A staple in our kitchens, oils play an important role both in our day-to-day cooking and as part of a healthy, balanced diet. However, recent attention-grabbing headlines have declared seed oils ‘toxic’, raising concerns over the safety of store cupboard staples such as sunflower, rapeseed and corn oil.
What are seed oils?
Seed oils (aka vegetable oils) are plant-based cooking oils made from the seeds of the likes of sunflower, sesame and grapes – as opposed to oils made from fruit such as olive and avocado, or nuts such as walnut and almond. A sprinkle of seeds over your breakfast cereal or enjoyed as a snack can be a healthy addition to your diet, so you’d think their oils would be too.
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Critics argue that most of these oils are industrially refined. This may involve adding a chemical solvent such as hexane to extract the oil, followed by cleaning, bleaching and deodorising; it’s these processes that potentially leave harmful residues in the oil. The oils may also be subjected to high temperatures that can alter and damage their delicate structure as well as compromise their nutritional value.
Most of us use these oils in home cooking; they’re often the oil of choice for commercial food outlets and they’re used in a wide range of processed and ultra-processed products and even ‘healthy’ snack foods that are now commonplace in our diets.
As a consequence, we’re eating a lot more of them. Thanks to the significant changes seen in food production over the past 50 years combined with our time-poor lives, we include more of these processed and ultra-processed foods (UPFs) in our weekly shop.
An increasing number of studies, including one from 2023 published in Advances in Nutrition, suggest an association with high intakes of UPFs and health conditions such as type 2 diabetes, high blood pressure and obesity. That said, there are many factors beyond their seed-oil content that may explain why UPFs are not good for our health. These may include their high sugar, salt and starch levels, the use of additives, the change in the food’s natural structure and many more besides.
Do seed oils cause inflammation?
Seed oils are high in linoleic acid, a type of omega-6 polyunsaturated fatty acid; this is an essential fat because we need it in small amounts and we have to obtain it from our diet. Our bodies use these fats for brain and heart health as well as for our skin and hormones.
When we eat too much omega-6 compared to another essential fat called omega-3, it can lead to imbalances in the body and promote inflammation. A 2022 World Health Organisation meta-analysis reported that a higher ratio of omega 6:3 was associated with a greater risk of cognitive decline, as well as gut issues such as colitis.
Over time the inflammation brought about by this imbalance can increase our risk of heart disease and cancer. We need both of these essential fats, but our Western diets are heavily skewed towards omega-6 at the expense of omega-3 – this is where the problem lies.
Can I still use seed oils?
Oil is central to many of our favourite recipes, from dressing a salad to sautéing, roasting and frying, so it may come as some comfort to know that a 2025 observational study of 200,000 people reported in JAMA Internal Medicine, concluded that substituting butter with seed oils was associated with lower mortality and had other benefits, too.
Moreover, although residues from processing may be detected in some oils, they’re at levels far below the maximum residue levels (MRLs) set for food and considered safe.
The best advice is to use refined seed oils in moderation; for salad dressings and lower temperature cooking opt for cold-pressed seed oils that are not subject to chemical processing.
Where you do use seed oils consider rapeseed because it’s backed by more research, it contributes some omega-3 and it offers positive effects on cholesterol levels.
In addition to seed oils and if your budget stretches to it, try to include a variety of healthy fats including olive, avocado and nut oils. Also include food sources of omega-3, the very best of which are oily varieties of fish, including mackerel and sardines.
Finally, limit how often you reheat oil. Repeatedly heating oil causes oxidation; this damage may lead to the build-up of harmful compounds and also makes the flavour and smell of the oil less appealing.
Top image credit: Getty
