Is It Safe to Eat Vegetable Oil Often?

Is It Safe to Eat Vegetable Oil Often?
(coffeekai/iStock)
Camille Su
1/29/2023
Updated:
4/27/2023
Compared with cold-pressed virgin olive oil in glass bottles, the plastic jugs of vegetable oil offer a real bang for your buck, which may be why they can be found in almost every kitchen. But how often can you use vegetable oil to fry, sauté, or deep-fry your meal without causing health issues?

An Issue of Carcinogenic Toxins

The raw materials of commercially available vegetable oils, such as soybean oil, corn oil, sunflower oil, rapeseed (canola) oil, and grapeseed oil, aren’t as high in oil content as olives, so it isn’t possible to simply press out the oil—it has to be extracted through a much more complex process.

The method involves soaking the raw materials in an “organic solvent” such as hexane to extract the oil. Then the materials are heated and the solvent is removed through evaporation. Often, this process also includes degumming, deacidification, decolorization, and deodorization. In other words, by the time you get your bottle of vegetable oil, those initial raw ingredients have been intensively processed.

Degumming removes lecithin and other colloids in soybeans, for example. The lecithin is usually then processed into soy lecithin health products.

Deacidification involves adding alkali (such as sodium hydroxide) to convert free fatty acids, which aren’t conducive to long-term storage of oil products, into soapstock through a “saponification” reaction, and then removing them. This can increase the stability of the oil.

Deacidification is usually followed by decolorization. There are a lot of impurities, pigments, and soapstock left over from deacidification, so activated clay will be added and heated to more than 212 degrees F under a vacuum. This allows the activated clay to adsorb the substances, thereby clearing the oil.

The final step is deodorization. All the substances remaining in the oil, such as objectionable odors and decomposed peroxides, will be sucked away after being exposed to a heat of more than 390 degrees F under high vacuum pressure. Once all the steps are complete, refined and pure vegetable oils remain.

Tung Chih-hong, deputy director and senior research scientist at the Food Industry Research and Development Institute in Taiwan, said that unless packaging emphasizes that the oil is cold-pressed, these oils will have gone through this high-temperature refining process, which is more common because it ensures the stability required for mass sale and long-term storage.

Unfortunately, this method produces some unhealthy byproducts, including trans fats, monochloropropanediol (MCPD), glycidyl fatty acid esters, and 4-hydroxynonenal.

Trans Fats

Trans fats can’t be metabolized and used by the body, so they accumulate inside of it. Trans fats have been shown to increase the risk of coronary heart disease. They also have adverse effects on the brain and nervous system and may play a role in the development of Alzheimer’s disease and cognitive decline with age.
Studies have also shown that trans fats are directly related to breast cancer, colon cancer, diabetes, obesity, and allergies.

MCPD, Glycidyl Fatty Acid Esters

MCPD and glycidyl fatty acid esters are converted into free MCPD and glycidol after entering the body. Glycidol is a group 2A carcinogen, which is a substance that is probably carcinogenic to humans and has genotoxicity and carcinogenicity. MCPD is a group 2B carcinogen and is possibly carcinogenic.

4-Hydroxynonenal

Vegetable oil is high in linoleic acid (omega-6), which has a low heat resistance and will produce 4-hydroxynonenal at high temperatures.
Four-hydroxynonenal is toxic and has been linked to a number of conditions, including inflammatory and degenerative diseases such as atherosclerosis, liver damage, Parkinson’s, Alzheimer’s, and cancer.

All oils contain omega-6 fatty acids, just in different proportions. Oils high in omega-6 include safflower oil, grapeseed oil, sunflower oil, corn oil, cottonseed oil, soybean oil, sesame oil, peanut oil, etc. Although omega-3 fatty acids are generally healthy, too many omega-6 fatty acids is problematic, especially without enough omega-3 fatty acids to balance them out.

The high-temperature refining method of vegetable oil has some unhealthy byproducts. (Shutterstock)
The high-temperature refining method of vegetable oil has some unhealthy byproducts. (Shutterstock)

The highly industrialized extraction process used for these oils also comes with unintended consequences.

“All foods processed at high temperatures undergo some reaction and may produce some trace substances. This is a normal phenomenon,” Tung said. “It will not cause harm to the body if it is kept below a certain amount.”

Is it true that these byproducts can only be controlled and reduced in amount but can’t be completely avoided?

Chao Ming-wei, a toxicologist and associate professor in the biotechnology department at Chung Yuan Christian University in Taiwan, answered frankly, “It is really difficult.”

