What harmful compounds can be generated in oil when restaurants repeatedly heat it for deep-frying foods?

Trans fats or high levels of saturated fats.

The process of deep-frying itself introduces significant cardiovascular risks due to the interaction between the food and the cooking medium, especially when oils are used repeatedly at high temperatures by commercial food manufacturers or restaurants. This repeated heating can chemically alter the fats, leading to the generation of trans fats or a significant increase in saturated fats within the food absorbed. Trans fats are especially deleterious because they actively worsen the cholesterol profile by raising LDL (bad) cholesterol and decreasing HDL (good) cholesterol. Furthermore, even seemingly healthier oils can degrade under high heat, forming advanced lipoxidation end products that promote internal inflammation.