The efforts of Edo-era doctors to figure out beriberi in Japan
atlasobscura.com
atlasobscura.com
Similarly, Doctors did not use to wash their hands before surgeries, as they were considered "Gentlemen", and thus "clean". It took a lot of evidence before most surgeons started routinely washing their hands before an operation.
Sadly, the man who first noticed and promoted this practice died before the medical community finally accepted his ideas... Poor Ignaz! (https://en.wikipedia.org/wiki/Ignaz_Semmelweis)
Note the screechy ire with which he was pushed around for his dissent and basically destroyed. This is one of the dark corners of human psychology, which we need to look out for and prevent. This is why Free Speech is vital in spirit, as well as in law. Sometimes it's the unpopular idea which has value.
Because he was so successfully un-personed, and it was basically because he was hurting people's "feels." If that's the way one does "Free Speech," it's just crappy. Get your hands on Manufacturing Consent -- If your press is technically free, but it still fits the propaganda model then it's not really free. If society supports free speech within the law, but huge swathes of private life extract huge consequences, it's not free speech. In that case people are being suppressed, not convinced, and both good and bad ideas are being suppressed.
That kind of society is authoritarian, not liberal.
I can think of a few general counterexamples:
* Fad diets (people flit from fad to fad, not outright reject all fad diets from then on)
* Losing your faith, but then being fascinated by a foreign religion (i.e. The trope of the ex-Christian falling in love with Buddhism)
I think the same can apply to religion; someone might be completely turned off of Abrahamic religions by their experiences with only one of them, regardless of whether this is a "fair" assessment.
If they've finally changed their mind about a deeply seated belief, that means they must have had some pretty compelling reason to do so... why would they backtrack casually?
White rice has traditionally been seen as the "main dish" of a meal. Many people, even younger people in their 20s, follow this line of thinking. Here are a few facts in no particular order:
1. Rice is placed on the left of a set meal (https://i.imgur.com/n29o2Dh.jpg), with miso soup on the right. Traditionally, the left side is the "higher" position.
2. In a set meal, everything that's not rice is generally referred to as "okazu," or "side dish." (https://en.wikipedia.org/wiki/Okazu)
3. A meal consisting of a bowl of rice with some kind of flavored salty flakes (furikake, https://i.imgur.com/hDAqndf.jpg) is considered a perfectly complete meal.
4. You ask "why no meat . . . even for the emperor" -- this is sort of a cultural blind spot for foreigners, I think. We think "oh man, meat, this is an honor!", but for a long time in Japan, meat was not only difficult to obtain, but was also seen as unclean in some ways (https://www.kikkoman.co.jp/kiifc/foodculture/pdf_09/e_002_00...).
4a. Butchers, for example, were of the untouchable (burakumin, https://en.wikipedia.org/wiki/Burakumin) class for dealing with slaughter. This perception even exists today, to some extent, especially in wealthy families.
Anecdotally, there is even a continuing modern perception of meat as "unhealthy" and "a sometime treat," even among younger people (say, in their 20s). People laugh at me for eating ~100-200g of chicken, pork, or beef per day and often point to that as the cause of my being overweight, seriously suggesting that I should instead eat 100-200 grams more of white rice per day.
It makes me really curious how much of our current medical (or other) knowledge is in a similar way.