At the end of The Opal Deception, Opal loses her magic by inserting a pituary gland in herself and turning herself into a human.
The simple truth was that Opal had been too clever for her own good. By placing a human pituary gland in her own skull, she had effectively humanized herself. Gradually the human growth hormone was overpowering the magic in her system.
Artemis Fowl: The Opal Deception, chapter 11: "A Last Good-Bye"
But we see in The Lost Colony that humans did have magic, but forgot how to use it.
He felt the magic ignite in his nerve endings, filling him up, elevating his consciousness to another place. It was a euphoric experience. Artemis realized there were sections of his brain opening up that hadn't been used by humans for millenia. He also realized that humans must have had their own magic once, but had forgotten how to use it.
Artemis Fowl: The Lost Colony, chapter 15: "Home Again, Home Again"
If human brains are capable of retaining magic, especially if they once had their own, why would the human growth hormone overpower the magic in Opal's system?