probably the best place in the country for someone to
be LI!

Steve Carper, author of Milk Is Not for Every Body: Living with Lactose
Intolerance
http://ourworld.compuserve.com/homepages/stevecarper