Is it really true that Hyaluronic Acid holds 1000 times its weight in water?

It’s funny, you see something repeated often enough that you start to accept it as true, without question. Maybe it’s because life gets busy, maybe it’s because it doesn’t really seem that important anyway or maybe it’s because you really believe it to be true. But then, one day something changes and all of a sudden that well known truth falls short of the pub test. Let’s look at that…

Hyaluronic Acid is definitely a water binder. It occurs naturally throughout our bodies including our skin (dermis and epidermis – mostly in the epidermis). In the skin it makes up part of our extracellular matrix and functions as part of our water management system. Here it helps keep cells hydrated so that they can talk to each other, stay plump and ensure the barrier stays sealed.

Chemically Hyaluronic Acid is made from a type of sugar, in this case a disaccharide.

Disaccharides are sugars that can be broken down further into monosaccharides a common example of which is Sucrose. Sucrose is made from glucose and fructose bound together by a sugar bond (glycosidic link).

Hyaluronic acid isn’t made from sucrose, instead it is made from D-glucoronic Acid and N-Acetyl-D-glucosamine.

Humans make D-glucoronic Acid in our livers and use it to help our bodies remove toxins. Chemically this is a glucose-type sugar with a twist. Instead of the last carbon being CH2OH it is COOH thus turning it from an alcohol group to an acid and changing its functionality. I put this in just to give you a bit of context and possibly an idea of where future water may bond (or stick) to when we get to that bit).

N-Acetyl-D-glucosamine is a bit more fancy. This is also built from a glucose structure but this time it has turned into an amide (as in amino acids, proteins etc). This molecule is commonly found as a structural brick in the cells of many things including fungi, insects, bacteria and crustaceans. For those who have heard that much of the Hyaluronic Acid we use in cosmetics is made by microbes you may now have a bit more of an insight into how that can be so given that they already have access to these bricks.

So when those two exciting monosaccharides join together they make a disaccharide. When these disaccharides find other identical disaccharides they can use their functional groups (hands) to hold onto each other thus forming chains. These chains can be very long, very short or somewhere in-between and it is now we call this combo ‘Hyaluronic Acid’. We typically express the weight of Hyaluronic Acid polymers in a unit called the Dalton which for the purposes of cosmetic science is taken to be equivalent to another unit of measure chemists use (g/mol) so 1Da (dalton) = 1 g/mol.

Being as though we are now going to talk about water binding capacity it may be useful to know that water has a weight of 18 Da while the Sodium Salt of a Hyaluronic acid disaccharides weighs 401.30 Da. However, when you make a polymer from the above you lose the Na+ (Sodium ion) and it is replaced by a hydrogen. This makes the weight of each disaccharide 391 Da although I really don’t think it is vitally important to get this exact in this case as cosmetic HA polymers are so variable in their weigh anyway – any figure we come to will only ever be approximate.

Holding Water.

It turns out that when you look and really think about it, talking about Hyaluronic Acid being able to ‘hold’ x amount of water isn’t a clear cut as it seems even with respect to the word ‘hold’.

As a chemist the first thought I had of how to confirm or deny this internet ‘truth’ was to work out how many water-bonding sites (hydrogen bonds) exist in hyaluronic acid. My theory being that if you know how many water molecules a hyaluronic acid disaccharide unit can hold (as a maximum), and you know (roughly, as an average) the HA’s molecular weight, you could get a number that gives you an approximate Water: HA ratio. If that turned out to be 1000:1 we have a winner – the internet is right! So I set out to do that.

Looking at the disaccharide I could see an easy 10 bonding sites in each monomer. This paper, which looks legit and has probably spent more time thinking about this than me confirmed that there are between 10-15 sites for water to attach to – happy to accept that range.

We now have all the information we need to do some calculations.

In finding this out I have my first answer to my water holding question. If water holding = water bonding (hydrogen bonding) then each disaccharide unit of the HA polymer (we also call these individual units Monomers) can hold up to 15 water molecules. or 15:1.

If I know the molecular weight of the HA that I’m planning to use I can work out how many monomers that contains (as an estimate as the MW of polymers used in cosmetics is typically an estimate). So if my HA was 1,000,000 Da it would contain approx:

1,000,000 (total weight of HA) / 391 (monomer / Disaccharide weight) = 2557 disaccharide units per polymer chain.

Each unit holds (H Bonds) with up to 15 water molecules so:

2557 x 15 = 38,363 water molecules attached to this polymer.

We also know that each water weighs 18 Da so now:

38,363 x 18 = 690,537 Da water weight in total for the water attached to that polymer chain.

So what do we end up with?

After doing that I can see that for every 1 x 1,000,000 Dalton weight of HA polymer it can hold (Hydrogen Bond with) 690,537 Dalton weight of water.

That is not 1000 x its weight, that’s not even equivalent to its weight. In fact, that’s less than 70% of its weight in water. How odd!

