lipids of the skin barrier , especially of the outermost stratum corneum , are highly impermeable to water when functioning at their best . These barrier lipids hold water within the skin , limit transepidermal water loss ( TEWL ), and prevent dehydration . TEWL is , in fact , often used to measure the functionality of the skin barrier . In humans , the average TEWL is about 300-400 ml / day , although this may vary considerably depending upon factors both within the skin and outside of the skin , such as humidity . 1
Although the absolute amount of water present within the skin is very important , lipid levels also control this to a large extent . If lipids in the skin barrier decline too much , then excess water is lost , and dehydration occurs .
Amounts of water in skin vary at different levels within the skin . More water is present in deeper layers of the skin . Water content within interior cells of the skin and body is about 70 % and is necessary for normal enzymatic processes within all cells . As cells of the epidermis move upwards during their normal transit , keratinocytes change in quality and lose water . When these reach the surface , they have become flattened , devitalized , and devoid of nuclei . At this point , they are no longer living but they do make a great skin barrier ! They also have lost a great deal of their intracellular water . This is a normal process of forming the protective skin barrier . A healthy skin barrier should still contain at least 10 % water or risk losing functionality . 2
Skin that appears flaky is often called dry . However , this flakiness is an abnormal desquamation process linked to any number of factors that could include low hydration levels as one of these . Corneocytes are normally shed in small enough clumps to be invisible to the human eye . When these are shed in larger flakes as the result of abnormal desquamation , then dryness may be assumed to be the only culprit when it is not . 3
The skin uses several key molecules and processes to maintain correct water balance .
Water – The correct amount of water must be present both within cells and between cells . Some ingredients – Imperata cylindrica root extract , for example – affect the sodium-potassium pump and the movement of water across cell membranes . This can be helpful in balancing water content both within and without cells .
Ceramides – Ceramide structures are key components of the skin barrier , along with other lipids such as cholesterol and free fatty acids . It is the lipid structure of the skin barrier that holds water within the skin – and within