Vegetable Oil: Half-Century-Old Dietary Recommendation May Be Linked to Cancer

When the term “cooking oil” is mentioned, the immediate image that comes to mind for most people is pale and yellow liquid in large transparent bottles labeled “vegetable oil.”

Though vegetable oils are a staple cooking oil for many Americans now, they are only a recent invention. Decades ago, common fats used in cooking were animal fats such as lard, butter, and suet, all of which tend to have a higher saturated fat ratio.

The current switch to vegetable oils can be traced back to researcher and physiologist Dr. Ancel Keys, who hypothesized in the 1950s that replacing animal fats higher in saturated fats with vegetable oils, which tend to be higher in polyunsaturated fat. The vegetable oils would lower blood cholesterol levels and in turn, reduce heart disease.

Yet this hypothesis may not be entirely accurate.

While vegetable oils do lower blood cholesterol, they do not necessarily reduce coronary heart disease mortalities. In fact, most may put consumers at risk of other harm.

Nevertheless, Keys’ hypothesis has persisted.

Starting in the 1960s, the American Heart Association put forward recommendations to switch from animal fat to vegetable oils. The idea influenced the first edition of Dietary Guidelines for Americans in 1980. Both the American Heart Association and the Dietary Guidelines for Americans, valid from 2020 to 2025, have recommended polyunsaturated fatty acid consumption over saturated fat.

What Are Vegetable Oils?

The name “vegetable oil” can be a bit misleading. People may think that because of the word “vegetable,” it is made from plants, which makes it healthy. However, vegetable oil isn’t actually made from plants.

Most common vegetable oils are actually made from both edible and inedible plant seeds. Oils that are made from edible vegetable seeds include corn, soy, peanut, and sunflower oils, while oils made from inedible seeds include canola, cottonseed, and safflower oils. The name “seed oil” is less palatable than vegetable oil. These seed oils are high in polyunsaturated fat and are therefore prone to oxidation.

There are also oils made from the flesh of fruits, such as olive, palm, and coconut oils. These oils differ from most of the common vegetable oils as they have a low polyunsaturated fat content.

Most Vegetable Oils Have 2 Main Problems

Contrary to food guidelines, most vegetable oils on the market may not be suitable for cooking due to two main problems.

1. Prone to Oxidation

Many people know the word oxidation but don’t really know what it is. Oxidation is a chemical reaction where atoms and compounds lose their electrons. These atoms become unstable and thus seek to recover their electrons, so they steal electrons from other compounds. Other compounds then steal from others, and this continues in a vicious cycle.

Oxidation harms the body. If a cell’s DNA becomes damaged from losing electrons to oxidation, the DNA can become unstable and mutate. Cells that contain mutated DNA are at significant risk of becoming cancerous. This is why antioxidants like vitamins C and E that stop oxidation are vital to health.

The problem with polyunsaturated fats, like those found in vegetable oils, is that they are highly oxidative. Polyunsaturated fats have two or more double—or unsaturated—bonds. These double bonds are highly reactive and oxidize when exposed to oxygen.

While polyunsaturated fatty acids spontaneously oxidize at room temperature, oxidation increases by several folds during cooking. This has been shown in the work of Martin Grootveld from De Montfort University in the UK, a professor specializing in bioanalytical chemistry and chemical pathology. He further found that the higher the proportion of polyunsaturated fat, the greater the number of toxic oxidation products formed.

Oils that contain a high amount of omega-3 fatty acids are prone to oxidation, Sally Morell, founder of the Weston A. Price Foundation and author of 10 books on nutrition, told The Epoch Times. This is because omega-3 fatty acids contain the most double bonds in their structure, presenting many opportunities for oxidation. Naturally occurring omega-3 fatty acids have three to six double bonds.

Oils high in omega-3 fatty acids include flaxseed, walnut, and canola oil.

Oxidized oil also goes rancid quickly, making it particularly odorous and unpalatable. For this reason, many mainstream vegetable cooking oils on the market have been refined.

2. Usually Highly Refined, Introduces Potential Toxins

The refinement of vegetable oils involves many industrial chemicals and processes. First, the oil is extracted using a solvent. Hexane, a crude oil constituent solvent, is a popular choice for this process (pdf).

Hexane helps pull the oil from the vegetable and seed pulps, potentially leaving behind hexane residues. Acute exposure to hexane can cause irritation.

The mixture is then degummed for impurities and winterized to separate waxes from the oil. The final steps are bleaching and deodorizing, during which the oil mixture goes through cycles of high heating to remove chemicals and give the final product a better appearance and neutral flavor.

The actual harms of refined oils are unknown, though deodorizing has been shown to cause the formation of trans unsaturated fats, the worst fats to consume, as well as glycidol, a known carcinogen (pdf).

Vegetable Oils and Cancer

Oxidation is linked to cancer, so some experts worry that consuming oxidants from heated vegetable oils may pose a greater cancer risk.

In the 1950s, several randomized controlled clinical trials began testing a low-animal fat diet that switched participants from animal fats, such as milk and cheese, to a polyunsaturated vegetable-fat diet.

While these studies saw reductions in cholesterol levels and often a decrease in heart attacks, long-term and follow-up studies showed that those who had traded animal fats for vegetable oils and proteins often reported higher overall mortality, often with increased cancer deaths.

Starting in the 1980s, several studies also linked low blood cholesterol with cancer, though it is still undetermined why this link exists.

“The people on the high vegetable oil died at higher rates from cancer, and there were a series of very high-level meetings at the National Institutes of Health throughout the 1980s trying to understand this very worrisome outcome,” said investigative journalist Nina Teicholz. “Those concerns were not resolved.”

Teicholz is the first to label seed-derived vegetable oils as “seed oils” and spent a decade researching fats and oils to write her New York Times bestseller “The Big Fat Surprise.”

Although other health experts have also linked vegetable oil with obesity and diabetes, Teicholz said the evidence is usually from observational studies. In contrast, the cancer link was seen in randomized controlled trials, making it more robust.

Saturated Fats Are More Suitable for Cooking

Fats high in saturated fat tend to be the most stable when cooked, Grootveld said. Because saturated fats contain no double bonds, they are much less reactive towards heat and oxygen.

Grootveld, who ranked cooking oils based on their oxidative abilities, said that oils high in saturated fat, such as animal fats like lard and tallow, are the least prone to oxidation. Both lard and tallow contain around 40 to 50 percent saturated fat, accompanied by monounsaturated fat and meager amounts of polyunsaturated fat.

All animal fats contain low amounts of omega-3 and omega-6 fatty acids, but these fats generate only very low levels of oxidation products because of their very high saturated and monounsaturated fat contents.

Some plant-derived oils, such as coconut, are predominantly composed of saturated fat, and are also resistant to oxidation. Palm oil is also resistant since it contains large amounts of both saturated and monounsaturated fats.

Oils high in monounsaturated fat, such as olive and some types of sunflower, are less resistant to oxidation than animal fats and may be suitable for low-temperature cooking at short intervals. These oils are more resistant to oxidation than polyunsaturated seed oils.

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