Seed Oils: What They Are and
Why You Should Avoid Them

What are seed oils?

Seed oils (often marketed under the generic name "vegetable oil") are oils extracted from the seeds of plants rather than from whole fruits (like olives or avocados) or animal fats. Common examples you'll find in UK supermarkets include sunflower, rapeseed (often called canola internationally), soybean, corn, and grapeseed oils.

Unlike butter or olive oil, which can be produced by simple churning or pressing, seed oils require an industrial process to extract. This involves high heat, chemical solvents like hexane, and multiple refining steps including bleaching and deodorisation. By the time it reaches your pan, the oil has been through a process far removed from the natural source.

Why are they a problem?

There are two primary concerns with seed oils: their high concentration of linoleic acid (an omega-6 fatty acid) and their inherent instability. Linoleic acid is a polyunsaturated fatty acid (PUFA). While we need small amounts of it, the modern diet provides it in massive, unnatural quantities.

PUFAs are chemically unstable because they have multiple double bonds in their structure. This means they oxidise easily when exposed to heat, light, or air. When you heat sunflower oil in a frying pan, it begins to break down into inflammatory compounds called aldehydes. Consuming these oxidised fats regularly triggers systemic inflammation, which is a driver for many modern chronic diseases.

The omega-6 to omega-3 ratio

Humans evolved eating a diet where omega-6 and omega-3 fats were consumed in roughly equal measure — a 1:1 ratio. Modern Western diets, largely due to the dominance of seed oils in processed foods and restaurant cooking, have pushed that ratio to somewhere between 15:1 and 20:1.

This chronic imbalance keeps the body in a pro-inflammatory state. While a low-carb diet naturally improves your health by reducing insulin, you only get the full benefit if you also address the quality of the fats you are using to replace those carbs.

What about "heart healthy" claims?

You'll still see "heart healthy" labels on bottles of rapeseed and sunflower oil. This guidance stems from observational studies in the 1960s and 70s that showed replacing saturated fat with polyunsaturated fat lowered LDL cholesterol. For decades, it was assumed that lowering LDL automatically meant fewer heart attacks.

However, more recent analysis of controlled trials — including the Sydney Diet Heart Study and the Minnesota Coronary Experiment — found that while LDL did indeed fall when people switched to vegetable oils, the risk of death from heart disease actually stayed the same or even increased. It is an active and often heated area of research, but for many in the low-carb community, the evidence for avoiding these highly processed, unstable oils is compelling.

Seed oils and low-carb eating

When you switch to a low-carb or keto way of eating, you naturally increase your fat consumption. Fat becomes your primary fuel source. This makes the quality of that fat more important than ever.

Cooking with oxidised seed oils while your body is trying to heal its metabolism is like trying to put fire out with petrol. Combining a high-fat intake with unstable, inflammatory PUFAs is potentially more damaging than a standard high-carb diet. If you're going to eat more fat, it must be the right kind of fat.

Oils to avoid

Oils to use instead

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Hidden seed oils in processed food

Once you start reading labels, you'll realise that seed oils are everywhere. They are the base for almost every supermarket mayonnaise, salad dressing, and pesto. They are in protein bars, crisps, biscuits, and even "healthy" nut butters.

One of the easiest wins for your health (and your wallet) is making your own mayonnaise. Standard supermarket mayo is almost 80% rapeseed or sunflower oil. We've done the maths on this: making your own mayo costs a fraction of the price of premium brands.

Eating out in the UK

Restaurants almost universally use rapeseed or sunflower oil because it's cheap and has a high smoke point. Choose grilled meats over fried ones, and avoid anything battered or deep-fried. Ask for extra virgin olive oil and vinegar for your salad rather than the "house dressing." While chains like Nando's or Wagamama provide allergen info, they rarely specify the oil type, so sticking to simple, grilled options is always the safest bet.

Practical tips for avoiding seed oils

• Audit your kitchen: Clear out the sunflower, vegetable, and rapeseed oils. Replace them with butter, olive oil, and coconut oil.
• Read every label: Make "no seed oils" a non-negotiable rule when buying packaged food. If it contains vegetable oil, put it back.
• Buy butter in bulk: It keeps for a long time and is significantly cheaper per kilogram when bought in larger packs from shops like Aldi or Lidl.
• Make your own: Dressings and mayonnaise take minutes to make and allow you to control exactly what goes into your body.