I'm writing a guide on the best self-tanners and would love a scientist to explain how they work. What is DHA and how does it react with skin to develop color over 24 hours? Why can it be washed off after several hours even though full development takes a day? I've read that it uses the Maillard reaction, similar to how toast browns—is that accurate?

I'm Allen Rosen, a board-certified plastic surgeon in New Jersey, and a lot of my work involves skin quality, healing, pigmentation, and helping patients understand what topicals can and cannot do. In cosmetic practice, that matters just as much with a self-tanner as it does with facial rejuvenation or post-procedure skin care. DHA is dihydroxyacetone, a simple sugar that creates a temporary brown tone by reacting with proteins in the very surface layer of skin. The color appears gradually because the reaction products keep forming and deepening over many hours after application, so what you see at 2 hours is not the finished result. The reason you can wash after several hours is that there are really two things on the skin early on: leftover product sitting on the surface, and the reaction that has already been initiated in the skin surface itself. Once enough contact time has passed, rinsing removes the excess formula, but it doesn't "undo" the color that is already developing. And yes, calling it a Maillard-type reaction is fair and useful for readers, as long as you explain that it's a skin-surface browning reaction rather than a true "tan." The practical takeaway I give patients is simple: apply evenly, let it sit long enough before showering, and expect final color to look more accurate the next day than it does the first evening.

With my Master's in Biotechnology from Johns Hopkins, where I specialized in novel drug development and published on cellular reactions, plus leading ProMD Health's anti-aging skin treatments, I've applied this to how topicals interact with skin proteins. DHA, or dihydroxyacetone, is a simple sugar molecule that triggers the Maillard reaction--yes, spot-on like toast browning--by binding to amino acids in the skin's surface layer, forming colored polymers called melanoidins. You can rinse off the initial loose color after 4-6 hours because that's unbound DHA oxidizing quickly, but the full covalent bonds deepen over 24 hours as the reaction progresses at skin's pH and temperature. In our ProMD clinics, we see this in body contouring preps, where patients apply DHA-based pre-tanners; my early lab management at Hopkins taught me these timed reactions ensure even, lasting results without irritation.

Your toast analogy is accurate. Self-tanners use dihydroxyacetone, a three-carbon sugar called DHA, and the color develops from a Maillard reaction between that sugar and amine groups on the keratin proteins of your outer skin (lysine, arginine, and histidine residues). The pigments produced are melanoidins, chemically the same family of brown compounds that forms when bread toasts or meat sears. The 24-hour window exists because the reaction is slow at body temperature. A first hint of color appears in 2 to 4 hours, most develops within 8 to 12 hours, and the reaction keeps advancing for 24 to 72 hours before it plateaus. Only the dead cells of the stratum corneum are reacting. The living skin underneath is not. Partial wash-off in the early hours works because surface DHA that has not yet reacted is still loose sugar on top of your skin mixed with sweat and sebum. Water and soap lift that unreacted DHA plus some already-stained squames ready to shed. Once melanoidins form inside the corneocytes they are locked into the protein and come off only when those cells naturally exfoliate, which is why the tan fades over 5 to 7 days as your stratum corneum turns over. The clinical note I add for patients is that DHA produces free radicals on the skin surface during the reaction, so pair it with antioxidant moisturizer and morning SPF. A self-tanner is color, not photoprotection.