Cooking garlic and onions at high heat can product harmful fats

Trans-fatty acids (TFAs) are harmful fats linked to serious health problems, especially heart disease.

They can build up on artery walls, restricting blood flow and increasing the risk of heart attacks.

According to the World Health Organization (WHO), TFAs cause more than 278,000 deaths every year worldwide. To reduce this risk, the WHO advises that TFAs should make up less than 1% of our daily energy intake.

TFAs are often associated with processed and fried foods, such as margarine, ghee, biscuits, and cakes. These fats are created during a chemical process called hydrogenation, where vegetable oil is treated with hydrogen to make it more solid and increase its shelf life.

While TFAs in processed foods have been widely studied, researchers have found that these fats can also form at home during cooking.

When unsaturated fatty acids (UFAs) in vegetable oils are heated to 150°C or higher, their molecular structure can change, producing TFAs. Vegetables that contain sulfur compounds, such as garlic, onions, and cabbage, are also suspected to influence the formation of TFAs during cooking.

These sulfur compounds are known to cause similar changes in other molecules, like carotenoids, which are natural pigments that give vegetables their bright colors. Scientists wondered if sulfur compounds might also encourage the transformation of UFAs into TFAs during regular cooking.

To explore this question, a research team in Japan, led by Mr. Junji Obi from Nissui Corporation and Dr. Masaki Honda from Meijo University, studied the effects of sulfur compounds on TFA formation. Their findings were published in Food Research International on November 27, 2024.

The researchers started by testing how sulfur compounds interact with vegetable oils in controlled lab conditions.

They examined various factors, such as temperature, cooking time, the type of sulfur compounds, and the presence of antioxidants, which are substances that can reduce harmful chemical reactions.

Then, they tested real cooking scenarios using sulfur-rich vegetables like garlic, onion, leek, cabbage, horseradish, and broccoli sprouts, along with common vegetable oils like soybean and olive oil.

The study showed that sulfur compounds significantly promote the formation of TFAs when vegetable oils are heated above 140°C. Vegetables such as garlic and onions, which are rich in sulfur compounds called polysulfides, played a major role in this process.

However, the researchers also found that adding antioxidants, such as vitamin E (α-tocopherol), could reduce the formation of TFAs when certain sulfur compounds, like isothiocyanates, were present. Unfortunately, antioxidants had little effect on reducing TFAs caused by polysulfides.

These findings suggest that cooking vegetables rich in sulfur compounds, like garlic and onions, with vegetable oils at high temperatures can lead to the production of small amounts of TFAs. However, the increase is likely to be minimal under normal cooking conditions—only a few percent at most.

“People don’t need to be overly cautious about this,” said Dr. Honda, the lead researcher. “The release of TFAs during regular cooking is very small. Still, it’s good to know that using sulfur-rich vegetables in high-heat cooking might slightly increase TFA intake.”

This research highlights the importance of understanding how everyday cooking habits might contribute to TFA exposure. While the risk is low, being aware of these factors can help people make informed decisions about their cooking methods to maintain better heart health.

If you care about nutrition, please read studies that whole grain foods could help increase longevity, and vitamin D supplements strongly reduce cancer death.

For more information about nutrition, please see recent studies about natural coconut sugar that could help reduce blood pressure and artery stiffness, and whey and soy protein may reduce inflammation in older people.

The research findings can be found in Food Research International.