From what I understand about proteins, if the hair is in not-so-great condition, the protein fills in the bad spots and makes hair seem healthier. If the hair is already in good condition, the protein just coats the hair, making it look dry, straw-like, etc.
Excuse my very basic explanation, it's all I can remember right now. But I think it can help explain why some hair responds well to proteins and some responds badly.
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