The consumption of trans fats (those found in industrial pastries, potato chips, ultra-processed foods, etc.) is associated with risk factors for cardiovascular disease, including increased levels of cholesterol and triglycerides in the blood. It is also associated with increased insulin resistance, adiposity and endothelial dysfunction, increasing the risk of diabetes and cardiovascular disease.

Approximately 645,000 deaths per year worldwide are attributed to a diet rich in trans fats. In view of their adverse health effects, the WHO recommends that intake should be minimized and contribute no more than 1% of total daily energy intake. However, their consumption in many parts of the world represents up to 6.5 % of total daily energy. 

The trans fatty acids we ingest can come from different sources.  They can be natural or produced industrially due to partial hydrogenation of vegetable oils (these are the worst). However, they can also be produced in the cooking method of food. The heating of oils during cooking can alter their physicochemical properties and generate trans fats from unsaturated fatty acids during cooking procedures (deep-frying, pan frying or sautéing). 

Some studies report increases in trans fat content in oils during cooking. However, others show little or no change. A new study has evaluated this on 21 types of oils to conclude the debate. The general conclusions are that heating oils below 200º hardly forms trans fats. However, between 200º and 240º the formation of these compounds increased, and the longer the heating time, the worse it gets.

Once again, we confirm that raising the temperature of the oil too much and/or heating it for a prolonged period of time is a practice that is hardly or not at all recommendable. Reusing reheated oils is not recommended at all.