Interpretation 1:
Some animals are unhealthy for humans.

Summary: 

Unclean animals are those that are unhealthy for humans because they spread viruses, and thus may cause sickness, allergies, etc., and were therefore designated by God as unclean.

Arguments in favour of this view:

1. Science has shown that eating the animals listed as unclean in Deuteronomy is unhealthy for humans. Besides the fact that these animals can transmit diseases, their meat is also bad for our blood vessels because of its high fat content.

2. A pig’s stomach has a high acidity, but because of eating so much, the stomach acid becomes diluted. Thus the stomach acid is no longer able to kill the parasites and bacteria. These parasites and bacteria then get into the pig’s meat and can therefore affect humans when their meat is consumed.

3. The protein structure of pork meat is very similar to human flesh. Our immune system does not recognize it as a foreign substance, allowing waste products to enter our blood more easily.

4. The birds that are declared unclean are omnivorous and have the same strong stomach acid as pigs.

5. With the other unclean animals, the same issue applies as with pigs.

6. The animals declared as unclean may not be unhealthy in themselves, but they were declared unfit as food because of the hygienic conditions in the Middle East when God gave this law to Israel. 

Arguments against this view:

1. All arguments that seek to show that unclean animals were not to be eaten for their threat to Israel’s health fail on the argument that our Lord Jesus Christ fulfilled and lifted the law of Leviticus 11:1–47, repeated in Deuteronomy 14:1–29.

2. In Mark 7:19b, Mark informs us—though many commentators assume on good grounds that Mark recorded in writing what Peter had told him—that Jesus, in a conversation with his first disciples about clean and unclean, declared every food to be clean. Even though there are, in the manuscripts, two different readings, this conclusion remains the same.

3. In the vision that Christ showed Peter after Pentecost (Acts 10:9–16; cf. Acts 11:1–12), he again also declared animals that had previously been labelled as unclean now as clean.

4. If unclean animals were unhealthy for humans, why did the LORD allow the Israelites to serve or sell the meat of these animals to a foreign guest (cf. Deuteronomy 14:21)?

5. The LORD does not merely warn against eating animals declared unclean by him. He abhors the eating of those animals (Deuteronomy 14:3). And he wants his people to abhor it as well (Leviticus 11:10–12, Leviticus 11:20). This choice of words is too strong for something that is unhealthy.

Interpretation 2:
Some animals evoke the thought of death.

Summary:

Unclean animals were all animals and insects that could somehow evoke the thought of death.

Arguments in favour of this view:

1. In the Old Testament law unclean does not mean unhealthy, a threat to our health, contagious. Women became unclean when they were menstruating, or after childbirth (Leviticus 12:2); men who had an ejaculation became unclean as well (Leviticus 15:16). Likewise, anyone who had to deal with a person who had died recently became unclean as a result (Numbers 9:6). But this did not mean that they had become a threat to the health of those in their surroundings.

2. God’s word also refers to people of unclean lips (Isaiah 6:5). This has nothing to do with bad breath or the danger of infecting another person with a virus. It signifies that evil, ungodly language comes across those lips.

3. Several times in the book of Ezekiel the Lord calls his entire people unclean. This is not because they are all sick and can infect others, but because not he but all kinds of idols have been given the central place in their lives, and because they shed innocent blood (Ezekiel 22:3, Ezekiel 22:4, Ezekiel 22:15; Ezekiel 24:13; Ezekiel 36:25, Ezekiel 36:29; Ezekiel 39:24). Because of Israel’s sins, their land also became unclean (Ezekiel 36:17).

4. For the LORD anything was unclean that served as a striking reminder that this life, as a result of our fall into sin, is no longer as he intended it to be in paradise. That could be diseases in which a person vomits, open wounds, burst sores, leprosy, and the like; but also a spontaneous ejaculation—potential new life is lost; a menstruation—the bleeding makes it clear that no pregnancy has taken place; a childbirth—another person received and born in sin comes into the world. And so, God also labelled any animal that can kill others, human or animal, or that is itself killed by a predator, or that eats dead animals—scavengers—as unclean.

5. God also marked all slimy aquatic animals without fins or scales as unclean because these animals usually stink, reminiscent of a decomposing corpse. And insects that give nasty stings to humans or that descend on open wounds and dead bodies were to be considered unclean.

6. In Deuteronomy 14:1–29, the prohibition against eating unclean animals immediately follows the prohibition of pagan mourning customs, e.g., shaving a slave’s hair off his head: In fact we are all slaves to death; or another example: cutting one’s skin with a knife so that your blood drips into the grave: If only I were dead too (Deuteronomy 14:1). The children of God may not mourn in that manner. They are no longer under the power of death but have been ransomed by the God of life (see 1 Thessalonians 4:13).

7. By extension, the prohibition against eating unclean animals must be read: God’s people had to show symbolically either in the kitchen or at the table that they are children of the God of life; and that through him, life does not end at some point for every human being. Instead, it leads to an eternal, restored life. So this law accentuated the distinction between God’s people and the Gentiles.

8. Thus, on the one hand, this provision of the law was a constant (!) reminder that life here on earth is subject to the power of death. On the other hand, it also pointed forward to the One who, by his death, would dethrone the one who has the power over death, the devil, and that One is Jesus Christ (Hebrews 2:14–15). The longing for him needed to be kept alive.

9. The Israelites could observe this provision as long as they lived together as one people in their own land. Daniel and his friends could also endure in Babylon (Daniel 1:1–21). But when our Lord Jesus Christ sent out his apostles among the Gentile nations starting at Pentecost and also established his churches everywhere there, this law was abrogated (see Acts 10:9–16; see also Acts 11:1–12; Acts 15:22–29; and Galatians 2:11–14).