I've heard that reusing cooking oil can be unhealthy. For example:
Never re-use cooking oil once it has been exposed to heat. It is a recipe for trans fats and cancer-causing carcinogens. -Source
Is this true?
Go Ask Alice, a Q&A site for health from Columbia University addressed this question:
However that entry is from 2002. Since then there have been studies that indicate the re-use of cooking oils can have detrimental effects on health:
A link to the study
A separate study in 2005 found that a toxin called 4-hydroxy-trans-2-nonenal (HNE) forms when cooking oils are reheated and has been associated with increased risks of stroke, Alzheimers, Huntingtons disease and other health problems.
Quoting from the study:
So, there is evidence that re-heating cooking oils can have a negative health impact. It is unclear to what extent the impact would be on humans at this stage. Given that the study experiments continuously heated the oil for an extended period, re-using cooking oil once or twice is probably OK.
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Not necessarily. Assuming by the use of "cooking oil" implies you are using vegetable oils which are predominately unsaturated, the usage of it during cooking will increase the oxidative rancidification process that oils naturally go through. Rancid cooking oils usually don't smell/taste good and some studies have linked rancid oils with cancer but its not the most conclusive.
This would not typically be true of coconut oil though as it's high smoke point and the fact that it is almost entirely saturated tends to reduce the rancidity of the oil after usage.
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