Researchers have discovered for the first time in food the presence of certain aldehydes suspected of being behind neurodegenerative diseases and some types of cancer. These toxic compounds appear in some oils when heated to frying temperature and even more so when reused. Reheating oils isn’t really healthy, and here’s what you need to know about it.

Heating and reheating oils at frying temperature creates toxic compounds.

If you’re like most people, you probably reuse your frying oil many times. After all, it can be expensive and it may seem pointless to throw it away.

However, reheating oils poses health risks. How many times you can safely reuse it will depend on what type of oil it is, at what temperature it was heated, and for how long.

Every time the oil is heated, its fat molecules break down a bit. This brings it to its “smoke point”, the temperature at which it literally begins to smoke and emit a bad odor, more quickly each time it is used.

When this happens, unhealthy substances are released both into the air and into the food that is cooked.

The study.

The work confirms the simultaneous presence in the frying oil of several toxic aldehydes from the group of α, β unsaturated oxygenates, such as 4-hydroxy- [E] -2nonenal.

Furthermore, two of them are described for the first time in foods (4-oxo- [E] -2-decenal and 4-oxo- [E] -2-undecenal). Until now, these substances had only been cited in biomedical studies, where their existence in the body is related to various types of cancer and neurodegenerative diseases, such as Alzheimer’s and Parkinson’s.

Toxic aldehydes are generated as a result of the degradation of the fatty acids in the oil and, although some are volatile, others remain in the oil after frying. This way, if you tend to reheat oils, they can be incorporated into cooked food. As they are very reactive compounds, they can react with proteins, hormones and enzymes in the body and prevent their proper functioning.

Health risks of reheating cooking oils.

Toxic fumes.

An oil heated to high temperatures releases toxic fumes. The fumes are emitted even before the smoke point is reached, but increase dramatically when the temperature exceeds the smoke point.

One study found that canola oil, which has a high smoke point, emitted the lowest level of toxic fumes. The coconut oil gave off the most smoke and at the lowest temperature. Extra virgin olive oil and safflower oil were in the middle.

Another experiment consisted of heating three types of oil to 190 ºC in an industrial fryer: olive, sunflower and flax. The operation was carried out for 40 hours (8 hours a day) in the first two and in 20 hours in the case of flax oil. The latter is not commonly used for cooking in the West, but has been selected for its high content of omega 3 groups.

The results reveal that sunflower and flax oils – especially the former – are the ones that generate the greatest number of toxic aldehydes and in less time. On the contrary, olive oil, which has a higher concentration of monounsaturated acids (such as oleic), generates less and later these harmful compounds.

Free radicals.

The free radicals are also produced when an oil reaches its smoke point. These are molecules that damage the cells of our body and can potentially cause cancer.

They are absorbed in foods that are fried and then can cause cancer or other illnesses in people who eat those foods.

Trans fat.

Trans fats are “bad” fats that increase your risk of heart disease. At high temperatures, some of the fats in the oil turn into trans fats. When the oils are reused, the amount of trans fat increases even more.

A study that looked at olive and peanut oils found that trans fats increased significantly in both when they were reheated.


As mentioned above, HNE (4-hydroxy-2-trans-nonenal) is a toxic substance that can increase the risk of Alzheimer’s, Parkinson’s, stroke, and LDL (“bad”) cholesterol. Heating oil at 190 ° C or above causes HNE to form. While using oil only causes HNE to form, reusing it will cause even more HNE to build up.

How to know when it’s time to change your cooking oil?

By now you may be thinking: “But I’ve always reused the oil! Can I really use it only once? ”

The safest thing is to use new oil each time. However, there are some things you can do to limit the risks of reusing it.

  • If the oil looks cloudy, has foam, or has a bad taste or smell, it’s time to throw it away.
  • If it’s still fine and you plan to reuse it, filter the oil through a few layers of cheesecloth to remove any food particles before storing. Store the oil in a sealed, light-proof container and keep it in the refrigerator if possible. Storing it in a cool, dark place is the next best option.
  • Don’t mix different types of oil together.
  • Fry foods that do not exceed 190 C. Use a thermometer to check the temperature.
  • Shake off excess batter from food before frying to limit food particles left in the oil.
  • Choose stainless steel pots or pans for frying. Avoid iron or copper, as these will damage the oil more quickly.
  • Finally, turn off the heat right after you’ve finished cooking. The longer the oil is heated, the faster it will damage.
  • Even if the oil still seems fine, experts recommend discarding it after 3 months.

Choosing the healthiest oil for frying.

While it may seem logical that healthier oils, such as olive oil, are the best options for frying, this is not always the case.

The healthiest oil to use will depend on the temperature you are going to use. Each oil has its own approximate smoke point:

  • Unrefined palm oil at 235ºC.
  • Safflower, peanut and soybean oil at 232ºC.
  • Canola at 224ºC.
  • Corn, Olive, Sesame and Sunflower at 210ºC.

When an oil is heated above its smoke point, toxins increase considerably. Therefore, the choice of which oil to use must take into account both the health of the oil itself and the temperature that will be used.

For example, while olive oil is healthy at lower temperatures, if you are going to fry food above 210 C, it would not be a good option.

Palm, peanut, safflower, or soybean oils can be safely heated to the highest temperatures.  However, these oils contain a large amount of unhealthy saturated fat.

Canola oil may be the best option in many cases. It contains the most unsaturated fats of any oil (even more than olive oil) and has a higher smoke point than olive oil. Also, it is generally much less expensive than olive oil.