The real spiritual meaning that these Old Testament dietary laws symbolized was that we were supposed to be holy (separated) unto God in everything - even what we eat.
Under the Old Testament law, it was forbidden to eat or to touch certain animals, not because there was anything wrong with the animals, but to illustrate the point of being separated unto God and to serve as a constant reminder of this separation.
Under the New Testament, we see that no animal is, or ever was, unclean of itself. The Old Testament designation of certain animals as unclean was purely symbolic, and this is why Jesus could make this statement in Mark 7:15.
Jesus' statements here refer to more than just the clean and unclean beasts of the Old Testament. He said nothing which enters into a man through his mouth can defile him. This statement cannot be interpreted, however, as condoning any type of abuse we would like to give our body such as gluttony, drug abuse, alcohol, etc.
Jesus is simply explaining that the condition of the heart of man should be given more importance over the physical body. The heart of man controls the body of man, not the other way around.
Sin doesn't make our heart corrupt, but a corrupt heart makes us sin.
God looks on our hearts, and our cleanliness or defilement in His sight is dependent solely on whether or not we have been made clean in our spirits by the blood of the Lamb. The blood of Jesus cleanses us from all unrighteousness.
