Skin cell turnover: how long the cycle really takes and why it slows with age

SKIN OVER 40

How long does skin cell turnover take?

In young adults, skin cell turnover takes around two to three weeks for the outermost layer and a little over five weeks on average for the epidermis as a whole. The process is always the same: cells divide in the lowest layer of the epidermis, the basal layer. The new cells move upwards, flatten, lose their nucleus and become corneocytes. These form the stratum corneum, the skin’s protective outer layer, which is eventually shed as tiny, invisible flakes. How long this journey takes depends on exactly what is being measured. A classic study in the Journal of Gerontology marked the skin’s surface with a fluorescent dye and observed how quickly it disappeared. In young adults, it took about 20 days to pass through the stratum corneum. A study in the Journal of Investigative Dermatology tracked cell division using labelled cellular building blocks and arrived at an average turnover time of 39 days for the epidermis as a whole, from the basal layer up to the stratum corneum.

So the figures do not contradict each other; they describe different stretches of the same journey. The one thing to bear in mind is that the surface you see and apply cream to is the final stage of a process that began weeks earlier further down.

Is the 28-day rule of thumb accurate?

As a rough order of magnitude for young skin, the 28-day rule holds; as a precise figure, it does not. It sits between the value measured for the stratum corneum of young skin and the value for the epidermis as a whole, and it is a good way of explaining why skin does not look different overnight. It becomes a problem when it is treated like a timetable, along the lines of: after four weeks your skin is completely new, so after four weeks every product must show visible results. That is not how skin works. First, the speed varies from person to person. Second, it varies from one part of the body to another, even within the face. Third, it changes with age, and not at an even pace. If you use 28 days as your yardstick at 55, you are counting on a pace your skin often no longer quite reaches. The more honest way of putting it is this: the surface of your skin replaces itself within a few weeks, and in mature skin that is more likely to take a little over a month than three weeks. That is enough to go by, and it takes the pressure off the question of whether everything has to look different after exactly one month.

Why does skin cell turnover slow down with age?

Skin cell turnover slows down with age because the cells in the basal layer divide less often, so fewer new cells come through. The Journal of Gerontology study mentioned above shows this quite directly: in older adults, transit through the stratum corneum took more than ten days longer. Because the number of cell layers in the stratum corneum stayed the same with age, the researchers interpreted this as a sign of reduced cell division. The pattern is striking. According to the analysis, the decline was not spread evenly across adult life; turnover stayed relatively constant in the younger years and dropped markedly after the age of 50. This is the stage of life when many people notice that their skin looks rougher and that minor irritation stays visible for longer.

Slower renewal is only part of the picture, though. Over the decades, UV damage adds up, and the skin becomes thinner and holds on to moisture less well. Which of these factors you can influence is covered in the article on how to slow down skin ageing. Why skin also loses thickness over the years is explained in the article on thin skin on the face.

What does this mean for how your skincare works?

For your skincare, skin cell turnover means one thing above all: part of the effect can be felt immediately, while the visible part only follows over several cycles. Moisture works directly in the stratum corneum. It makes your skin feel softer after you apply it, and this effect lasts for as long as you keep up your routine. Anything that goes beyond the moment, such as a more even or calmer-looking complexion, takes time, because the stratum corneum you apply cream to today is only gradually replaced by new cells coming up from below. That is why it does not show after a week but over several cycles – and in mature skin, those cycles take longer. You will find specific timelines for each active ingredient in the article on how long skincare takes to work. In practice, this means consistency counts for more than quantity. One pack of The Duo lasts around two months when used morning and evening. Even with slower turnover, that is roughly two passes through the stratum corneum – enough time to judge fairly how your skin responds.

Can you speed up skin cell turnover?

Skin cell turnover can be sped up, mainly with retinoids and acids, but faster is not automatically better. Retinoids such as retinol act on cell division in the epidermis. Acids such as AHAs loosen the bonds between the outermost corneocytes so that they shed earlier. Both can make the surface look smoother, but both can also cause irritation, especially on sensitive or dry skin. Acids also make skin more sensitive to UV radiation. If you remove corneocytes faster than new ones come through, you thin out the protective layer instead of renewing it. Particularly after 50, when the supply of new cells is already slower, that is a poor trade.

• Daily sun protection: does not speed anything up, but protects the cells coming through from additional UV damage.
• Retinoids: the best-studied way of influencing turnover, with an adjustment period and some potential for irritation.
• Acids only in moderation: what they do and when you can skip them is covered in the article on the AHA peel.
• Look after your skin barrier: moisture and lipids do not speed anything up, but they make sure the stratum corneum can do its job.

Why does mature skin need patience rather than more exfoliation?

Mature skin needs patience rather than more exfoliation because it replaces lost corneocytes more slowly, so overdoing it stays visible for longer. An exfoliant removes whatever is sitting loosely on the surface. That feels smooth for a short while, but it does nothing to change how quickly new cells are formed further down. If you exfoliate too often or too harshly, your skin loses part of its protective layer, and mature skin takes longer to make up for that loss. Typical consequences are tightness, redness and stinging from products that used to be fine. The article on whether you need to exfoliate your face at all helps you answer that question. For most mature skin, the answer is: rarely or not at all. Realistic expectations help here. In her review on Judge.me, Nicole B. writes: “The fine lines don’t go away, which isn’t really the idea anyway” (review translated from German). That is exactly the right attitude for the pace of mature skin.

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