What is going on?

A couple of things spring to mind, firstly it’s more likely that I’m interpreting the word ‘hold’ differently to that which is quoted online. Secondly I could have got my calculations wrong – but is it likely they are so far out given the fairly logical way I’ve tried to approach this (I feel?). Thirdly, the internet is full of shit so why should this be any different!

Being the eternal optimist I am going with one, also that’s more interesting a thing to think about.

Holding – a different definition.

I started to consider that holding could easily mean something else here. I know from my general chemistry that water molecules can each hold onto another four water molecules without any other chemicals around to incentivise them so it quickly became imaginable to me that what the Hyaluronic Acid is doing is providing a starting point framework from which hundreds if not thousands of water molecules can be anchored. Then I realised that HA is not necessarily existing in neat regimented lines that behave independent of each other, that they flow, create three dimensional structures, latices, meshes, networks and maybe even water traps. Before I knew it it seemed not only possible but logical that HA could be a mega water binder, a binder that far exceeded its structural hydrogen bonding confines – after all, HA is not making a bucket that has a known volume, it’s more like a fishing net, something that has the opportunity to expand and hold water in a whole range of ways.

But where is the evidence?

I found this paper which confirms HA as a structure maker – one that is influenced by both pH and concentration. This to me is interesting and it is this detail that can help us cosmetic chemists make the most out of the HA that we formulate with. In simple terms the paper points out that we should always think of HA in an applied sense, considering and allowing for how it interacts with the environment in which we put it. In cosmetic science this would mean that I can’t expect the same from my HA serum formulated at 0.5% activity and pH 3 as you can your cream that has 0.1% HA and a pH of 5. The HA will structure its self and bind water differently in each situation. This is deeply interesting, this is the detail that we chemists strive to master and we haven’t even mentioned the potential for molecular weight to play a part yet (and that’s seemingly ALL the internet focuses on).

I found a few other papers too including this very old one that talks of HA as an adhesive or water-glue. Then this one which talks of its ability to deliver drugs through the skin- something I often talk about in my classes. It’s clear we’ve had an intellectual relationship with HA for a long time but have we really imagined it as well as we might have? Have we instead just reduced it to something basic and in doing so forgotten to look for its beauty and nuance?

But hang on, we still need to know where these numbers came from. Any ideas?

So the last thing I want to share in this post is this. During my reading (which took in a few more papers that those listed above) I found many references to another ‘fact’, that 1g of HA can hold up to 6 litres of water. This was something I did manage to trace back and I found it in a paper that has been cited over 750 times since:

Novel and established applications of microbial polysaccharides by Ian W Sutherland published in 1998.

You have to pay to read this paper and you can do that by following this link and then going to the publishers page (I did this, cost me $36 I think). To give you an idea of his research one free paper he wrote in 2001 is available here.

Now Ian doesn’t give a reference in this paper as to where that figure comes from but he does give a lot of other interesting references in this and other papers he’s written, is an expert in cell biology and has spent a considerable amount of time studying biopolymers such as these. Then there is his book on the subject (which I am yet to read). In spite of the fact that I’m convinced Ian knows what he’s talking about I still find it odd (to say the least) that he didn’t elaborate on this figure in this much-quoted paper. I’m also weirded out a bit by the fact that hundreds of other people have just quoted this paper without (from what I’ve read and I haven’t read them all) explaining where this figure came from), this has even been observed in patents.

I feel inclined towards the idea that HA could indeed hold 1000 times its weight in water or even more but I would prefer this to be stated as an idea rather than a truth until I either find some more evidence or create some myself. In any case, believing it as an idea makes more sense given that HA is not one thing, it’s a family of things (things being cosmetic ingredients) that could all subtly differ in their water-binding capacity. Further I’m less concerned about the actual number now, it seems far less important somehow.

My conclusion so far is that this is less likely a case of internet exaggeration and more likely one of over-simplification. We seem so often to be encouraged to turn our attention and focus onto simple, measurable and singular somethings rather than creating ourselves space for pondering the details of the everything.

Just one more thing…

On my troll around the internet I did find one person who’d taken up the challenge of trying to investigate whether this 1000 x weight thing was true by focusing on what happens to the HA when you add water to it. This person concluded that if HA can bind 1000 x its weight in water it would mean that 1g of it would turn 1 litre of water into a gel (or a more gel-like substance than just watery water). That didn’t happen of course and that lead to the conclusion that the internet and cosmetic brands are lying. I admire this person for trying and feel the experiment had merit but it is important to note that while a polymers molecular weight has been found to correlate positively with its water holding capacity, we now appreciate that water holding and water gelling are not one in the same. To measure how HA influences water requires spectroscopy rather than our eyes – with chemistry there’s always more than what we see.

And with that I’ll say goodbye for now.

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