The trouble with vegetable oils

8 min readApr 18, 2022


Other than not coming from vegetables

Photo by Pete Godfrey on Unsplash

Plant oils would be a better term. They come from seeds (grapeseed, sunflower seed), grains (maize, rice bran), fruits (avocado, olive), nuts (almond, walnut) or legumes (peanut, soy), none of which are usually classified as vegetables. A product labelled vegetable oil will be a blend of refined plant oils. Presumably, refined plant oils are referred to as vegetable oils for health-promoting marketing purposes.

Oil is extracted from plant matter mechanically by compression and/or centrifuge, or by industrial processing. Mechanically extracted oils retain nutrients and antioxidants, and undesirable compounds have not formed under heat or chemical processing. However, the bulk of mass-produced oils come from industrial processing.

Canola oil is one of the most popular of the industrial plant oils. It is recommended by health authorities because its fatty-acid profile is a good match with the US Department of Agriculture (USDA)’s notion of what is healthy. It also has an interesting, if somewhat disquieting, side-history. I will use it as an example. The extraction process is similar for other refined plant oils.

The recent history of canola oil

The rape plant (a brassica, like broccoli or cabbage) is one of the few crops that can thrive on the Canadian prairies. Up until the mid-20th century, rapeseed oil was used industrially. It was popular because of the way it interacted with water, making it the ideal lubricant for steam engines, however this use died out with the engines themselves. While it had other industrial uses, it was not generally sold as food (why would it be?) because it had a high level of the fatty acid erucic acid. This was considered harmful. Nevertheless, it was consumed in some parts of the world (although it had a bitter flavour).

It was not until the 1970’s that Canadian researchers and farmers developed a strain of rape that was low in erucic acid. To distinguish it from earlier rapeseed oils, it was called Low Erucic Acid Rapeseed (LEAR) oil. This was not thought to be a promising marketing name (any more than rapeseed oil was), so the acronym CANOLA was devised from the description: CANadian Oil, Low Acid. To solve the problem that canola oil didn’t come from canola plants (but rather rape plants) the new strain of rape was renamed canola.

To spread into the lucrative US market and beyond, the Canadian government and manufacturers campaigned heavily (and rumours say expensively) for US Food and Drug Administration (FDA) approval for their new oil. The FDA normally requires lengthy scientific studies on the safety of any new food product. It can take decades. However, they can also issue a classification called GRAS — Generally Recognised As Safe. This is quicker and does not require definitive scientific evidence. Common foods such as coffee, vinegar, bread etc are GRAS. It is unusual to classify a new product GRAS. The Canadians, in their urgency, wanted GRAS.

Normally, the FDA issues a GRAS classification on rather vague criteria. Their criteria: “through experience based on common use” and “substantial history of consumption for food use by a significant number of consumers.” Neither of these applied to canola at the time. However, the FDA can issue a GRAS classification if it thinks, on balance, that there is enough evidence to conclude that a product or ingredient is likely to be safe to eat. The scanty evidence can be provided by the manufacturer. This is known, in the vernacular, as ‘the loophole’.

In 1985, the FDA gave canola oil their coveted GRAS classification. The GRAS status was later extended to the use of canola oil in infant formulas. This was re-approved in 2014.

How is rapeseed refined to make oil?

The grain is mashed and mechanically pressed to extract some oil and then rinsed with an industrial solvent (hexane) to dissolve the remaining oil. The oil/solvent mix is heated to evaporate the solvent and to leave behind the oil (and some hexane residue). The oil then undergoes caustic refining, bleaching (with acid-activated clays), de-waxing (the wax is recycled for shortening) and de-gumming using temperature and further questionable chemicals (peroxides for example). Because the oil is highly unsaturated (>90%) and thus prone to rancidity, it must undergo deodorization (steam injection under a high vacuum and temperature), which removes much of the omega-3 fatty acids, converting some into trans fats. To further inhibit rancidity, vitamin E (an antioxidant) is added. There you have it— canola oil. The healthy fat.

The refining process is described more graphically in this video.

Some problems with refined plant oils:

1. Recency

They are new to our diet. We haven’t been eating them for more than about 100 years at most, and in many cases much less (e.g. canola oil ~30 years). They are untested.

Compare that to the time we were eating animal fats — maybe 2 million years. Even olive oil (a few thousand years). While we may have consumed small amounts of plant oil as a result of eating plants, the current levels of consumption are unprecedented for our biology — we didn’t evolve with this stuff. The long-term implications for our health are not known, and indeed very few studies have, or are being, carried out. They are just assumed to be GRAS. It is possible that research is blocked in various ways.

2. Nutritionally bereft

The refining process strips any co-nutrients from the plant oils. They are bereft of vitamins (except any that are added back in), minerals and other beneficial nutrients that are normally present in animal fats or cold-pressed oils such as olive oil.

