Time and again online you come across the statement that 60% of what you apply to your skin is absorbed into it. Something that so many people are saying must be true, must it not?
Well no it isn’t. But I admit it does sound sort of believable. You apply a cream to your skin and rub it in and it disappears. Where is it all going?
To get at the answer to that we need to have a look at the structure of the skin. The top layer of the skin is known as the stratum corneum and this comprises a highly selective barrier. It will let water across it. In theory the water from the cream could penetrate across the stratum corneum, but in practice water is continually being lost from the body’s core across the skin and into the atmosphere. So the water in the cream – a skin cream will be about 70% water typically – will just get caught up in the continual flow of water out of the body.
It is important to remember that water is being lost all the time from the surface of the skin. This simple fact has all sorts of impacts, and one of them is that any chemical that you want to get to cross the skin is effectively swimming upstream against the current.
Another feature of the skin that makes it an extraordinarily good barrier is its extremely large surface area. This is too small to see with the naked eye, but it is made up of huge numbers of tiny cells tightly squeezed against one another. It is a bit like a sponge that is able to absorb a great deal. This is how most moisturisers do their stuff. They absorb into the stratum corneum and reduce the speed that water can get across it.
The stratum corneum is continually shedding itself so what you’ve added will sooner or later be lost from the surface. This is why you have to continually reapply your moisturising cream.
Some of what is applied will make it through into the blood stream, but it will be way way lower than 60%. This whole process is of huge interest to pharmaceutical companies who would love to be able to apply drugs through the skin. Millions have been spent on studies into every aspect of skin penetration you can imagine. In fact a very small proportion of the money spent ended up in my bank account when I worked on this very area. If you read the very wide literature on the subject you will find out that we have a pretty good idea of what does and doesn’t penetrate the skin. And in fact most things don’t. That is why you don’t see insulin patches or aspirin patches.
I can only think of three drugs of any significance that have the right properties to be successfully applied in patches. Caffeine, nicotine and glyceryl trinitrate. Nearly everything else is effectively blocked by the stratum corneum. But even for the small number of ingredients that can get through the skin at all, there is still one more line of defence. The skin contains enzymes that break down chemicals into smaller and easier to handle materials. So the small proportion that gets through is quite likely to have been transformed before it reaches the blood stream.
So bearing all this in mind it is impossible to believe that 60% of what you apply to the skin gets through to your bloodstream. I don’t know what the true figure is. It might be a couple of percent. If I was basing it purely on my own work I’d put the figure below 1%.
But if you really want to know you could always do your own experiment. One day instead of eating the traditional way through your mouth have a go at getting your food to absorb through your skin. If the 60% figure is correct this really shouldn’t be very difficult. Good luck!