Online only
As well as Jennifer Bowden’s columns in the NZ Listener, listener.co.nz subscribers can access her fortnightly Myth-buster column which explores food and nutrition myths. Here, she looks at what the science says about seed oils - like sunflower, canola, soybean and grapeseed - and how it applies to everyday cooking choices.
Seed oils have become a controversial topic, with claims that they cause inflammation, chronic disease and even colorectal cancer. But how much of this is backed by science and how much is speculation?
One of the main concerns about seed oils is their high omega-6 fatty acid content. Omega-6s are polyunsaturated fats that, in theory, can contribute to inflammation in the body. However, the reality is more complex. While omega-6 fatty acids play a role in the body’s inflammatory response, inflammation is not inherently bad; it’s a natural part of our immune function and healing.
The more relevant question is whether omega-6 intake is balanced with omega-3 fatty acids, which have anti-inflammatory properties. Large-scale studies have found that if people consume enough omega-3s (from sources like fish, avocado, nuts and certain seeds), omega-6-rich seed oils do not appear to promote harmful inflammation.
Still, headlines sometimes link seed oils to colorectal cancer, so where does this idea come from? Many concerns about seed oils come from mechanistic studies – research done with cell cultures or animals – to examine potential biochemical effects. While useful, these studies don’t always translate directly to human health.
For example, a recent US study investigating inflammation in colon cancer tumours did not specifically examine diet as a risk factor. However, because the associated press release mentioned how Western diets are typically high in ultra-processed foods and seed oils, this led to the mistaken assumption that the study was reporting that seed oils cause problematic inflammation in colon cancer.
There is no clear scientific consensus that seed oils directly cause cancer. However, there are two valid concerns:
1. Cooking stability: When heated to high temperatures, particularly in frying, Polyunsaturated Fatty Acids (PUFAs) in seed oils can generate harmful lipid oxidation products (LOPs). These compounds, such as aldehydes, have been linked to oxidative stress and inflammation, which can contribute to cancer development.
2. Ultra-processed foods: Many processed foods contain seed oils alongside refined carbohydrates, additives and preservatives. Research suggests that a high intake of ultra-processed foods may increase the risk of colorectal cancer. So, the concern is a dietary pattern of processed foods rather than the seed oils themselves.
Repeatedly heating oils, as in commercial deep-frying, can degrade them and produce harmful compounds. For home cooking, this is less of an issue if you choose the right oils with an appropriate smoke point – which is the temperature at which the oil begins to break down and gives off smoke, causing a deterioration in flavour and quality:
· High-heat cooking (frying, roasting): Avocado oil, refined canola oil, refined sunflower oil, refined peanut oil, refined sesame oil.
· Medium-heat cooking (sautéing, baking): Light or refined olive oil, refined canola oil, refined peanut oil.
· Low-heat cooking (salad dressings, drizzling): Extra virgin olive oil, flaxseed oil, walnut oil.
· Oils to avoid for high-heat cooking: Highly unsaturated oils like soybean and safflower oil, which oxidise and polymerise at high temperatures.
The World Cancer Research Fund (WCRF) offers dietary advice for colon cancer prevention based on various research studies, including large-scale epidemiological studies and clinical trials, not just mechanistic studies. The WCRF guidelines recommend:
· Eat whole, minimally processed foods.
· Consume plenty of fibre from vegetables, fruit, legumes and whole grains.
· Limit processed and red meat intake.
· Minimise alcohol intake.
In short, there is little evidence that seed oils are inherently harmful when included in a balanced diet. The real concerns stem from how they are used, particularly in high-heat cooking or within ultra-processed foods.
So, to make the best choices, diversify dietary fat sources by regularly including olive oil, avocado, nuts, seeds and fish. Also, use the appropriate oils for different cooking methods. Finally, focus on creating a sustainable dietary pattern of whole, minimally processed foods for life rather than concentrating too much on individual ingredients.
