What Are Seed Oils?

What seed oils are, how they're made, and why they dominate the modern food supply

Seed oils are industrial cooking oils extracted from the seeds of plants like soybeans, corn, canola (rapeseed), sunflower, cottonseed, and safflower. Unlike olive oil or coconut oil, which can be pressed out mechanically, seed oils require heavy chemical processing to produce. They're cheap to make, and that's why they've quietly become the most consumed fats in the modern diet -- showing up in almost every packaged food, restaurant meal, and fast food fryer.

If you're just starting to pay attention to what you eat, seed oils are one of the simplest things to become aware of. They're listed on nearly every ingredient label, usually as "soybean oil," "canola oil," or "vegetable oil."

How Seed Oils Are Made

The production of seed oils is a multi-step industrial process that looks nothing like pressing olives or churning butter:

Hexane extraction: Seeds are crushed and soaked in hexane, a petroleum-derived chemical solvent, to pull out maximum oil [1]. Trace amounts of hexane can remain in the final product.
Degumming: Phosphoric acid or water is added to remove phospholipids and other impurities.
Bleaching: The oil is passed through bleaching clays to strip out color, pigments, and remaining impurities.
Deodorizing: The oil is heated to extremely high temperatures (around 450-500 degrees F) under vacuum to remove the unpleasant smell and taste that the processing creates.

The result is a clear, neutral-tasting oil with a long shelf life -- but one that bears little resemblance to anything found in nature.

The Omega-6 Problem

Seed oils are extremely high in omega-6 fatty acids, particularly linoleic acid (LA). Omega-6 isn't inherently bad -- your body needs some -- but the ratio of omega-6 to omega-3 in your diet matters enormously [1].

For most of human history, humans consumed omega-6 and omega-3 in roughly a 1:1 to 4:1 ratio. Today, thanks largely to seed oils, the typical Western diet has pushed that ratio to somewhere between 15:1 and 25:1 [1]. This dramatic imbalance is associated with increased inflammation and chronic disease [2].

Why They're in Everything

Seed oil consumption in the U.S. increased more than 1,000-fold during the 20th century [3]. Several factors drove this:

Cost: Seed oils are far cheaper to produce than animal fats or olive oil.
Government dietary guidelines: Starting in the 1960s, health authorities recommended replacing saturated fats (butter, lard, tallow) with "heart-healthy" polyunsaturated vegetable oils -- advice that is now being reconsidered.
Food industry convenience: Seed oils are shelf-stable, neutral-tasting, and work well in processed food manufacturing.
Agricultural subsidies: Corn and soybeans are among the most heavily subsidized crops in the U.S., making their oils artificially cheap.

Evidence Review

The shift from traditional fats to seed oils represents one of the most dramatic changes in the human diet over the past century. Simopoulos (2002) documented how the omega-6 to omega-3 ratio shifted from approximately 1:1 in ancestral diets to 15-25:1 in modern Western diets, driven primarily by increased consumption of vegetable oils rich in linoleic acid [1]. This work established the framework for understanding how fatty acid ratios -- not just total fat intake -- influence inflammatory pathways.

Mani and Kurpad (2016) examined the parallel rise of refined vegetable oil consumption and metabolic syndrome prevalence, noting that the increase in omega-6 polyunsaturated fatty acid intake from seed oils may be a contributing factor to metabolic dysfunction [2]. While this is correlational rather than causal, the temporal relationship is striking.

USDA Economic Research Service data shows that soybean oil alone accounts for roughly 7% of total caloric intake in the American diet -- a figure that was essentially zero before 1900 [3]. This represents an unprecedented dietary shift in evolutionary terms.

DiNicolantonio and O'Keefe (2018) proposed the "oxidized linoleic acid hypothesis," arguing that the oxidation products of linoleic acid from seed oils -- rather than saturated fat or cholesterol -- may be a primary dietary driver of coronary heart disease [4]. This hypothesis remains actively debated, but it has gained traction as the limitations of the traditional "diet-heart hypothesis" have become more apparent.

Where the evidence stands: The dramatic increase in seed oil consumption is well-documented. The biological plausibility of harm from excess omega-6 and oxidized linoleic acid is supported by mechanistic research. However, large-scale randomized controlled trials specifically isolating seed oil consumption as a variable are limited, and some researchers argue that linoleic acid in unoxidized form may be benign or even beneficial. This is an area of active scientific debate.

References

The importance of the ratio of omega-6/omega-3 essential fatty acidsSimopoulos AP. Biomedicine & Pharmacotherapy, 2002. PubMed 23107521 →
Increase in the intake of refined vegetable oils may be the cause of increased prevalence of metabolic syndromeMani I, Kurpad AV. Journal of Nutrition & Intermediary Metabolism, 2016. PubMed 22570770 →
U.S. Trends in Food Availability and a Dietary Assessment of Loss-Adjusted Food AvailabilityUSDA Economic Research Service. USDA ERS, 2016. Source →
Omega-6 vegetable oils as a driver of coronary heart disease: the oxidized linoleic acid hypothesisDiNicolantonio JJ, O'Keefe JH. Open Heart, 2018. PubMed 32898098 →