I wouldn't recommend reusing cooking oils or eating foods cooked in oils that have been reheated, whether you're dining at home or in restaurants. A recent study found that a toxin called 4-hydroxy-trans-2-nonenal (HNE) forms when such oils as canola, corn, soybean and sunflower oils are reheated. Consumption of foods containing HNE from cooking oils has been associated with increased risks of cardiovascular disease, stroke, Parkinson'?s disease, Alzheimer's disease, Huntington's disease, various liver disorders, and cancer. Once absorbed in the body, HNE reacts with DNA, RNA and proteins affecting basic cellular processes.
A study presented at a May 2005 meeting of the American Oil Chemists Society found that HNE shows up in oils that are reheated in home cooking a day or two after they were originally used. Based on the findings of the study, which was conducted at the University of Minnesota, researchers recommended avoiding foods fried in polyunsaturated vegetable oils at home and in restaurants. I've been giving that same advice for years.
I worry more about reheated cooking oils used in restaurants rather than at home because home cooks generally don't use as much oil as restaurants do and probably don't reuse the oil as much. Unfortunately, restaurants don't operate under industry-wide standards that specify what kinds of cooking oils are best to use or how many times the oils can be reheated. And since cooking oils are expensive, there is economic incentive for restaurants to reuse them.
My bottom line: never eat anything deep-fried in a restaurant.
Andrew Weil, M.D.