Cooking certain vegetables in oils at high temperatures may create unhealthy fats linked to the risk of heart disease, according to a new study.

The exact relationship between unhealthy fats and heart disease is debated among scientists and nutritionists, but they agree that one type of fat is unhealthy: trans fats.

Trans fats are often created when manufacturers make ultra-processed foods such as margarine, potato chips, frozen pizza, and store-bought baked goods such as cookies, cakes, and donuts.

The process involves adding hydrogen to unsaturated fats—which are liquid at room temperature, such as vegetable oil—to create a trans fat that is shelf-stable and solid at room temperature.

Eating a lot of trans fat can raise the level of LDL (known as “bad”) cholesterol and lower the level of HDL (known as “good”) cholesterol in your blood, so the American Heart Association recommends that Americans limit the amount of trans fat they eat to reduce their risk heart diseases.

Heart disease is the leading cause of death in the US, accounting for 1 in 5 deaths in 2022 and causing one death every 33 seconds, according to the Centers for Disease Control and Prevention.

But new research from Japan suggests it’s not just ultra-processed foods that could be a problem for trans fats in the diet.

A study by food scientists at Meijo University found that cooking certain vegetables in vegetable oils at high temperatures can encourage the oils to become trans fats.

In particular, they found that this occurred when garlic, onions, and leeks were cooked in unsaturated fats such as olive oil at temperatures above 285°F.

Brassica vegetables such as broccoli, cauliflower, Brussels sprouts and cabbage could also support this process, called trans-isomerization: when healthy unsaturated fatty acids (UFAs) are turned into unhealthy trans-fatty acids (TFAs).

The researchers concluded that this was because these vegetables were high in natural sulfur compounds.

“We were interested in the effect of temperature, reaction time, concentration of sulfur compounds, type of sulfur compounds and the addition of antioxidants on UFA isomerization,” Juni Obi, first author of the paper from Nissui Corporation, said in a statement.

Specifically, the researchers found that sulfur-rich compounds in certain vegetables, namely isothiocyanates—found in cruciferous vegetables such as broccoli and cabbage—and polysulfide—found in garlic, onions, and leeks—can form trans fats at high temperatures.

However, adding antioxidants – anti-inflammatory compounds such as vitamin E – to isothiocyanate vegetables could disrupt this process.

“The release of TFA under normal cooking conditions is expected to be minimal,” said Dr. Masaki Hondo, corresponding author of the paper from Meijo University, in a statement. “In this case, excessive caution is unnecessary.

“However, it is important to understand that cooking with ingredients rich in natural sulfur compounds can increase the risk of TFA intake.”

In other words, even though trans fats were created, they were still a much smaller fraction of the fats that would be in the case of processed foods, and for most people there is nothing to worry about.

But for those already at risk for heart disease, it may be beneficial to cook these vegetables at lower temperatures to reduce the amount of trans fat in the diet.

This article was published in a scientific journal Food Research International.

Link

Obi, J., Sakamoto, T., Furihata, K., Sato, S., Honda, M. (2024). Vegetables containing sulfur compounds promote the trans-isomerization of unsaturated fatty acids in triacylglycerols during the cooking process, Food Research International, 200(115425). https://doi.org/10.1016/j.foodres.2024.115425