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The Vitamin C Numbers Game

June 28, 2017

It doesn’t take the average beauty hunter long to work out that when it comes to good skin, vitamin C can deliver:





  • Ascorbic Acid is a cofactor required for the function of several hydroxylases and monooxygenases.
  • Vitamin C is not synthesised in humans and has to be provided by diet.
  • The word “Ascorbus’ means no scurvy.
  • Topical application of Vitamin C has been found to positively impact collagen synthesis.
  • Study participants with a diet low in vitamin C reported the largest change in collagen function via topical application. This was because those eating vitamin C were already able to produce collagen at an optimal level for their age and skin type.
  • Ascorbic Acid is the vitamin C of choice for the skin but it is highly unstable. The phosphate and Glucoside form of the vitamin work via slightly different pathways but give similar collagen boosting results. They can be classed as pro-vitamins (Provitamin is a vitamin pre-curser that can be converted into its active form through normal metabolic processes).

However, when it comes to buying a Vitamin C product, the beauty hunter is faced with a myriad of choices and decisions, not least is the decision of which percentage of Vitamin C is good enough to get the job done. I wan to discuss that a little now.

Let’s start at the very beginning, with Ascorbic Acid, the original (and maybe still the best?) Vitamin C.

Vitamin C Ascorbic Acid

Ascorbic acid is a white, granulated acid with an acidic pKa of 4.17 which means at pH 4.17 it will exist in a balance between how it looks above and as above but with one proton missing (H).

But that’s not all, Ascorbic Acid also has a second Pka at 11.8 due to it being what we call a Di-Acid,  although this second change is less relevant to us as cosmetic chemists as we rarely formulate at such high pH:

Ascorbic acid at pKa

For a formulator, knowing an acids PKa is important as this is the point where the acid solution naturally sits as a 50:50 mix. Any reduction in pH increases the acidity dramatically, pushing more of the molecule into its reactive side while any decrease in acidity (increase in pH) pushes the acid towards its less active side.  It is important to note though that this pH shifting is not going to ‘use up’ the molecule, the arrows in the above equation show that this reaction can go both ways if the conditions (pH) are right. So you could formulate a product at a higher pH and rely on the lower pH of the skin to facilitate the change towards the more active form.  This is theoretically possible but with the skin pH being somewhere between 4.5-5.5 it is not likely to result in a very active product.

So, when it comes to Ascorbic Acid, our vitamin C of choice for this article, we know that pH 4.1 is the sweet spot and below that it only gets more potent (and less protonated) as the balance shifts in favour of the AScH2 side.  Another way of thinking about this is that as the pH rises, the chemical changes within Ascorbic Acid increase. Chemical changes = chemical reactions = potential rise in instability and decrease in activity.

While interesting (hopefully) and relevant to what I’m going to delve into next, pH is not the key focus of this article, the key focus is concentration and that’s what I’ll look at now, linking it to pH at the end (if I remember).

When it comes to vitamin C products one could be forgiven for thinking that it is ALL about concentration. There is an abundance of products out there spruiking their Vitamin C percentage loud and proud, none more so than the 20% bunch.

20% seems to be the golden number (at the moment) when it comes to vitamin C serums and is a percentage active point that most customers want to formulate to which begs the question, why?

I’ve been reading heaps of papers on topically applied Ascorbic Acid and the one that gave me the best insight into the origins of this 20% number was this: Topical L-Ascorbic Acid Acid: Percutaneous Absorption Studies. Dermatologic Surgery  Volume 27 (2) – 1 Feb 2001.    This study was carried out on white yorkshire pigs (live) that had been shaved prior to application (potentially increasing transdermal delivery via abrasion of surface – like exfoliation although they did have 24 hours to recover from that shave). As an aside, the realisation that a live animal experiment has given us this data is one of the issues that, for me, complicates the animal testing argument. I am forever wrestling with the questions that arise from this including the question of whether a brand that uses this data to perfect their formulations is somehow complicit in the animal testing being as though they then profit from it?  I really don’t know as it makes no sense to ignore data we discover via animal testing don’t you think?    Anyway, the net result of this study found that skin absorption increases up to 20% concentration of ascorbic acid under the experimental conditions but didn’t increase over that.  The bottom line being that 20% is the OPTIMAL concentration for skin penetration and adding more may be quite pointless.

