I have been able to gather a little information that might help you
out. I assume when you say intolerance you do mean the intolerance
that is caused by the lack of enzymes in the stomach needed to digest
a food (which causes the symptoms). Not a food allergy (many sites and
articles will use the two terms interchangebly even though they are
two completely different ailments).
You are correct when you say that symptoms of the intolerance can
occur days after the consumption of the food in question, and this
does make it hard for one to pin-point the food that causes their
symptoms. Infact it eliminates any 100% conclusive test. There are
methods however that many people and physicians use to determine what
intolerances a person has.
The elimination diet was the main tool I found used. Here is a summary
of how it works.
"Basically, taking an Elimination Diet means cutting back to a very
restrictive diet for 4 - 6 weeks. Allowed foods are lamb, rice, some
vegetables and a few fruits. The purpose is to clear the body of all
suspected foods. Unfortunately there are hundreds of foods which can
cause food intolerance, so none of these can be eaten during an
Elimination Diet. And dining in restaurants is also very difficult.
Many people find an Elimination Diet tedious and difficult to endure,
and there is the risk of missing out on nutrients. Therefore the
dietician or doctor may prescribe supplementary vitamins or minerals
for the duration of the diet."
After the body is purged of all the foods, the person is slowly eased
back into a normal diet, systematicly eating the foods that are known
to trigger food intolerances. Once it is triggered the person will be
able to identify which substance was responsible.
As for your question on wether or not a person can become intolerant
to a food over night. It is a yes/no answer. I don't believe a person
can wake up on day and suddenly be intolerant but what can happen is
an existing intolerance can worsen and become apparent. At one point a
person may not suffer any symptoms at all and over a period of time it
can worsen and cause symptoms. This can also work the other way and a
person may build up a tolerance for foods that they were once
intolerant, thus getting rid of their symptoms.
Symptoms usually aren't permanant and nethier is damage to the body
but there are exceptions. People who have chronic symptoms should
first check with a doctor to see if there is an underlying cause if
not they may look into food intolerance. Since there are 4 major types
of intolerance: Dairy (lactose) intolerance, Fructose
intolerance,Yeast sensitivity, Gluten intolerance (includes wheat).
You can see that someone who is gluten intolerant and unaware may
easily ingest wheat products every day and end up with chronic
symptoms that are caused by this intolerance. Once again intolerance
can vary depending on stess, hormone levels and some of the offending
food may even be eaten with no symptoms at all appearing. Another
'ongoing' symptom could be caused by damage to the body, although rare
gluten intolerance can result in damage to villi in the small
intestine, which makes it difficult for the body to absorb water and
nutrients from foods. Which could cause an overall decline in health.
http://www.allergyclinics.co.uk/food.htm (Mostly deals with allergy
but has some information that pertains to intolerance)
http://en.wikipedia.org/wiki/Food_intolerance (Not a lot of
Information, but was a referance)
http://curezone.com/diet/ (General information on food intolerance)
http://www.foodintol.com/eliminationdiet.asp (Chart showing frequency
of specific food intolances and some information on the elimintion
explination of the elimination diet)
Hope I was of some help,