[ Journal · The Substance ]

Seed oils in skincare: what the research says

The rejection of seed oils started on the dinner plate and then moved to the bathroom. The problem is that the argument doesn't survive the journey: eating an oil and spreading it on your skin are two completely different things. However, there is a real issue, narrower and much less discussed, and it is not about the oil itself but its state.

Where the panic comes from

The subject was born in nutrition, in the United States, driven by the ancestral movement and amplified by social media. The argument, in short: sunflower, rapeseed, soybean, or corn oils are rich in omega-6s, our diet contains too many compared to omega-3s, and this imbalance is said to maintain an inflammatory environment.

This debate exists and is legitimate, but it concerns what we ingest, in quantity, every day, for years.

The shift toward cosmetics happened through simple semantic contamination: the same words, the same vocabulary, therefore supposedly the same problem. This is where the reasoning breaks down.

Eating is not applying

Three differences make the transposition impossible.

• The route of entry. An ingested oil is digested; its fatty acids enter circulation and become metabolic building blocks. An applied oil stays for the most part on the surface of the skin and in the upper layers of the stratum corneum.
• The quantity. We are talking about daily tablespoons on one hand, and a few drops on the other, only a fraction of which penetrates.
• The function. On the plate, oil is fuel and a metabolic signal. On the skin, it is a barrier material. A good food oil and a good cosmetic oil are not judged by the same criteria.

In other words, one can perfectly decide to limit seed oils in the kitchen while continuing to use them on the skin without contradicting oneself.

What research says about linoleic acid

This is the point that bothers the activist narrative, and it is firmly established.

Acne-prone skin is actually deficient in linoleic acid. A study published in the Journal of Investigative Dermatology compared the composition of acylceramides in comedones and on the skin surface of acneic and non-acneic subjects. The authors noted a lower proportion of linoleic acid in the former.

Its topical application reduces microcomedones. A trial published in Clinical and Experimental Dermatology measured the effect of a topical supply of linoleic acid via image analysis. The result supports a decrease in the size of microcomedones after one month.

Yet, linoleic acid is precisely the primary fatty acid in the most criticized seed oils. Sunflower contains 55% to 70%, safflower up to 75%.

The real problem, first point: oxidation

What distinguishes polyunsaturated fatty acids is not their intrinsic harm, but their fragility.

Each double bond is an attack point for oxygen. The more bonds an oil has, the more unstable it is. Concretely:

• An oil rich in polyunsaturates goes rancid when exposed to air, light, and heat
• The process begins in the bottle, long before it reaches the skin
• An oxidized oil does not just lose its benefit: its degradation products are irritating
• On the skin, oxidation continues, fueled by ultraviolet rays and pollution

This is where the only solid criticism lies. Not "seed oils are bad," but "oils rich in polyunsaturates are perishable, and no one tells you the state of yours."

The chart above shows the essential point: it is not the plant or animal origin of a fat that determines its fragility, but its share of polyunsaturates. Olive oil is much more stable than sunflower oil, and a predominantly saturated fat has almost nothing to oxidize.

Second point: refining, invisible on the label

The majority of industrial vegetable oils are not mechanically pressed. They are solvent-extracted, most often with hexane, then refined, deodorized, and sometimes bleached.

This process makes the oil inexpensive, seemingly stable, and odor-neutral. It also removes some of its minor compounds, notably tocopherols, which are its natural antioxidants.

And none of this appears on the label. The INCI name for a cold-pressed sunflower oil from a mill and that of a hexane-extracted oil from a refinery are strictly identical: Helianthus Annuus Seed Oil. You cannot distinguish them.

Third point: what the INCI doesn't say

The ingredient list is a real step forward, but it has three blind spots.

• It says nothing about freshness. An oil pressed three weeks ago and one stored for two years bear the same name.
• It says nothing about extraction. Cold-pressed or hexane, same line.
• It says almost nothing about proportions. The order is descending, but below 1%, ingredients can be listed in any order. An oil highlighted on the packaging may only be present in trace amounts.

How to spot an oil that has gone bad

No laboratory needed; your nose is enough.

• Smell. A rancid oil smells like wet cardboard, old grease chalk, or paint. It is the most reliable signal.
• Color. An unusual browning or yellowing compared to when purchased.
• Texture. A sticky or thickened sensation that wasn't there originally.
• Skin sensation. Tingling or redness from a product that was previously well-tolerated.

Good habits are simple: buy in small quantities and replenish often, store away from light and heat, and close carefully.

What it changes in practice

There is no reason to banish seed oils from your bathroom. There are good reasons to look at three things.

• Packaging. An opaque or tinted glass bottle protects better than a transparent one. A pump container limits air entry compared to an open jar.
• Presence of an antioxidant. Tocopherol in an INCI list is not an admission of weakness; it is a sign that a formula rich in polyunsaturates has been properly thought out.
• Date. The period after opening, the little open jar symbol with a number, is underutilized information. On a formula rich in fragile oils, it matters more than on others.

A remark in passing, because it comes up often: jojoba oil is not a seed oil in the sense of this debate. Chemically, it is not a triglyceride but a liquid wax, and it contains almost no polyunsaturates. The question of oxidation does not apply to it in the same terms.

The verdict

The discourse currently circulating confuses three very different things: the nutritional question of omega-6s, the industrial quality of refined oils, and the chemical fragility of polyunsaturates. Only the third truly concerns your skin, and the second exacerbates it.

What to take away from this:

• Seed oils are not bad for the skin, and linoleic acid is actually beneficial for it
• They are perishable, and an oxidized oil irritates instead of nourishing
• The label does not allow you to know what state yours is in
• A formula poor in polyunsaturates bypasses the problem rather than solving it, and that is another way to approach the issue

The debate deserves better than one camp against the other. The right question is not "seed oils, yes or no," but "what do I really know about what I put on my skin?"