I can't remember where I read it, years ago, but it stuck with me. Oil that has been repeatedly
heated and cooled is altered so as to make it un-healthy. Chemical changes take place, BUT the same oil makes for great deep-frying, producing crisp brown results. I remember the word "oxidation" as part of the article. The better restaurants changed their oil often, but keep a little of the oxidized oil to facilitate the frying.
I know most sources agree that "cold-pressed" oils are the healthiest, so this idea holds with that theroy. (Can someone with some hard, quotable facts rescue me here?)
Meanwhile, I just had pan-fried duck confit for dinner with potatoes fried in the fat (I had a salad too to justify the whole thing) I read in some other source that in the Southwest of France where duck and goose fat are the norm for cooking, the folks there fare very well, the same as in the South of France where olive oil is king. The poultry fat is said to be much healthier than the butter-based cooking of Northern France, but I wonder if it isn't the fact that duck/goose fat is home-rendered, NO solvents, no emulsifiers. I also read (somewhere) that home-rendered lard is high in Omega-3 or whatever, but when it is hydrogenated like Crisco to make it a smoother consistency, all of that benefit is lost.
Just rambling................about fat...................it's late........