Fe writes:

<< I've heard that the body takes four or five years to don't considere  some
dairy products as a poison any more....

 maybe you spent this time, came back to eat them, and then you reach the
"lactose limit"  again... >>

I'm sorry, but this is simply untrue for any dairy-related condition except
for secondary LI, which may disappear when the intestines heal after whatever
has caused them damage. Adults with primary (i.e. natural age-related) LI
will never get their lactase-making ability back. Adults with dairy allergies
will have them for life. Milk is not in any way, shape, or form a "poison".

Elaine and Juliann -

I've also had mothers tell me both that their LI disappeared during pregnancy
or lactation or that they had become LI at those times. There is nothing in
the medical literature that I know of that explains either one.

David Pohle --

<<This is very interesting, but where do people who develop LI long after
weaning fit in.  [snip]  Is there perhaps a gene that functions to produce
lactase only until
a certain period in life, after which it shuts down in whole or in part?>>

LI was at one point called adult-onset LI for this reason, until more
research brought home the point that in many non-western European cultures,
the majority of people who become LI do so as children. But in cultures where
milk drinking is common, even those who do become LI tend to do so as adults.
(Which makes me wonder whether two or more genes aren't combining their
effects, but this doesn't seem to be supported by the genetics literature.)

The gene that is talked about in the Nature article is this gene and may at
one time have been timed to stop lactase production at the age of weaning,
but no longer does so for many people. People may become LI at any time in
their lives after weaning. Cases are known of those aged 70 becoming (or at
least first noticing that they've become) LI. BTW, weaning is naturally
around age 4.

Steve Carper
Steve Carper's Lactose Intolerance Clearinghouse
http://ourworld.compuserve.com/homepages/stevecarper
author of Milk Is Not for Every Body