As a formulator I can attest to the difficulties in formulating 20% of a sticky water soluble powder into a product.  For one, there’s 20% of your water phase gone, made more dense, more active and more acidic than it otherwise would be. With 20% of stuff turning your water into Syrup be prepared for a very sticky dry-down phase and a few issues with emulsion stability – both solvable but not in any ‘piece of cake’ way.  We haven’t even gone into oxidative stability – Vitamin C solutions won’t last for very long without help – but that’s a matter for another day.

It turns out that the aforementioned study also found that the pig skin became saturated with Ascorbic Acid applied at a 20% concentration once a day after three days of application.  Interesting indeed and possibly cause for us to pause and consider whether we could get away with less vitamin C after all given these fast and dramatic results?

Where the cosmetic industry parts ways with science is called the marketing department. OK, so that’s not entirely true, I know plenty of marketing people who love (and are qualified in) science but all too often the words ‘well that’s what the market wants’ are thrown out there as if the market was a real, separate thing with a mind of its own……

I personally see no reason why, all other things being equal, we can’t present the market with 1%, 5%, 10% and 20% Ascorbic Acid products (and everything in-between) given that we accept a skin cycle of roughly 28 days and application rates of once or twice a day.   The same study as above looked at concentrations of 5, 10 and 15% finding incremental increases in skin concentration relative to the Ascorbic Acid concentration in the product – a step wise increase.  One could hypothesise that if 20% reaches saturation within 3 days, 10% might reach saturation within 6 days, 5% in 12 days and 2.5% in 24 days, 2% saturated within a skin cycle of 28 days and 1% becoming saturated at around 60 days.  Given that the skins cells are not all synchronised on the same 28 day cycle (we are not snakes that shed all at once) this hypothesis seems reasonable enough meaning that practically ALL concentrations of Vitamin C are helpful and able to saturate the skin in time.   One other thing to consider is what we call the ‘half life’ of the Vitamin C in the skin – how long can it hang around for before it is used up/ oxidised/ excreted etc.  I haven’t found a number for the skin yet but have found that it has a half life of around 83 days in the kidneys which is where our long-term storage of oral Ascorbic Acid goes.

But the market wants bigger, better, stronger, faster. 

I saw today a company selling a 30% Vitamin C product and will look out for the science to support a need for MORE. I secretly hold the thought that I will be waiting for that science for quite some time…..

So what’s a brand to do? Just focus on numbers or attempt to broaden the conversation?

Shoppers have a notoriously short attention span so I get the need for a short, sharp injection of information to get their dollars into your wallet but these days shoppers are not just shopping at the point of sale, they are also shopping around for information, depth, authenticity, knowledge, evidence and, dare I say it, SCIENCE.  I feel there is still so much more scope within the Vitamin C market for conversations and information that will really help people choose a product that works for them and yes, I do think that includes products that contain less than 20% vitamin C, in fact I think that’s exactly what we need!

Some other, non numbers games you can play/ ask about.

Ascorbic Acid may be the cheapest and most directly biologically active form of vitamin C that there is but it isn’t the only form.  Why don’t we start broadening our product offerings and conversations to include these things? Not just because they are often easier to formulate with (stabilise) but because they might just bring something else to the party, open up different markets or facilitate the formulation of different types of products?   In pursuing this, the 20% addition figure becomes less relevant (it only relates to topically applied ascorbic acid)  but real evidence can potentially fill that gap – after all, it isn’t necessarily the amount you put into a product that matters (which is what we are focusing on here) it’s more about what it can deliver and where!

Product format is another area of focus,  improving skin feel, exploring different ways to enhance dermal penetration and looking at building additional benefits into the base (rather than it be just a vitamin C vehicle) can be powerful things to focus on given that customers can really feel a difference and have their expectations challenged.

And lastly we have pH.

If you do manage to read the paper that I’ve shared here you will see that the team not only looked at Vitamin C concentration but they also looked at the product pH.  Different forms of Vitamin C have different pH ‘sweet spots’ so again, adopting a one-size-fits-all pH for a Vitamin C product isn’t necessarily going to cut it.  When it comes to Ascorbic Acid the sweet spot is actually the very acidic pH 2 (according to that paper).  Now pH 2 is far too acidic for a consumer product and would be better suited to a professional range such as that found in a medi spa or equivalent. The reason Ascorbic Acid works best under these acidic conditions is that its charge density is lowered with decreasing pH and in lowering the charge density you make it easier for the Ascorbic Acid to penetrate deeper into the skin.  Remember that Ascorbic Acid is water soluble and the intracellular cement binding the skin cells together is an oily soup – oil and water don’t usually mix and a high charge density will be actively repelled.  Anyway, the bottom line is that we opt for a pH around the Pka of Ascorbic acid because that is a good balance between the activity of the acid and the potential for it to penetrate.  Ascorbic Acid can still build up in the skin when the pH is higher and at pH 5 you are still getting just over half the penetration as you get at pH 3.5 but with potentially way less stinging (important for people with a compromised skin barrier) and more formulating versatility.  So the bottom line here is that pH is a formulation dependent factor on how effective a product could be and that concentration alone is not enough to make a product ‘good’.