Refined plant oils are nothing more than fat. Compare that to the nutrient profile of suet (from the USDA database): 52% saturated, 35% unsaturated (nearly all mono-unsaturated); 1.5% protein; 4% water; Vitamins — E, K, B6, B12, folate, thiamin, riboflavin, niacin; Minerals — zinc, sodium, phosphorus, potassium, magnesium, iron, calcium. That’s food.

3. Hexane

Some steps in the refining process are disturbing. For example, hexanes are petrochemicals. They are colourless and odourless liquids at room temperature, and not easily detectable. It is recognised that some hexane will survive the refining process and be present in the final product. The FDA has guidelines for inhaled hexane (for oil refinery workers) but not for ingested hexane. There is no ceiling imposed for hexane in refined plant oils. Instead, the FDA uses “current good manufacturing practices“ — the level manufacturers should be able to stay beneath if they make an effort (another loophole). The US Environmental Protection Agency, EPA, has classified hexane as Group D — not classifiable as to human carcinogenicity. Neither agency sets dietary recommendations because there is no data available to set them. Presumably, gathering such data is not a funding priority.

4. Inflammation

Refined plant oils are inflammatory. They are high in omega-6 (⍵-6) fatty acids. While this is an essential fatty acid in small doses, it is inflammatory at high doses. Inflammatory processes are implicated in many modern diseases, including heart disease, cancers and neurological disorders. Furthermore, ⍵-6 competes with omega- 3 (⍵-3), so the more ⍵-6 we eat, the more ⍵-3 we need. The ⍵-3s are anti-inflammatory.

5. Trans fats

Refined plant oils are usually high in polyunsaturates. An unsaturated fatty acid is a chain of carbon atoms in which some adjacent carbons form a double bond. Most of these bonds have what chemists call the cis form. With heating (i.e. in cooking), some of these double bonds can change to the trans form, that is, a trans fatty acid is created. Even if a manufacturer produces a refined plant oil with negligible levels of trans fats, these can be created during the cooking process. The more double bonds there are, i.e. the more polyunsaturated it is, the more likely it is for trans fats to form.

6. Breakdown byproducts

The double bonds are weak points in the fatty acid carbon chain, and make the fatty acid prone to reactions such as oxidation. Every double bond increases the likelihood of an oxidative reaction (so polyunsaturates are most prone). These reactions increase with heating (as in cooking). Even sunlight is enough to start some reactions (and why speciality oils are sold in dark bottles). Some of the byproducts of these reactions include free radicals, aldehydes, formaldehydes and unspecified polymers.

Aldehydes can be very reactive and damage DNA and disrupt cell functioning leading to cell death. The aldehyde, hydroxynonenal (HNE), present in heated plant oils, may have a role in atherosclerosis. The less said about formaldehyde the better. The polymers are a known problem in fast-food outlets, where they sink to the bottom as sludge, or escape the oil as a fog-like substance and form shellac-like residues on surfaces. A new product had to be invented to clean them off the surfaces. They are inflammable. Other byproducts are the monochlorpropane diols (MCPDs), currently being investigated by the European Union.

In general though, there are few scientific studies into these byproducts, and seemingly little interest in funding such studies. I realise that there will also be breakdown products in animal fats, and that chemicals with complex names can sound alarming (often unnecessarily). So how much any of this really matters for our health I cannot say, but then again, in the absence of data, neither can anyone else.

7. Marketing

Be aware that marketing departments specialise in factual illusions. Many seemingly-pressed oils, including olive oil, could have been refined. Labels like ‘pure’ and ‘light’ fit into this category (‘virgin’ might also). In some cases the oil is refined and some cold-pressed extra virgin oil added back in so that the phrase ‘extra virgin’ can be added to the label. Refining is attractive to a manufacturer because they get the most out of the raw material.

8. You can’t escape

Finally, and sadly, there is no escaping refined plant oils. Of particular concern are food outlets that reuse these oils in their deep frying vats, and maintain them at high temperature for extended periods. No food outlet or restaurant (except high-end ones) would dare substitute tallow or lard for their frying oil, even though these fats, high in saturated fatty acids, are both natural and stable. They would be harassed and arraigned.


At first we were told to get rid of saturated fat, then partially hydrogenated oils (trans fats), now all we have left are plant oils, which themselves have health concerns. Food scientists, stuck with plant oils and needing a more solid version for pastries, biscuits etc, are resorting to exploring interesterification and other techniques or products that are unknowns. Health authorities have created a monster. As a result of their simplistic nutritional advice together with institutional regulation and vested pressure, we have a health situation that these authorities are powerless to acknowledge they have a role in, let alone able to fix.

The first step in the fix is to return animal fats to our diet. You can start when you’re ready, quietly, in the privacy of your home… no one will know.

Recommended reading

Nina Teicholz, “The Big Fat Surprise, Why Butter, Meat & Cheese Belong in a Healthy Diet

Related 6XC links

A beginners guide to fats in 10 questions

Heritage fats — a primer

A guide to rendering heritage fats

Our body-fat composition: the fatty acids we store and how we make them




Science of cooking, eating and health. Retired neuroscientist.