The sun and skin ageing.

It is well-known that sun exposure can be harmful to the skin, ranging from simple redness to melanomas and carcinomas, including more or less intense sunburns. However, the sun's impact on the skin does not stop there and can also result in premature skin sagging accompanied by brown spots, true markers of age. The sun can also accelerate the onset of signs of photoaging such as dryness, roughness, wrinkles, dilation of blood vessels (telangiectasia)... Depending on phototypes, the impact of the sun's rays is relatively significant.

For fair skin, some scientists believe that 80 to 90% of ageing signs result from sun exposure.

The different rays of the sun's spectrum have an impact on the organisation of the skin's layers and their constituents, particularly UV rays, which emit in the 100-400 nanometre wavelength range. While UVB rays (280-320 nm) stop at the epidermis, UVA rays (320-400 nm) can penetrate more deeply and reach the dermis, where they cause a range of damage. In particular, studies have shown that this radiation can up-regulate the activity of the transcription factor AP1-1, a stimulator of the expression of matrix metalloproteinases (MMPs). These, in turn, are known to degrade the collagen and elastin, the structural proteins of the dermis. The induction of MMPs could thus be a primary mechanism of photoaging.

Furthermore, upon penetrating the skin, UV rays induce an excess generation of free radicals, which are highly reactive chemical species. Naturally produced during cellular energy metabolism, free radicals are responsible for oxidative damage to skin components (proteins, lipids, and DNA), ultimately leading to a visible chronic change in its appearance. Although free radical damage is part of the normal ageing process, the additional damage caused by UV-generated molecules plays a significant additional role in photoageing.

Under normal circumstances, the balance between free radicals and antioxidants is maintained by regulatory mechanisms. However, with age and repeated exposure to UV rays, the number of free radicals increases and oxidative stress sets in. Several studies have indeed confirmed that UV rays produce enough free radicals to overwhelm the body's endogenous antioxidant defences. Furthermore, these free radicals play a role in the UVA-induced induction of MMPs and promote the degradation of the dermis's protein fibres, resulting in skin that is less firm, less elastic, and less supple.

Another marker of skin ageing related to sun exposure is the appearance of telangiectasias on the face, which are small dilated blood vessels. Indeed, studies have shown that UV rays are responsible for the hardening of blood vessels. Combined with the progressive reduction in the density of the supporting perivascular connective tissue over time, this phenomenon leads to chronic dilation, causing the gradual appearance of red spots and telangiectasias.

Furthermore, the sun stimulates the melanogenesis, the skin's pigmentation system. Under the effect of UV rays, the transfer of melanosomes to keratinocytes intensifies, leading to an increase in melanin production. In addition, epidermal melanocytes are highly vulnerable to oxidative stress, a phenomenon that disrupts melanogenesis and leads to a disorganised and excessive production of melanin. These various factors, coupled with the gradual depletion of the melanocytes' pigment potential, lead to the appearance of brown spots, also known as solar lentigines, usually around the age of 40.

Finally, a recent clinical trial has highlighted the existing link between sun exposure and skin ageing. This was conducted with a very large sample of 298 women with a light phototype, aged between 30 and 78 years. The participants were divided into two groups based on their sun exposure history: 157 women were characterised as sun-seeking (the "sun" group) and the remaining 141 were classified as sun-avoiding (the "shade" group). Several criteria related to skin ageing, such as the presence of wrinkles and the existence of pigmentation and vascular disorders, were evaluated according to the age groups of the participants. The results are compiled in the table below and clearly show the impact of the sun on skin ageing.