Most of us have reused cooking oil at some point, either to save money, reduce waste, or even enhance the flavour of the next meal. In fact, some dishes seem to taste better when prepared with oil that has been used before. But is the oil that remains after frying safe for reuse?
Several studies suggest that cooking with used oil may be harmful to your health. Clinical nutritionist Valentine Ida Chepngeno explains that once oil is exposed to high heat, it begins to break down through processes like oxidation and thermal degradation. This alters the structure of fats, destroys protective antioxidants such as vitamin E, and leads to the formation of harmful compounds.
“What many people don’t realise is that oil doesn’t just get used, it transforms chemically. So even if it looks usable, its nutritional quality is already compromised after the first heating,” she says.
Valentine says there is no exact number of safe reuses. “Oil degradation depends on temperature, how long it is heated, and the type of food being fried. Though significant deterioration can occur after just one or two high-heat.”
Some deep-fried Bhajia cooked at a fast food joint on the streets of Kibera| Nation Media Group
Photo credit: Francis Nderitu| Nation Media Group
Repeated heating triggers several damaging processes. Oxidation occurs when oil reacts with oxygen, producing toxic compounds such as aldehydes. Polymerisation thickens the oil, while the formation of trans fats further reduces its nutritional value. Over time, these changes turn cooking oil into a source of substances that can negatively affect health, contributing to ageing, weakened immunity, and chronic disease.
Cooking oils, which are basically vegetable oils extracted from coconut, sunflower seeds, palm, cotton seeds or ground nuts, are especially vulnerable to these changes. With each reheating cycle, the concentration of toxic substances such as aldehydes and lipid peroxides increases. These compounds are linked to oxidative stress and cellular damage.
“The more you reuse oil, the more you increase your body’s toxic load, even if the food tastes the same.”
Short-term effects of consuming reused cooking oil include indigestion, bloating, nausea, and general gastrointestinal discomfort. However, she says not everyone will notice immediate symptoms.
“The absence of symptoms does not mean safety. Many dietary risks are silent but progressive,” she adds.
Regular consumption of food prepared in reused oil is associated with an increased risk of cardiovascular disease, hypertension, Type 2 Diabetes and certain cancers.
Children, pregnant women, and individuals with existing health conditions are more vulnerable to the effects of degraded oils. “These groups already have higher nutritional demands, so oxidised fats can worsen deficiencies and health outcomes.”
Valentine warns that straining leftover cooking oil does not make it safe. "Straining only removes visible food particles. It does not eliminate the harmful chemical compounds formed during heating.”
Even relying on your senses can be misleading. Oil that has gone bad may appear darker, thicker, or smell rancid, but it can also appear perfectly normal while still containing harmful substances.
However, Valentine notes that not all cooking oils behave the same way under heat. “Some, like palm oil or high-oleic oils, are more heat-stable, while others rich in polyunsaturated fats break down more quickly. But all oils degrade with time,” she says.
So what is the safest approach? “Avoid reusing cooking oil altogether. If reuse is unavoidable, limit it to one reuse, avoid overheating, and store the oil properly,” she advises.
