I think it depends on the definition of "harmful". Certainly grinding oats into oats flour does not make them dangerous for the body just because of the grindi

I think it depends on the definition of "harmful". Certainly grinding oats into oats flour does not make them dangerous for the body just because of the grinding process. But it is probably that when they are ground, you use more of X to keep them together, and X is not good for the body. Not a poison but something that makes your body suffer when eaten in too much quantity (say, sugar). So all in all the processed food brings to your body stuff that it should not in such quantities. This is like a beer once in a while, which is fine, but two daily will eventually make you sick.