One "fixed idea" in medicine is that nutrition does not play a significant role in overall health. It used to be important not long ago.
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It's impossible, becomes of several basic precepts.
Even cases where there is currently a debate on the facts, such as butter vs margarine, red meat vs on red meat, artificial sweeteners...the debate itself falsifies your assertion. Whichever is true, it's not fixed, right?
FYI, I was talking about the medical profession, specifically the medical education.
Read the content provided in the links in my previous post. Some medical schools don't teach nutrition at all.