The margarine and vegetable oil v butter, lard, and tallow ... From around the WWII era .... "modern medicine" and nutrition theorist pushed the vegetable side. Then, around a decade 'r so ago ... the "experts", "physicians", and "nutritionists" made a full 180. Now, it's the complete other way around.
I keep waiting for "science" to determine that smoking three packs a day is a recommended portion of a daily lifestyle. There was a time that "physicians" used to prescribe smoking for various maladies.