Don't cook garlic, onions on high heat! It may harm your heart health, claims study

Cooking garlic and onions in vegetable oil at high temperatures can generate trans-fatty acids (TFA) and can prove to be detrimental to heart health

Cooking garlic and onions in vegetable oil at high temperatures can generate trans-fatty acids (TFA) and can prove to be detrimental to heart health, claims a study by Japanese researchers.

TFAs are harmful fats that can accumulate along artery walls, restricting blood flow and increasing the risk of heart attacks.

While TFAs are commonly present in processed foods, evidence suggests that they can also be created at home during cooking. Studies indicate that unsaturated fatty acids (UFAs), usually considered beneficial, can undergo trans-isomerisation -- a molecular reconfiguration that transforms them into TFAs when heated at 150 degrees Celsius or higher.

To explore, researchers from Meijo University assessed the role of isothiocyanates and polysulfides -- sulphur-containing compounds found in vegetables like garlic, leeks, onions, scallions, and shallots -- in promoting the trans-isomerisation of vegetable UFAs during cooking.

The team first evaluated the effects of sulphur compounds on triacylglycerols (TAGs) in a model system using reagents. Then they tested garlic, onion, leek, cabbage, horseradish, broccoli sprouts, and vegetable oils like soybean and olive oils to simulate actual cooking processes.

The results, published online in the journal Food Research International, showed that sulphur compounds significantly promote heat-induced trans-isomerisation of UFAs in vegetable oils. This occurs especially when cooking temperatures are above 140 degrees Celsius.

Further, the team also assessed the role of antioxidants such as alpha-tocopherol in reducing the isomerisation of UFAs in triglycerides like triolein and trilinolein.

While the addition of antioxidants significantly reduced the promotion of UFA's isomerisation by isothiocyanates, they did little to inhibit the promotion of isomerisation by polysulfides.

“This explains that cooking polysulfide-rich vegetables such as garlic and onions in vegetable oil at high temperatures can generate TFAs. The study demonstrates that garlic and onions significantly promote the trans-isomerisation of UFAs,” said the team led by Dr. Masaki Honda at Meijo University.

On the other hand, the release of TFAs under normal cooking conditions is minimal.

"Cooking with ingredients rich in natural sulphur compounds may increase the risk of TFA intake," the researchers said.

According to the World Health Organization (WHO), TFAs are responsible for over 278,000 annual deaths worldwide. The global health body recommends that TFA consumption should be limited to less than 1 per cent of daily energy intake.

Also Read: Love coffee or tea? These beverages may lower risk of head and neck cancer, claims study

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