In summary I’d encourage you to think holistically about Vitamin C and to move beyond the marketing mindset that views this distills years worth of science into a quest for the best percentage.  That’s just not going to cut it in my view.

Amanda x

PS:  I thought that if you had time you might like to see some of the scientific data presented on a slightly different type of Ascorbic Acid – Ethyl Ascorbic Acid.  It isn’t often that I laugh at a material data sheet but this did crack me up a little.  I think it’s worth its own blog post 🙂








6 Comments leave one →
  1. June 28, 2017 3:20 pm

    Thank you for writing this. A while back I tried The Ordinary Vitamin C Suspension 23% and it burned the shit out of my skin. I didn’t, and still can’t quite understand why the concentration had to be so high. I’m sure there are others who would purchase a formula with a much lower concentration, as my complaints regarding this product are not unique.

    • RealizeBeautyEd permalink*
      June 30, 2017 6:18 pm

      Oh bugger Siobhan, that sounds terrible for you. Some people are quite reactive to vitamin C anyway for some reason (I haven’t worked through that yet) so it isn’t necessarily that there is anything inherently wrong with the product you mention, it might just not be right for you or you might have used too much I guess. In any case I feel it is important that people realise that while 20% is great for fast results on people who can tolerate it, you will get a better result with a lower concentration product IF you are a sensitive soul as you will comply with using it for longer (plus there is less residual stickiness with a lower active product. One thing I didn’t mention in the blog post is the role of penetration enhancers. Vitamin C doesn’t really need to penetrate into the deeper skin layers too much to be an antioxidant as oxidation happens at the surface but some penetration is useful for its other properties, skin brightening, collagen production. If you are mainly looking for a vitamin C product for environmental protection it might be best to go with one that isn’t too heavy on penetration enhancers such as propylene glycol (or propanediol, the natural alternative) as the more penetrating a product, the greater its potential for irritation.

  2. June 28, 2017 4:27 pm

    Reblogged this on Oriental Skincare.

    • RealizeBeautyEd permalink*
      June 30, 2017 6:14 pm

      Thank you 🙂

  3. Ric permalink
    June 28, 2017 11:31 pm

    What is not mentioned is that most of the “high” percentage Ascorbic Acid formulations are based on anhydrous formulae (no water) where the ascorbic acid (a water soluble vitamin) is a suspension (not dissolved in the product matrix). When applied to skin the only ascorbic acid that penetrates skin is that which dissolves in the skin’s surface moisture. In drier climates this may only be a very small fraction of the percentage listed on the label with the remainder being wiped off or “lost on drying”. The case mentioned by Siobhan S could be attributed to the somewhat “common” case where the 23% Ascorbic Acid product is applied immediately after washing or application of a moisturiser and the amount of ascorbic acid dissolved is very high. Ascorbic acid is an alpha-Hydroxy Acid (AHA) and can act as an exfoliant in high concentrations the resulting situation feeling like a severe burn.
    Note; Anhydrous formulations are used because ascorbic acid is more stable when there is no water present. The reason why the level is so high is just that not all is bio-available (due to the reason detailed above) and high levels are designed to supply sufficient ascorbic acid in normal conditions, although “normal conditions” do not consider the the case where high levels of water may be involved.
    My recommendation that no more than 10% pure ascorbic acid should be used, although with some derivatives higher values can be used (NB this is not often the case due to high cost of some derivatives). I agree with Amanda that levels as low as 2-3% can be very effective when used frequently and should reiterate that 20-23% (particularly in anhydrous formulae) is a costly waste and can be quite dangerous.

    • RealizeBeautyEd permalink*
      June 28, 2017 11:36 pm

      So true. I think the anhydrous route can be good in some cases but in others it is highly likely to either elevate irritation or be a complete waste of money. As an aside I’ve spent a long time looking into the rationale behind anhydrous formulations and feel that the issues in stabilising vitamin C have been somewhat over-stated. It isn’t that hard given a bit of elbow grease and you do end up with quite a useful little formula if you get it right 🙂

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