Vitamin C, Explained: A Q&A with Dr Clare Kiely

Vitamin C is one of the most familiar ingredients in skincare  known for its antioxidant properties, its association with brighter-looking skin and its role in supporting collagen.

But Vitamin C isn’t quite as simple as picking the highest percentage on the shelf. There are different forms, different formulation considerations and different reasons why one Vitamin C product may behave very differently from another.

We asked The Skin Diary co-founder and Consultant Dermatologist Dr Clare Kiely to explain what actually matters.

What does Vitamin C actually do for the skin?

Dr Clare Kiely: “Everyone knows that Vitamin C is a great antioxidant. Vitamin C and Vitamin E are two of the most potent antioxidants in the very top layers of your skin, so they’re really important for skin function.

“As an antioxidant, Vitamin C can also help with pigmentation, which is why it’s known for brightening.

“But what I think people don’t really understand is how important Vitamin C is for collagen function in the skin.”

Vitamin C plays an essential role in the processes involved in forming and stabilising collagen,  helping create the strong, supportive collagen structure our connective tissue needs.

Dr Clare: “A really good example of how important Vitamin C is can be seen in severe Vitamin C deficiency. In scurvy, the connective tissue isn’t strong enough and you can get bleeding into the tissues because the collagen structure becomes weak.

“So Vitamin C is really important for the strength of that structural protein, collagen, in your skin.”

Why are there so many different forms of Vitamin C in skincare?

Dr Clare: “This is something that really confuses people.

“The classic form that everyone knows is L-ascorbic acid. But since the advent of L-ascorbic acid in topical skincare, there have been lots of different iterations of Vitamin C.

“The reason for that is that L-ascorbic acid is very unstable. To formulate it in a stable serum, you need to formulate it at a very low pH, which can sometimes be a little bit irritating.”

Different Vitamin C derivatives have therefore been developed with different properties around stability, solubility and tolerability.

Is there a form of Vitamin C you particularly like?

Dr Clare: “One of the forms we particularly like at The Skin Diary is THDA - tetrahexyldecyl ascorbate.

“We like it because it’s a lipid-soluble form of Vitamin C. It’s also very stable and it’s very well tolerated.”

Being lipid-soluble gives THDA different formulation characteristics from traditional water-soluble L-ascorbic acid, including the ability to be formulated without relying on the same very low-pH environment.

For us, that combination of stability, skin delivery and tolerability makes it a particularly interesting form of Vitamin C to work with.

What percentage of Vitamin C should we be looking for?

Dr Clare: “This is a nuanced one.

“Because Vitamin C can be unstable in a formulation, you could have a Vitamin C serum that says it’s 20%, but if it hasn’t been stabilised properly, you may not actually be getting that level from the product.

“So really, it’s about looking for a form of Vitamin C that has been properly stabilised.”

The percentage on the bottle can be useful information, but it doesn’t tell you everything about the finished formula.

Should we always be looking for the highest percentage?

Dr Clare: “No. The higher percentage of an ingredient has no bearing, on its own, on how well that ingredient is working within your skin.

“Sometimes you can have too high a level and it isn’t actually benefiting your skin. It may even be causing irritation.

“So you really need to trust the formulators and scientists behind the skincare brands. And if you’re interested, ask the brands you’re buying your Vitamin C from why they’ve chosen that particular level.”

In other words, a bigger number doesn’t automatically mean a better product.

The form, concentration and complete formulation all need to make sense together.

Vitamin C comes in creams, serums and other formats. What do you recommend?

Dr Clare: “I tend to like using Vitamin C in the morning as a serum formulation.

“The reason for that is that it’s such a good antioxidant, so it works well at the beginning of the day.

“You can, of course, use it at night as well, but there may be other things that you want to incorporate into your skincare routine at night. Rather than layering too many things on, I like Vitamin C in the morning and then products containing things like retinoids at night.”

Vitamin C can complement a morning skincare routine, but it doesn’t replace daily sun protection. Antioxidants and sunscreen perform different roles within the skin.

Why do some people find Vitamin C irritating?

Dr Clare: “This tends to be mainly down to the formulation.

“Classic L-ascorbic acid has to be formulated at quite a low pH, and sometimes that lower pH level can irritate people’s skin, especially with consistent use.

“That’s why other forms of Vitamin C have been developed that are less irritating, more stable at a more neutral pH and more lipid-soluble.”

So a previous reaction to one Vitamin C serum doesn’t necessarily mean that Vitamin C as a whole won’t suit your skin.

Different forms  and different finished formulations  can behave very differently.

The takeaway

Vitamin C has truly earned its place in your skincare routine. It is an important antioxidant, has a role in pigmentation and is fundamental to healthy collagen formation.

But the words “Vitamin C” on the front of a bottle only tell you part of the story.

Rather than simply looking for the highest percentage, consider the form being used, why that concentration has been chosen, how the Vitamin C has been formulated and whether the finished product is suitable for your skin.

When it comes to Vitamin C, more isn’t automatically better. Thoughtful formulation matters.