Musar על יומא 77:2

This is because the forbidden foods bring in Tuma (spiritual contamination) in a person's heart and soul so that the holiness of G-d, blessed be He, departs and withdraws from him. This is also what they stated (Yoma 39a): "'becoming defiled with them' (Vayikra 11:43) - read not ve-nitmetem [that you will be defiled], but ve-nitamtem [that you will become dull-hearted]".