Cooking oil is often used excessively, which can be unhealthy. In other words, when food is fried in hot oil, the oil degrades, as evidenced by its darker color and rancid flavor.
 
Oil that has been heated will chemically react in various ways, including polymerization, oxidation, and hydrolysis. These reactions produce polar molecules such as free fatty acids, aldehydes, ketones, monoglycerides, and diglycerides, which are referred to as polar compounds. These compounds can increase blood pressure and may even cause cancer.