Guide 4 min read

Why is ferulic acid in a vitamin C serum?

Ferulic acid stabilizes L-ascorbic acid and vitamin E, and it roughly doubles what the pair can do. In the 2005 study that defined the formula, adding 0.5% ferulic acid to 15% vitamin C and 1% vitamin E raised photoprotection against simulated sunlight from about 4-fold to about 8-fold (Journal of Investigative Dermatology).

What does ferulic acid do in the bottle?

L-ascorbic acid is the form of vitamin C the body uses and the form nearly all of the topical research was done on. It is also the form that oxidizes if you look at it wrong. Once it oxidizes it turns yellow, then brown, and stops being useful. Vitamin E is more stable but not immune.

Ferulic acid is a plant phenolic from rice bran, oats and coffee. Added to a solution of L-ascorbic acid and alpha-tocopherol it improves the chemical stability of both vitamins, so more of the active survives to be applied. That is the boring half of the story and it is the reason the serum works on day sixty as well as day one.

What does it do on skin?

The interesting half. In 2005, Lin and colleagues at Duke tested 15% L-ascorbic acid and 1% alpha-tocopherol with and without 0.5% ferulic acid, applied for four days before solar-simulated irradiation. Adding ferulic acid doubled the photoprotection, from 4-fold to approximately 8-fold, measured by both erythema and sunburn cell formation. The formulation also reduced the induction of caspase-3 and caspase-7 and efficiently reduced thymine dimers, the DNA damage sunlight leaves behind.

That study was done on pig skin, which is the standard model for this kind of work, and we say so because a receipt that hides its species is not a receipt. The human confirmation came in 2008. Murray and colleagues applied the same 15%, 1%, 0.5% solution to normal-appearing human skin for four days, irradiated it with 2 to 10 minimal erythema doses, and found significant protection on every measure they used: erythema, sunburn cells, thymine dimers, p53 and the ultraviolet-induced cytokines including IL-1 alpha, IL-6, IL-8, IL-10 and TNF-alpha.

Why 15%, 1% and 0.5%?

Because those are the studied levels. The 2003 paper that came first showed 15% L-ascorbic acid with 1% alpha-tocopherol reaching an antioxidant protection factor of 4-fold; either vitamin alone was protective, and the combination was superior. Ferulic acid at 0.5% is the level used across the entire series. Above 20% vitamin C there is no added benefit in the published work, and stability gets harder.

The pH matters as much as the percentage. L-ascorbic acid needs to be at roughly pH 3.5 or below to be in the uncharged form that crosses the stratum corneum. That is why a well-made vitamin C serum tingles for a moment on fresh skin, and why it does not belong on skin that is broken or freshly resurfaced.

Does a vitamin C serum replace sunscreen?

No. This system reduces the oxidative damage ultraviolet light causes, and it absorbs no ultraviolet light at all. The 2008 paper frames it explicitly as complementary to sunscreen. Serum first, sunscreen on top, every morning. Serum instead of sunscreen is how people get a suntan with antioxidants in it.

Why is the serum amber, and when is that a problem?

Ferulic acid darkens over time and gives a well-made vitamin C serum its faint amber cast from the first day. That is different from the deep orange of oxidized ascorbic acid. Pale amber is the ferulic acid. Orange turning brown is the vitamin C telling you it has retired. Keep the bottle capped, cool and out of the light, and use it up rather than saving it.

What we made with it

Bright Side is 15% L-ascorbic acid, 1% vitamin E and 0.5% ferulic acid at a pH near 3. Unfragranced, because the actives had enough going on. It is the second step of the morning routine, after Clean Slate and before Mixed Signals, and it goes under sunscreen, never instead of it.

Related questions

Is 20% vitamin C better than 15%?

Not in the published work. The Duke photoprotection series used 15% throughout, and above 20% there is no documented added benefit while stability gets harder and the sting gets worse. Fifteen is the studied level, which is why it is the level we use.

Why does my vitamin C serum turn orange?

Oxidation. L-ascorbic acid reacts with air and light, turning yellow, then orange, then brown, and losing its usefulness as it goes. A faint amber from day one is ferulic acid and is normal. Deep orange means the serum is past its useful life.

How many days before a vitamin C serum protects the skin?

In both the 2005 and 2008 studies the solution was applied for four consecutive days before the skin was irradiated, which is the schedule used to build up protection. Daily use is the point. A single application the morning of a beach day is not what was tested.

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