In the Judeo-Christian tradition, the biblical basis for dietary restrictions (and let's not forget dietary permissions, too!) is spiritual in nature. Just as in the moral and ethical realms there are restrictions such as those outlined in the Ten Commandments, so also in the practical and day-to-day living out of one’s faith—whether Jew or Gentile, there are both biblical restrictions and biblical permissions.
The restrictions in particular have both “spiritual” and “physical” ramifications, since people who are created in the image of God are not just spiritual creatures but a somewhat mysterious nexus of the spiritual and the material, mind and matter, soul and body.
The restrictions can also be very practical in nature, and in the early history of Israel as a nation they cover such things as the treatment of servants; the lending of money; various sexual do’s and don’ts; the treatment of strangers; the care of animals; contracts; bodily functions; the treatment of skin diseases; and the list goes on and on. The Old Testament books of Exodus and Leviticus are filled with teachings and protocols on such practical issues.
A key “Old Testament” concept concerns the distinction between clean and unclean. Again, those antonyms (viz., clean and unclean) have both physical and spiritual components and applications. Levitical priests, for example, could contaminate themselves by coming into contact with a dead person or a dead animal. In other words, they became, for a time, unclean. God, however, provided a protocol for the priest to be cleansed from his uncleanness and then become fit once again to represent God to the people.
All this to say, the Old Testament teaching about what foods were clean and what foods were unclean was highly practical. More important than practicality, however, was that 1) the teaching came from God, and he, above all, was to be obeyed and was worthy of Israel’s obedience; and 2) the rules by which the children of Israel were to live were designed to make them different, in a good sense, from people in the nations surrounding them. These nations and people groups, by and large, did not worship the one true God of the Israelites: YHWH (the LORD).
On the contrary, they worshiped many gods, and quite often that “worship” of their gods and idols involved not only superstition, myths, and evil practices, but it tended to contribute to the moral decay of their nations. Just as Christians today are to be "salt" and "light" in a corrupt and spiritually dark world, so too were the children of Israel to be a beacon of light in the midst of the cultural decay of their time.
In conclusion, Jesus’ teaching and the teaching of the apostles and writers of the New Testament is very clear: All foods are clean, and they may be eaten with thanksgiving toward God if they are eaten in faith and with a good conscience (see, for example, Mark 7:19; Romans Chapters 14 and 15; and 1 Timothy 6:17).
If, however, a Christian has a “hang-up” about eating something in particular, and his or her brother or sister in Christ does not have a similar “hang-up,” then neither of them is to judge each other for either eating or not eating. Why? Because 1) only God, ultimately, is fit to judge people in matters of conscience; and 2) to judge or condemn others regarding matters of conscience is to violate the law of love.