"Aruba in a Tuba," the plastic  tube in the cosmetics store   promised, and I was a believer. After months of pallor brought about by daily use of sunscreen and lots of office work, I was ready to go for the gold. On my way to the cash register, I thought of the Bain de Soleil model from the early 1980s. Remember her? In a sleek black bikini, relaxed and baked to golden-brown perfection with that "St. Tropez Tan?" I'm looking for that look without risking my health. Although I've given up on the bikini, I'm still shooting for bronzed legs without having to wear pantyhose.

Everyone knows by now that too much exposure to sunlight is unhealthy. It causes skin cancer and premature aging. So a natural tan is out of the question. Heck, even the Bain de Soleil model swore off the real thing years ago and does a fake bake these days. Now it was my turn. But as the clerk rang up my purchase, I had to ask, "What's that stuff?" She didn't know, and neither did I. So I found out.

Aruba in a Tuba is just one of a hundred or more brands of creams, gels, mousses, and sprays that you can find at drug- stores, department stores, and salons. They all work the same way--that is, they all have the same active ingredient: dihydroxyacetone (DHA).

DHA is a simple sugar and is nontoxic. In fact, it is an intermediate in carbohydrate metabolism in higher plants and animals and is more rapidly metabolized than glucose in the body. Specifically, this three-carbon keto sugar is a physiologic product of the body formed and utilized during glycolysis--that quintessential metabolic process that we've all had to memorize at least once. DHA used in self-tanners is usually prepared by fermentation of glycerine by Acetobacter suboxydans.

DHA's browning effects were discovered by accident. In the mid-1950s, at Children's Hospital at the University of Cincinnati, researcher Eva Wittgenstein was studying the effect of large oral doses of DHA in children who had glycogen storage disease. These kids were ingesting a lot of DHA--as much as 1 g per kg of their body weight. Sometimes the children spit up some of this sweet concentrated material, and it splashed on their skin. A few hours later, the kids had brown spots on their skin where stray splashes hadn't been wiped off.

Wittgenstein was able to do something with her observations other than berating the staff for not getting those kids cleaned up. Curious, she prepared aqueous solutions of varying concentrations of DHA and was able to reproduce the pigmentation on her own skin. (That's a lot better than letting the kids do it for her.) She was able to turn her skin brown, but how?

The answer lies in the Maillard reaction, perhaps known best to food chemists. In this process, named for Louis-Camille Maillard, who first described it in 1912, amino acids interact with sugars to create brown or golden brown compounds. This type of interaction is the cause of much of the browning that occurs during manufacture and storage of foods. It also makes beer golden brown.

Here is how it happens in self-tanners: The reaction of skin with DHA to produce an artificial tan proceeds through combination with free amino groups in skin proteins, and particularly by combination of DHA with the free guanido group in arginine. (Epidermal proteins have a very high content of the amino acids arginine, lysine, and histidine.) In related experiments, Wittgenstein found that arginine was the most reactive, with the appearance of a dark brown color within 30 minutes. Aqueous mixtures of DHA with glycine, lysine, and histidine also gave brown to yellow pigments. These pigments are called melanoidins. Melanoidins are polymeric compounds that are linked by lysine side chains to the proteins of the stratum corneum--which is the outermost, dead layer of human skin.

And DHA doesn't penetrate any further than the stratum corneum. Wittgenstein figured this out by tape stripping. After she treated her skin with a solution of DHA and before the pigment had developed, she put some tape on her skin and pulled it off. No pigment formed in the peeled area.

DHA is present in tanning products at concentrations between 2 and 5%. The deeper tanning formulas are more concentrated. The formulations are stable at pHs between 4 and 6. Above pH 7, brown compounds form in solution, so it doesn't work as a tanner. Therefore, mixtures sold as tanning agents are buffered to about pH 5 and are also kept relatively cool, because prolonged heating to even a little above body temperature affects their stability.

The cosmetic acceptability of these products seems to have improved in recent years. More natural-appearing brown or golden hues are produced, as opposed to a more off-color orange observed with older formulations. The shades obtained may be more acceptable to medium-complected people than those with dark or fair complexions. Purer supplies of DHA, refining of DHA-containing vehicles to allow better penetration, recognition of the need for a lower pH, and more rapid color change with a lower concentration of DHA may have all contributed to the improved outcome.

Although the formation of melanoidins is different from that of melanin--the natural substance in skin that causes real tanning--some of their properties are similar, especially their absorption spectra. This suggests DHA tans offer some protection from the sun, but not much--maybe equivalent to SPF (sun protection factor) 2 or 3. So you'll still need plenty of sunscreen, shades, and a big hat to stay safe. You can get a wicked sunburn through your artificial tan and defeat the whole purpose.

So play it safe: Your perfect tan is made in the shade.

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