Take 4-hydroxynonenal as an example: Vegetable oil will produce oleic acid and linoleic acid during the high-temperature extraction process, and linoleic acid contains 4-hydroxynonenal.

The key lies in the amount. There are now regulations around the world to ensure that the byproducts in these vegetable oils don’t exceed a certain number of parts per million or billion if they’re sold to the public. Legal manufacturers around the world try to reduce these byproducts in the process of refining oil products.

Cooking Makes Vegetable Oil Toxic

There are regulations governing the amount of toxins in vegetable oil, but those don’t protect us from what we do with it. That’s because the moment we use it in high-temperature cooking, we start another chemical change.
High-temperature deep-frying, pan-frying, or sautéing will all change these oils. In reality, vegetable oil with high omega-6 content is only suitable for stir-frying on medium heat because of its low heat resistance.
Dr. Yen Tzung-hai, a professor in the department of nephrology in the Clinical Poison Center at Chang Gung Memorial Hospital in Taiwan, pointed out that the frying temperature may reach up to 392 F. Omega-6-rich oils are prone to peroxidation during frying, resulting in the formation of 4-hydroxynonenal.

For cooking at higher temperatures, you need a different kind of oil. He mentioned that oils with high smoke points and high saturated fat, such as lard and palm oil, should be used when frying food, but make sure not to fry in them repeatedly.

East Asian cuisine often calls for sautéing at high heat when cooking, which involves stir-frying spices such as shallots and garlic in oil to force out the aroma. However, the oil temperature continues to rise during this process, exceeding the smoke point and emitting oil fumes. Chao said that during the process of high-temperature sautéing, the toxins in vegetable oil will start to oxidize and volatilize, exposing people to these chemicals. When the cook inhales, these fumes can even cause lung damage and increase the risk of lung cancer.
Vegetable oil isn't suitable for high-temperature deep-frying, pan-frying, or sautéing, as it can produce toxins. (Jack Jelly/Shutterstock)
Vegetable oil isn't suitable for high-temperature deep-frying, pan-frying, or sautéing, as it can produce toxins. (Jack Jelly/Shutterstock)

Besides avoiding high-temperature cooking, we also need to ensure that the vegetable oil is stored properly.

Vegetable oil should be kept in a cool place at room temperature to avoid oxidation due to exposure to light. Most bottles are made of transparent plastic, but it’s best to choose a dark bottle, or better yet, a glass bottle. “This is because oil products are indeed unstable,” Chao said.

The plastic bottles of vegetable oil sold in supermarkets and retailers are mainly made of type 2 (high-density polyethylene, HDPE) and type 5 (polypropylene, PP) plastics, which are resistant to corrosion, acid and alkali, high temperatures, and have no plasticizer. If you wish to repack oil at home, a glass bottle should be your first choice, followed by type 2 and type 5 plastic bottles. Don’t use type 3 plastic (polyvinyl chloride) bottles, as they contain added plasticizers to improve plasticity.

Is Cold-Pressed Oil Safer and Healthier?

Pressed oils such as olive oil and tea seed oil are healthier because they are unrefined and thus retain more of their original nutrients.

Chao said that, generally, consuming a little vegetable oil every day will not cause too much harm. However, given the choice, he recommends opting for cold-pressed oils such as olive oil, which are healthier.

But is cold-pressed oil necessarily better?

Tung pointed out that cold-pressed oil stored in a poor environment or with poor raw-material quality may also produce some aflatoxin or other problems.

Moreover, cold-pressed oil doesn’t necessarily preserve better than extracted oil. Although cold-pressed oils retain more of the original plant nutrients, they also retain some harmful substances and odors.

Take olive oil, for example. Because it retains its original polyphenols, odor, and pigment, it isn’t suitable for food processing because it isn’t as stable and doesn’t have as long of a shelf life as vegetable oils.

In addition, there are different grades of olive oil. The processes of making extra-virgin and virgin olive oils involve washing and crushing the olives and then centrifuging them to separate the oil.

Some of the oil remaining on the pomace can be pressed under stronger conditions or extracted with solvents—that is, through a high-temperature refining process. The refined oil is called pomace oil, and manufacturers will mix it with virgin olive oil to form what is marketed as “pure olive oil.”

If the olive oil isn’t labeled as cold-pressed virgin or extra-virgin on the package, it’s a blend of refined oil and virgin oil. As consumers, we should pay attention to labels when purchasing products.

Camille Su is a health reporter covering disease, nutrition, and investigative topics. Have a tip? [email protected]
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