The context:

7:5 Then the Pharisees and scribes asked him, Why walk not thy disciples according to the tradition of the elders, but eat bread with unwashen hands? 7:6 He answered and said unto them, Well hath Esaias prophesied of you hypocrites, as it is written, This people honoureth me with their lips, but their heart is far from me.

7:10 For Moses said, Honour thy father and thy mother; and, Whoso curseth father or mother, let him die the death: 7:11 But ye say, If a man shall say to his father or mother, It is Corban, that is to say, a gift, by whatsoever thou mightest be profited by me; he shall be free. 7:12 And ye suffer him no more to do ought for his father or his mother; 7:13 Making the word of God of none effect through your tradition, which ye have delivered: and many such like things do ye.

7:18 And he saith unto them, Are ye so without understanding also? Do ye not perceive, that whatsoever thing from without entereth into the man, it cannot defile him; 7:19 Because it entereth not into his heart, but into the belly, and goeth out into the draught, purging all meats?

7:21 For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, 7:22 Thefts, covetousness, wickedness, deceit, lasciviousness, an evil eye, blasphemy, pride, foolishness: 7:23 All these evil things come from within, and defile the man.

It is not saying that anything can and should be eaten, but putting things into perspective. The apostles were not observing the hand washing teachings of the Pharisees, because those teachings come from the Talmud, not the Torah, and are unnecessary. Jesus scoped it against what they do allow in their teaching (dishonoring father and mother in their age) and pointed out how it was against the spirit of the Torah.

We know, from a secular, practical standpoint that eating mercury is a bad idea. Shrimp are high in mercury. We can not purge it without assistance after we eat it. Mercury causes many diseases over time. Eating certain types of chitin cause diseases over time.

This is a conversation that the church as a whole needs to have, putting aside prejudices and the teachings of seminary-taught ideologies and interpretations, and earnestly digging into the scriptures, in context, and in full.