The cooking oil in your kitchen
might be the most harmful thing
you eat.

What are seed oils - and why does it matter?

Seed oils are vegetable oils extracted from the seeds of plants - sunflower, rapeseed (canola), soybean, corn, cottonseed, safflower, rice bran, and grapeseed oils are the main culprits. They're called "vegetable oils" on most UK supermarket shelves, which sounds harmless enough. They're anything but.

These oils became dominant in the Western diet in the mid-20th century for two reasons: they were cheap to produce, and they were marketed as a healthier alternative to saturated animal fats following the (now heavily disputed) lipid hypothesis - the idea that saturated fat directly causes heart disease.

The oxidation problem - why heat makes them dangerous

Here's the fundamental issue: seed oils are predominantly composed of polyunsaturated fatty acids (PUFAs) - particularly omega-6 fatty acids. Polyunsaturated fats are inherently unstable. They have multiple double bonds in their molecular structure, which makes them highly susceptible to oxidation when exposed to heat, light, or air.

When you cook with a seed oil - sunflower oil, for example - and heat it to frying temperatures, it undergoes oxidation. This produces a range of toxic compounds: aldehydes, lipid peroxides, and acrolein, amongst others. A 2015 study from De Montfort University (Leicester) found that heating sunflower oil and corn oil to frying temperatures produced levels of aldehydes up to 200 times higher than the safety limits set by the World Health Organisation.

To put that plainly: when you fry food in sunflower oil, you're producing compounds at 200x the safe concentration. These aldehydes are linked to cancer, cardiovascular disease, and neurological conditions including dementia.

The omega-6 to omega-3 imbalance - chronic inflammation

Beyond oxidation, there's a second major mechanism. Seed oils are extraordinarily high in omega-6 fatty acids - specifically linoleic acid. Our bodies need both omega-6 and omega-3 fatty acids, but the ratio matters enormously.

Historically, humans ate omega-6 and omega-3 in a ratio of roughly 1:1 to 4:1. The current Western diet delivers a ratio of approximately 15:1 to 20:1 - heavily skewed towards omega-6. This imbalance drives chronic, systemic inflammation. Omega-6 is pro-inflammatory by nature; omega-3 is anti-inflammatory. When omega-6 dominates, inflammation wins.

Chronic low-grade inflammation is implicated in virtually every major modern disease: cardiovascular disease, type 2 diabetes, obesity, autoimmune conditions, depression, and cancer. The rise in these conditions tracks remarkably closely with the rise in seed oil consumption over the past century.

The linoleic acid accumulation issue

Linoleic acid (the dominant fat in seed oils) doesn't just pass through your system - it gets stored in your fat tissue and incorporated into cell membranes. Studies show that linoleic acid now constitutes around 20–25% of the fat stored in human adipose tissue, compared with 1–2% fifty years ago. It accumulates over years, meaning the damage compounds over time.

What to cook with instead - the alternatives

Kerrygold - the specific call-out

If you're going to make one switch, make it to Kerrygold butter. Here's why it's worth specifying by name:

• Kerrygold is produced from Irish grass-fed cows. Irish pasture conditions mean the cows are outdoors and grass-fed for significantly more of the year than UK or US equivalents
• Grass-fed butter is measurably higher in omega-3 fatty acids, conjugated linoleic acid (CLA), and vitamin K2 than standard grain-fed butter
• CLA has demonstrated anti-cancer properties in multiple studies and supports healthy body composition
• Vitamin K2 is largely absent from most Western diets - it's the crucial partner to vitamin D3 for directing calcium into bones rather than arteries
• Kerrygold has a distinctive yellow colour from the beta-carotene in fresh grass - the same reason grass-fed meat is sometimes slightly darker

It costs roughly £1.40–£1.60 for 250g in most UK supermarkets. The nutritional difference over standard butter is genuinely meaningful.

How to spot seed oils on food labels

Seed oils are hidden in a vast range of processed foods - often under names designed to sound either neutral or healthy. Train yourself to spot these on ingredient labels:

• Vegetable oil - the most common hiding place. Usually a blend of rapeseed, sunflower, or soybean oil
• Rapeseed oil - widely used in UK products due to domestic production
• Sunflower oil - in almost every crisp, biscuit, and bakery product
• "Healthy" products to check: hummus (almost always sunflower oil), granola, protein bars, "healthy" crisps, salad dressings, mayonnaise, most supermarket breads, ready meals, and virtually all restaurant-fried food
• Emulsifiers: soy lecithin (E322) is a soy derivative - not technically an oil, but another highly processed soy product to be aware of

What about margarine - what it actually is

Margarine deserves its own section. It was invented in 1869 as a cheap butter substitute, made primarily from animal fat. By the mid-20th century it had transitioned to hydrogenated vegetable oils - a process that turns liquid seed oils into a solid by adding hydrogen atoms to the molecular structure.

This process creates trans fats - the most dangerous fats in the human diet. Trans fats raise LDL cholesterol, lower HDL cholesterol, promote inflammation, and are directly linked to heart disease. They're so harmful that they've been banned in many countries, and the US FDA completed their removal from food supply in 2018.

Modern margarines use different processes to avoid trans fats (interesterification), but the fundamental product remains a highly processed, industrial fat made from seed oils. It exists to replicate butter at a lower cost. Given what we now know about seed oils, the entire premise of "this is healthier than butter" was backwards from the start.