The very best moisturizers for dry skin are what nature provides us with.
Billions of microscopic water-retaining globules in the skin's outer layers called natural moisturising factors (NMFs).
When we're young we have them in just the right amount to keep the skin glowing, soft and fresh.
However, this perfect balance is upset at puberty when fluctuating hormone levels are apt to over stimulate oil production.
The balance tips the other way later in life, when the body's ability to produce sebum starts to fall.
Our natural moisturizing factors get depleted and the skin no longer holds onto moisture as effectively as it used to.
The result: dry, flaky skin.
So what's the best way to replenish the skin's moisture - and more to the point, restore the body's ability to keep your skin that way - if your NMFs can't keep up any more? Let's take a quick lesson in dermatology.
Slowing the moisture loss NMFs are responsible for preventing moisture from deep within the skin - called transepidermal water - from too rapid evaporation which in turn leads to dehydration or dry skin.
Water is constantly moving through the skin, from the skin's deep layers to the outer layers and then out into the atmosphere.
NMFs are like sentries that grab hold of this water and detains it as long as they can.
So if we're looking to replicate this process the best moisturizer is one that does the same job.
Conventional moisturising creams rubbed into the surface of the skin won't do the trick.
Water molecules are just too big to pass through the skin barrier.
In fact, any water left on your skin after using "light" (more water than oil) moisturisers or toners - or even after washing and not thoroughly drying yourself - will have the opposite effect.
Over time, this extra moisture can end up diluting your natural moisturising factors to the point where they can evaporate, leaving the skin dryer than ever.
Oil is the answer Thankfully, treating dry skin is quite simple.
All we need to do is replenish the skin's oil supply, either with a facial oil or with a face cream emulsified with as little water as possible.
But not just any oils will do.
We want something that can mimic the oils found in our natural internal moisturisers.
And the closest match happens to be plant-based oils rich in essential fatty acids.
You'll find plenty of moisturizing products that use mineral oils instead.
These are petrochemical by-products and have a very different composition from plant oils.
They're devoid of fatty acids and can even impair the natural function of the skin.
They are very common in commercial brands - so give a wide berth to any skin products that list paraffinium or something similar among their ingredients.
No more dry skin? There's a growing body of research that supports the view that the skin's moisture levels are proportional to the levels of fatty acids present.
So, taking that a step further, it makes sense to take advantage of plant oils rich in these fatty acids and let them work their magic on dry skin.
Their similarity to the body's own oils means that they are absorbed remarkably quickly, without leaving your face looking or feeling greasy.