“All disease begins in the gut” — Hippocrates
Seed oils — soybean, corn, canola, sunflower — are used everywhere in cooking and food manufacture, and the criticism of them is really a criticism of how they are made.
How the oil is extracted
Many seed oils are extracted using solvents such as hexane, which can leave residues behind. The oils are then refined: bleached and deodorised to improve appearance, taste and shelf life. Those processes strip out natural antioxidants, leaving an oil that is high in omega-6 fatty acids and prone to going rancid.
A high intake of omega-6-dominant oils is associated with inflammation, oxidative stress, and elevated risk of chronic disease.
What we used before
Most packaged food — dressings, mayonnaise, baked goods, snacks, anything fried — contains seed oils, because they are cheap and taste of almost nothing.
Historically, people cooked with lard, tallow and butter. Unlike seed oils, animal fats are stable under heat, carry a more even balance of omega-3 and omega-6, and bring vitamins A, D and K2 with them.
Skin and sunlight
There is an argument in the newer research that a diet high in polyunsaturated omega-6 fats makes skin more susceptible to sunburn, because the resulting inflammation and oxidative stress leave skin cells more sensitive to UV. Traditional fats, tallow in particular, are proposed as the better option — a close match for human sebum, and a natural moisturiser.
Seed oils and the gut
Heavy seed oil consumption appears to disturb the balance of gut bacteria. The omega-6 to omega-3 ratio is part of it, and the harsh refining that removes protective antioxidants is part of it. An unbalanced gut flora is in turn associated with digestive problems, weakened immunity and chronic low-grade inflammation.
The remedy is unremarkable: cook with fats that were already fats, eat whole foods, and keep the diet varied enough that no single oil dominates it.