Self-tanners. The role of dihydroxyacetone

Although we are increasingly aware of the risks of sun exposure, the idea that tanned skin is more attractive and healthier still prevails. For this reason, companies that are dedicated to solar cosmetics are focusing their efforts on launching increasingly safer tanning products. There are two types of these products on the market: tanning accelerators and self-tanners. By themselves, these products do not protect against solar radiation, unless they contain UVA and UVB filters in their formulation.

Tanning cosmetics enhance the synthesis of melanin, which is the molecule responsible for the natural tanning of the skin. They are formulated from active principles related to melanin or the melanization process in which tyrosine, tyrosinase, dopa, etc. intervene. Natural extracts of carrot, green walnut shell and citrus are also used.

In the case of self-tanners, the coloration of the skin is achieved without exposure to solar radiation. They react with keratin and other proteins in the epidermis. Products that trigger skin coloring reactions are usually hydroxyaldehydes or hydroxyketones. Among them, dihydroxyacetone (DHA) stands out very especially. Tea extract, carrot oil and oily walnut extract are also used.

In this article we will focus on self-tanning products, in particular those that contain DHA in their formulation, since this molecule is present in almost all self-tanning preparations available on the market.

It must be taken into account that the market for self-tanning products, although in its beginnings it did not have a great reception (since they gave a very orange tone, left orange stains on the skin and clothes and had a rather unpleasant), has evolved notably due to the improvements achieved in the product itself (they act without leaving stains, the color that is achieved is much more similar to a natural tan, they incorporate moisturizing agents, exfoliants, sun filters, antioxidants that delay aging, specific products for different skin tones and exclusive for the face) and by the growing awareness of the aggressiveness of the sun.

Discovery of the tanning effect of DHA

The first self-tanners containing DHA appeared on the market in 1959, shortly after the tanning properties of this substance were discovered.

The tanning properties of DHA were discovered by chance. At the Children’s Hospital of the University of Cincinnati (USA), the effect of giving high doses of DHA in children with glycogen storage disease was being studied. On occasion, children vomited the sweet concentrate that was being administered. It was found that in those places where the remains of vomit had not been completely removed, brown spots appeared after a few hours. From there, aqueous solutions of DHA at different concentrations were tested and their coloring properties confirmed.

Chemical structure of DHA

DHA (1,3-dihydroxy-2-propanone) is a sugar with 3 carbon molecules. It is a non-toxic product. In fact, it is an intermediate in carbohydrate metabolism in most plants and animals.

In self-tanning products, the DHA that is used mainly is that prepared by fermentation of glycerol by the use of Gluconobacter oxidans. DHA is found as a dimer in freshly prepared aqueous solutions. It reverts to the monomeric form on heating.

It is stable at pH between 4 and 6. Above pH 7 it loses effectiveness and its maximum activity is at pH 5-6, which is actually the normal pH of healthy skin.

Mechanism of action of DHA

The site of action of DHA is the stratum corneum of the skin. DHA interacts with free amines, peptides, and amino acids in the uppermost layers of the epidermis, resulting in the Maillard reaction. Chemically, the Maillard reaction consists of the reaction of amino acids with sugars, with which colored compounds are obtained. As a consequence of this reaction, DHA is converted to pyruvaldehyde with removal of water. Subsequently, the keto or aldehyde function reacts with the amine function of the skin’s keratin and forms an imine (ketoimine or aldoimine). This is followed by a series of complex reactions, not entirely known, but which as a result give cyclic and linear polymers of a golden or brown color. These polymers are called “melanoidins”, name that responds to the similarity of coloration obtained with respect to melanin. Although melanin and melanoidins are chemically very different, they have similarities in certain properties, especially their absorption spectrum.

Protective activity of DHA

The similarity between natural tanning and that produced by DHA can be misleading: unlike melanin, polymers derived from DHA do not absorb significant amounts of UV light and therefore do not protect against UVB rays. Instead, they do absorb UVA radiation. For this reason, the association of DHA with juglone or lawsona (the main component in henna dyes) had been recommended. The aforementioned union is composed of a 3% solution of DHA in 50% isopropanol, and also contains 0.035% of juglone or lawsona. This mixture tans the skin. It has been ensured that the acquired coloration absorbs 95% of UV radiation and 20% of IR radiation that reaches the skin. Protection is acquired only after color develops, and is then maintained by reapplying the mixture at 2-7 day intervals. The problem with this association is its instability, since it requires the sequential application of its components. It is currently in disuse, since the sequential application of the active ingredients leads to an uneven distribution of color on the skin. People who use self-tanners containing DHA need to be aware that, despite their tanned appearance, they are not protected against solar radiation.

Unlike melanin, DHA-derived polymers do not absorb significant amounts of UV light and therefore do not protect from UVB rays.

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