Why reusing cooking oil might be slowly affecting your health
By William Muthama, May 17, 2026Reusing cooking oil is a common practice in many households, especially when deep frying foods like chips, samosas, mandazis, or chicken. While it may seem economical and convenient, scientific research shows that repeatedly heating cooking oil can have serious long-term effects on health.
When oil is heated at high temperatures during frying, its chemical structure begins to change. The natural fats break down, and harmful compounds such as free radicals, aldehydes, and trans fats start to form.
These substances are not only toxic but can also accumulate in the body over time, increasing the risk of chronic diseases.
What research says
According to research published in the National Library of Medicine by Ajeet Saoji and Prachi Saoji, the repeated use of cooking oil is associated with increased formation of harmful compounds that may contribute to non-communicable diseases such as heart disease, diabetes, and obesity.
The study also highlights that oil reuse is common in many households due to a lack of awareness and economic reasons, making it a public health concern.
One of the biggest concerns linked to reused cooking oil is oxidative stress. Free radicals produced during repeated heating can damage cells in the body.
Over time, this damage may contribute to inflammation, which is linked to conditions such as heart disease, diabetes, and even certain types of cancer. Although the effects are not immediate, regular consumption of food cooked in reused oil may gradually affect the body’s internal systems.
Heart health risks
Another issue is the formation of trans fats. These unhealthy fats are known to raise bad cholesterol (LDL) and lower good cholesterol (HDL), increasing the risk of cardiovascular problems.
The research has shown that oils reused multiple times become thicker, darker, and more unstable, making them even more harmful when consumed.
“Repeated heating of cooking oil leads to the formation of toxic compounds that may increase the risk of chronic diseases,” the study highlighted.
In many households, oil is reused to save money, especially where food is deep-fried frequently. However, this cost-saving habit may come at a hidden health cost.
Foods prepared in repeatedly heated oil may also lose nutritional value while absorbing harmful compounds formed during frying.
The study recommends avoiding the reuse of cooking oil, especially after deep frying. If oil must be reused, it should be strained, stored properly, and used only once or twice at most, depending on the type of oil and temperature used. Discoloured, thickened, or foul-smelling oil should always be discarded immediately.