Beware! Repeated heating of vegetable oils can cause cancer, heart disease

The practice of ‘reusing’ vegetable oils for cooking is very common, both at homes and in commercial establishments. If you are also doing it. Please stop! It is very harmful and may lead to serious health problems including cancer and heart disease.

In a report released recently, Indian Council of Medical Research (ICMR) warned that repeatedly heating vegetable oil can increase the risk of cancer.

At high temperatures, some of the fats in oil change into trans fats. Trans fats are harmful fats that increase the risk of heart disease. When oils are reused, the amount of trans fats increases.

ICMR along with National Institute of Nutrition released 17 new dietary guidelines for Indians across different age groups to help them make better food choices. The guidelines aim to provide evidence-based recommendations to help Indians make informed food choices to maintain good health and prevent malnutrition in all forms. In addition to dietary recommendations, the guidelines provide information on physical activity, hydration, healthy weight management, food safety and food labelling.

According to the Dietary Guidelines for Indians released earlier this month, “The practice of ‘reusing’ vegetable oils for cooking, (which have been repeatedly heated during food preparations) is very common, both at homes and in commercial establishments. Repeated heating of vegetable oils/fat, results in oxidation of PUFA, leading to the generation of compounds which are harmful/toxic and may increase the risk of cardiovascular diseases and cancer.”

Ipsita Chakraborty, senior nutritionist at Hungry Koala explains, “If you repeatedly heat vegetable oils, it leads to the formation of harmful compounds such as polycyclic aromatic hydrocarbons (PAHs), aldehydes, and acrylamide. These compounds are formed through a series of chemical reactions, including oxidation, hydrolysis, and polymerisation, that occur when oils are subjected to high temperatures multiple times.”

Polycyclic aromatic hydrocarbons, she warns, are known carcinogens that can damage DNA and contribute to cancer development. Aldehydes, particularly acrolein, can also be toxic and irritating to the respiratory system. Additionally, acrylamide, which forms in starchy foods cooked at high temperatures, has been classified as a probable human carcinogen. These compounds pose significant health risks, emphasising the importance of safe cooking practices, the Indian Express reported.

Soybean, sunflower, corn oils more prone to degradation

Some vegetable oils like soybean, sunflower and corn oils are more prone to producing harmful substances. Oils with high levels of polyunsaturated fats, such as soybean, sunflower, and corn oils, are more prone to degradation and the formation of harmful substances when heated repeatedly. This susceptibility is due to the chemical structure of polyunsaturated fats, which contain multiple double bonds that are more reactive and prone to oxidation. 

On the other hand, oils with higher levels of monounsaturated fats, like olive oil and canola oil, and those with higher saturated fat content, such as coconut oil and palm oil, are more stable at high temperatures. Factors contributing to the susceptibility of oils include their fatty acid composition, the presence of antioxidants, and the temperature and duration of heating.

Chronic Diseases

Reheating and reusing oil can have several negative health consequences beyond just cancer. As the oil breaks down with heat, it can release harmful toxins like aldehydes and become more acidic. These toxins can irritate the digestive system and potentially contribute to chronic diseases.

Additionally, reheated oil can have a higher level of trans fats, which are linked to an increased risk of heart disease. The breakdown of the oil also generates free radicals, unstable molecules that damage cells and contribute to inflammation, a risk factor for many health problems. Finally, reheated oil can simply taste bad, imparting a burnt or rancid flavour to your food. While occasionally reusing oil likely won’t cause major problems, it’s best to minimize the practice and use fresh oil whenever possible for optimal health and flavour,

Safe practices

To minimise the risks associated with the repeated heating of vegetable oils, individuals should adopt several safe practices.

“Firstly,  I would  advise to avoid reusing oil multiple times, especially if it has been heated to high temperatures. One must use fresh oil for each cooking session as it can significantly reduce the formation of harmful compounds,” Chakraborty adds.

Secondly, people should choose oils with higher smoke points and greater stability, she elaborates, such as olive oil, avocado oil, or coconut oil, can help mitigate risks. Additionally, incorporating cooking methods that require less oil, such as steaming, baking, or grilling, can reduce the need for high-temperature frying.

Monitoring the temperature during cooking and avoiding overheating the oil can also prevent degradation and the formation of toxic compounds.

Alternative cooking methods 

According to Chakraborty, using air fryers or ovens for baking instead of deep-frying can achieve a similar texture with significantly less oil. “Steaming and boiling are also excellent methods that preserve nutrients without requiring oil.”

When frying is necessary, using oils with high stability at high temperatures, such as avocado oil or ghee, can reduce the formation of harmful compounds. Cold-pressed oils are preferable as they retain more antioxidants that can counteract some of the harmful effects of heating.

Furthermore, integrating more raw or minimally processed foods into the diet can reduce the reliance on vegetable oils altogether.

Consume within a day or two

Advising how vegetable oil can be reused, the apex body suggests filtering it for curry preparations and consuming such oil within a day or two.

“At household level, vegetable oil once used for frying, should be filtered and may be used for curry preparations but using the same oil for frying again should be avoided. Also, such oils should be consumed in a day or two. Storing ‘used’ oils for a long time should be avoided, as the rate of deterioration is high in such oils,” notes the report.

(With input from The Indian Express)