Seed oils are found in almost every processed food today: ready meals, crisps, margarine, baked goods and salad dressings. At the same time, criticism of them is growing. At its centre are polyunsaturated fatty acids, PUFA for short, and linoleic acid in particular. In this article you will learn what seed oils are, how they are made, why the biologist Ray Peat rejected them and what research says.
What are seed oils?
Seed oils are oils extracted from the seeds of plants. They include sunflower oil, soybean oil, corn oil, safflower oil, cottonseed oil, grapeseed oil and canola (rapeseed) oil. What they have in common is a high proportion of polyunsaturated fatty acids. Olive oil, coconut oil, butter or beef fat do not belong to this group. They come from fruit or animal tissue and contain far less PUFA.
PUFA and linoleic acid explained
Fatty acids are classified by their chemical structure. Saturated fatty acids have no double bond, monounsaturated ones have one, polyunsaturated ones (PUFA) two or more. The more double bonds, the more easily a fat reacts with oxygen. The most important PUFA in seed oils is linoleic acid, an omega-6 fatty acid. Sunflower and safflower oil consist largely of it. Canola oil contains less linoleic acid but more alpha-linolenic acid, an omega-3 fatty acid.
Linoleic acid is essential, meaning the body cannot make it itself. The requirement, however, is small. Due to the widespread use of seed oils, intake in Western countries rose sharply over the 20th century.
How seed oils are made
Most seed oils from the supermarket are refined. The seeds are pressed and extracted with solvents such as hexane to obtain as much oil as possible. The oil is then degummed, neutralised, bleached and deodorised at high temperatures so that it tastes neutral and keeps for a long time. Cold-pressed oils skip these steps but have the same high PUFA content.
What Ray Peat criticised about PUFA
Ray Peat saw polyunsaturated fatty acids as one of the most important disruptors of metabolism. His main arguments:
Oxidation: PUFA oxidise easily. This creates breakdown products such as aldehydes, which Peat regarded as harmful to cells.
Thyroid: Peat held the view that PUFA inhibit the action of thyroid hormones and thus slow down energy production.
Storage in tissue: Linoleic acid is stored in fat tissue. Peat argued that a reserve builds up over the years and is released during stress or fasting.
These arguments describe Peat's view. What he wrote on the subject can be found in Ray Peat on oxidation and Ray Peat on prostaglandins.
What does research say?
The evidence is not clear-cut. Official nutrition societies still recommend partly replacing saturated fats with unsaturated ones and point to studies on blood lipids. At the same time, it is well established that PUFA form more oxidation products than saturated fats when heated strongly and repeatedly. Whether a high linoleic acid intake causes harm in the long term is debated. What is clear: seed oils are a comparatively new part of the human diet, and most of them today come from heavily processed foods.
Which fats Peat used instead
For cooking, Peat preferred butter, ghee, coconut oil and beef fat. These fats are rich in saturated fatty acids and more stable when heated. He considered olive oil in small amounts less problematic than seed oils. If you want to cut down on seed oils, the biggest gain comes from eating fewer ready-made products, fried food and restaurant meals, where they are used most often.
Conclusion
Seed oils such as sunflower, soybean or corn oil supply a lot of linoleic acid and are usually industrially refined. Ray Peat rejected them because he saw PUFA as a factor that slows the metabolism. Science does not agree on this question. If you want to lower your intake, cook with stable fats and read the ingredient list on processed foods. What Peat's diet looks like overall is explained in the article Pro-metabolic diet in everyday life.
