People tend to have strong feelings about food. A friend of mine was cooking dinner for their family only for someone to exclaim “Are you trying to poison us?” The poison in question: sunflower oil, which, alongside “seed oils” in general, has been gaining a bad reputation on social media recently. Other seed oils include canola oil, peanut oil, and sesame oil. But why are these oils in particular so “poisonous”?

Seed oil sceptics’ critiques tend to focus on how the oils are made, a lack of research on long-term effects of seed oil consumption, byproducts created during the cooking process, and their use as industrial lubricants [1]. While some of these are valid concerns, there are a few shortcomings in these arguments against seed oils. Let’s start with the lack of long-term studies on seed oils. The ethics and practicality of feeding one group of people a diet high in seed oils and another group a diet high in other sources of fat make conducting long-term studies difficult [2]. Instead, two types of trial are commonly used: observational studies, in which the habits of large populations of people are analysed to see if there is a link between diet, lifestyle choices, and health; and randomised control trials, in which two groups of participants are fed different diets over a shorter period of time to see whether there’s any difference in health outcomes [3].

In terms of seed oils, one randomised control study that sceptics often cite is by the LA Veterans Administration from the late 1960s. Unfortunately, there were no links to this study in the original video [1], so I had to do some digging. Curiously, I found the study on the page of someone who was debunking a critic of seed oils [4]. How can one study be used to both support and condone seed oils?

The LA veterans study took place in a rest home of older (65+) men for around eight years. Initially, the researchers found that there was a drop in mortality from those adhering to the diet high in unsaturated fats (i.e., eating seed oils). The main cause of death in the diet high in saturated fats was heart disease. However, towards the end of the trial, those who were fed seed oils had a slightly higher mortality rate from cancer [5]. So depending on which piece of evidence you select, seed oils either appear good or bad for you. The reality, as with most science, is more complex.

More recent studies agree that the unsaturated fats from seed oils lower the risk of heart disease when compared to animal-based fats [6], [7]. A study comparing mortality between plant-based seed oils and lard found that consumers of seed oils had a lower mortality rate [8]. This study had a longer timeline (data taken every 2–3 years between 1989 and 2011) than the eight-year veteran study. But what about the increased cancer risk? Trans fats, such as those found in margarine in the 1960s [9], [10] have been found to increase the risk of cancer [11]. However, a recent review article on the types of fats found in seed oils indicated that, so long as used carefully while cooking, any increase in cancer risk was marginal [12]. In a moment, I’ll get to the ways in which the cooking process changes this. For now, let’s take a look at how seed oils are made.

Like many foods, seed oils come from a plant. The seeds of the plant are harvested, cleaned, and hulled to start exposing the oil, and then heated and crushed to further release oil [13]. Some artisanal oil brands will stop extracting oil here, but most large brands will try to get as much oil out of the solids as possible. At this point, the solid remainder of the seeds looks like a birdseed patty, called a cake [14]. This cake then undergoes a solvent extraction process [15], and here is where things start to get… messy. The solvent used is hexane, and it’s not a pleasant chemical to work with. It’s highly flammable, causes skin irritation, and may damage organs through repeated exposure [16]. However, the amount of hexane remaining in the final product is miniscule. In New Zealand, there must be less than 20 mg of hexane per kilogram of oil. To start to see any effects from the hexane left in oil, you would have to drink over 33L at once,1 at which point you’d be experiencing other issues!

What about the other differences in the chemical makeup of seed oils versus olive oil and tallow? All types of cooking oil are made out of long-chain fatty acids, which are often shortened to “fats”. The exact chemical composition of these oils depends on where they come from. Sunflower and soybean oil contain mostly linoleic acid, and palm oil contains palmitic acid [13]. Tallow and butter, on the other hand, contain more saturated fatty acids [17], [18]. As established, monounsaturated fatty acids in seed oils are considered to be better for you than the saturated versions found in animal fats. However, things start to get complicated when you heat them up.

At high temperatures, chemicals often start to change their structure. The unsaturated fatty acids in the oil break down, forming some concerning compounds: polycyclic aromatic hydrocarbons (PAHs) [19], [20]. Consuming some types of PAHs has been linked to cancer [20]. However, other reviews have found the effects of these volatile compounds on health negligible [2].

Peanut oil and sunflower oil tend to have higher smoke points than canola and rice bran, so may be better choices. When cooking, keeping the temperature of the oil low (below 200 ºC) helps to reduce the amount of PAHs released [2o], [21]. Repeated heating and cooling also lowers the smoke point of the oil, which breaks down more of the unsaturated fatty acids [12], [20]. Avoid this where possible.

Drawing the line between conspiracy and speculation becomes difficult with ingredients as ubiquitous as seed oils. The scientific uncertainty of health outcomes [22], vested commercial interest of large food corporations, and complex chemical processes in cooking allow speculations to grow rife.

One such speculation maintains that seed oils are the main cause of the obesity epidemic. While the obesity epidemic over the past fifty years has coincided with the uptake of seed oils during cooking, this doesn’t mean that seed oils are solely responsible [2], [4].

Oil, as a whole, isn’t good for your health [23]. This much is known. The chances are that seed oils aren’t any worse for you than any other type of fat used in cooking. The realistic alternatives to oil are minuscule, as animal-based fats are not only equally unhealthy, but more environmentally damaging [24]. And truthfully, food just doesn’t feel the same with the fat removed [25], [26]. In small amounts, cooking with oil can even improve the taste and texture of food.

And me? I’ll still be cooking my food with canola oil.