In many religions, even a major part of common traditions consists of different kinds of dietary regulations. Followers of many religions are often denied the use certain kinds of otherwise even very common foodstuffs. Modern study of religions is in unity about the fact these prohibitions and regulations mainly aim at differentiating the followers of certain religious ideologies from all other people.
There have been many attempts to rationalize, for example, the Jewish and Muslim prohibition of eating pork on a rational basis. It has been claimed that, for example, the fear of trichinella is the reason(http://en.wikipedia.org/wiki/Trichinella) or even the ecological impact of pork production.

However, it is widely accepted in the field of study of religions that that these prohibitions are not based on any kind of rational causes or any kind of logical thinking. Instead, these religious traditions are based on the need to show as clearly and easily as possible where the boundaries between the believers and unbelievers go.
Their important function is so to intensify the division between the true believers and all other people. When you have been taught from the early childhood that it is disgusting to eat something, you very easily see those people who are eating those disgusting things as foreign and even repulsive.

Eating is the very basic human activity. Divisions that are based on the eating habits also do work at the very basic level. That is the main reason why religions are so extremely interested in what their followers are eating and what not.
By giving up eating a good tasting and nourishing variety of meat or other foodstuffs, believers are also making a personal sacrifice. It makes the people who are making the same sacrifice more feel close to another.

A domestic sow and her piglet. - Wikipedia

Even more important is that when a real sacrifice had been already done, a person very easily starts to rationalize doing it to himself. If we buy a certain type of car, we want to see more information supporting our decision. We will simpply pass over the information that is not compatible with our decision.
In a similar vein, all the personal choices that are made because of a religion does make our the bond with that religion stronger. We start collecting information that will support our own earlier decisions and will start avoiding such information that does not support the decision that has already been made.

The human psyche is a very powerful tool. Very soon people can start thinking that these quite irrational prohibitions on foodstuffs that are so common in many religions must be somehow based on some kind of rational reasons. It has been noted that the true believers of Jewish or Muslim faith can have powerful and real physical reactions if that they are told that they have eaten pork by mistake.
These reactions are naturally not there if they are not told about this mishaps. Their psyche just has labeled the prohibited foodstuffs to being something really harmful. Their body will react to this information as if this fear generated purely in mind would have been a real physical danger.

In the Finnish Protestant and Lutheran world where I live there are practically no dietary regulations based on religions at all. The founders of the new Christian religion made a huge decision at the very start when they started accepting uncircumcised members. An even bigger decision was made when they overturned the age-old Jewish prohibition on eating pork.
These decisions embarked this new religion to its new road of becoming a self-supporting religion from its then current status of a mere Jewish sect. The founders of Protestant and Lutheran version of Christianity did later drop also, for example, the old Catholic prohibitions on eating flesh on Fridays and many other remaining restrictions relating to eating.

Doing it was also an easy way of rapidly intensifying by then already growing differences between these competing version of Christianity at a very basic level.
On the other hand, the Protestant and Lutheran churches may have by giving up regulating the foodstuffs their followers may eat greatly enhanced their chances of survival in the western societies. In these societies this kind of personal decisions are more and more made based on rational arguments in these societies.

The Protestant state churches of the west do not even want to try to regulate the daily eating behavior of their followers living in a modern free society. I suspect that they do not want to drive people into situations where they would start comparing the costs of following customs based on irrational beliefs with the benefits coming from not following them at all.

(This piece was completely rewritten 0n 21rd of May, 2012)