What are the health risks of cooking/frying with healthy oils past their smoke point?
I've heard that the oil is decomposed and toxin are formed. How does that affect our health when we eat fried foods?
The worst components of fats and oils from health point of view are "trans fats". These are unsaturated fatty acids with one or more trans configuration double bonds. MUFA (monounsaturated fatty acids) and PUFA (polyunsaturated fatty acids), on the other hand, are good types of fat and oil components.Mayo Clinic
Repeated reusing and heating oils to high temperature lead to formation of trans fats hence this should be avoided. See: Does Overheating Olive Oil Turn it to Trans Fat?
Trans fats increase the risk of atherosclerosis (plaque formation in the walls of arteries) that limit flow of blood in vital organs like heart and brain (see: Trans Fatty Acids and Cardiovascular Disease). Trans fats are thought to be even worse than saturated fats for their propensity to cause atherosclerosis.
This leads to life-threatening conditions like myocardial infarction (heart attack), brain stroke and limb gangrene. Harmful health effects may also extend to other organs and diseases, e.g. Diabetes and Alzheimer. See: Dietary fats and the risk of incident Alzheimer disease.
FDA has made 2018 the year by which trans fat should be completely eliminated from food supply.NY Times
It seems to be that the health danger of frying past a smoking point is absorbing cancer-causing chemicals from the fumes the oil produces or from ingesting the oil itself.
... it is believed that fats that have gone past their smoke points contain a large quantity of free radicals which contibute to (sic) risk of cancer.
When an oil is heated past its smoke point, it generates toxic fumes and free radicals which are extremely harmful to your body. When the smoke point is reached, you’ll begin to see the gaseous vapors from heating, a marker that the oil has started to decompose. Decomposition involves chemical changes that ...also create cancer-causing compounds that are harmful when consumed and/or inhaled.
Oxidative DNA damage was associated with exposure of Chinese restaurant workers to cooking oil fumes.
As side notes,
...the reuse of oils is one of the main reasons why eating at restaurants is discouraged,..."; "canola oil is always the worst choice, because it becomes toxic long before it reaches its smoke point. The high rates of lung cancer in China are largely due to the use of canola oil and rapeseed oil,...