What is dairy intolerance?
"The cow is the foster mother of the human race. From the time of the ancient Hindoo to this time have the thoughts of men turned to this kindly and beneficent creature as one of the chief sustaining forces of the human race” – W.D. Hoard
Strictly speaking, the expression “food allergy” is meant to apply only to a relatively rare, rapid onset reaction to dairy, which typically occurs within minutes of ingestion and may involve quite extreme symptoms up tincluding full blown anaphalaxis: peanut allergy would be the classic example of this kind of "true allergic" reaction. (Within the body, this kind of true allergy involves immune system components known as “IgE antibodies” or E type immunglobulins). Typical symptoms for true dairy allergy might involve swelling of lips and mouth area, streaming eyes, compromised breathing etc.
Food intolerance is different: it typically is less dramatic and much less acute and may take hours or even a day or longer after ingestion before symptoms present (usually as the food reaches the lower end of the gastrointestinal tract). Though not as dramatic as true allergy reactions, food intolerances can provoke long-term unpleasant symptomology which, in some cases, can be quite serious. Given the fact that symptoms may take a long time to appear and may be associated with foods eaten each day, typically sufferers may be so used to the ill-effects as to be completely unaware that things can be other than they have been. In other words, as some degree of symptomology is usually experienced all the time, the issue may go undetected, often for many years.
It is possible to do blood tests to detect the presence of the IgG antibodies associated with food intolerance reactions. However, these tests are both expensive and not always entirely reliable (false positives are quite common). Therefore, in general, most practitioners recommend an “elimination/challenge test” as the best and least expensive way to understand if you have an issue with cow dairy. This involves a period of total abstinence from all sources of dairy, both the obvious and traces in processed foods, usually for a period of around 2 weeks. Therafter, the "challenge" involves a sudden reintroduction of dairy (ideally some milk and some cheese). If there is an intolerance issue, quite clear symptoms will present within hours or maximum 1 to 2 days after the reintroduction challenge.
One note of caution about the "elimination phase": it is not a good idea to prolong the abstinene for longer than 2 weeks. If a body is starved of all lactose for a period of even just 20 days, the genetic expression of the lactase enzyme begins to diminish somewhat. In other words, prolonged elimination of all dairy can actually bring on a lactose intolerance as the body ceases to make enzymes it senses are not needed.
Finally, it is important to understand that there are 2 totally different variants of dairy intolerance, one due to lactose and the other casein. Details here: