Does self-tanning lotion protect my skin from the sun?

Theself-tanner is a cosmetic product used to induce a customisable matte colouration to the skin, similar to a tan but not quite the same. In this context, it replaces the action of the sun's rays and the activity of melanin, which is responsible for the skin's natural colouration. The process is carried out with active ingredients, most often DHA and erythrulose. The superficial colouration achieved, akin to natural tanning, is due to the covalent binding of DHA's ketonic or aldehydic compounds to free amino acid groups in the corneal layer (glycosylation) in a chemical reaction known as the "Maillard reaction".

This process results in the formation of melanoidins, polymeric pigments produced through a series of non-enzymatic chemical steps, which are fixed in the upper layers of the stratum corneum and remain there until the corneocytes are renewed. This colouration, which only affects the first layer of desquamating cells, is therefore semi-permanent, develops gradually and is well tolerated by the skin. It develops over approximately six hours.

Can self-tanners be considered as sun protection products?

Even though some self-tanning treatments also contain sunscreens, it should be noted that self-tanner does not replace sun protection. Indeed, sun protection treatments benefit from a protection factor (SPF), which gives them the ability to protect you from the sun's UV rays. The brown colouring achieved has nothing in common with the natural pigmentation due to melanin. Indeed, melanoidins do not have the same protective properties as melanin against solar rays.

Several studies have quantified the sun protection power of melanoidins obtained through the reaction between DHA and the main amino acids present in the corneal cells of the epidermis. The conditions necessary for the formation of melanoidins were then recreated, and the sun protection factor against UVB (SPF) and protection factor against UVA (PF-UVA) were determined.

However, regardless of the amino acid used, the concentration of DHA applied or the number of applications, the corresponding SPF and UVA-PF values are virtually nil (approximately SPF 2 and UVA-PF 2) and that the SPF value decreases over time, even though the colouration lasts several days. This property arises from the fact that DHA has the ability to absorb short-wavelength ultraviolet rays to the visible region of sunlight, with an overlap in the UVA part of the spectrum, thus potentially providing some protection against UVA.

Due to these low values, we can state that self-tanning formulas primarily containing DHA do not possess sun protection properties and cannot be considered as a sun protection agent as such.

Given the modest SPF of DHA, it is therefore necessary to add a sun care product to the self-tanner to provide sufficient protection against the sun. To recognise the level of protection offered by a cream, milk, oil or sun gel, look at the sun protection factor against UVB (SPF) but also that against UVA (PA-UVA), indicated on the product label. They assess the product's ability to delay the assault of UVB (sunburn, skin cancers) and UVA (premature skin ageing) on the skin, respectively. The higher the protection indices, the greater the protection provided will be. The selected index should take into account your skin type as well as the conditions of sun exposure (mountain, beach...). Sun protections are thus classified into 4 categories according to the SPF value provided.