Psychology isn't all bad, since a lot more folks with severe mental illness can now go on to lead a much more normal life due to medication and coping mechanisms. Same for folks that go through trauma and have after-effects. But we are still refining this stuff, so some of it is going to be crap and there are going to be lots of pseudo-psychology fed to the uneducated masses.
Nutrition is somewhat similar. We simply don't have the knowledge to understand its complexities right now. But it isn't all crap: We understand connections between malnourishment and stunted growth, for example. Understand folic acid helps prevent birth defects and vitamin C helps prevent scurvy. To confound matters more, we have a lot of misinformation, partially due to politics and business practices. This isn't really confined to nutrition, though. Society is prone to reject new ways of thinking. Heck, the US pushed against modern "european-style" medicine back in the 1800's - they didn't think they needed science in their medicine or something like that.