||Hey Erowid, what's up with that caffeinated soap? Does it really work? What kind of dose would one expect to get out of an average hand-washing or shower?|
||As far as soaps go, I can't think of any reason why caffeine would stop the soap from working as a soap, helping to dissolve dirt and oils deposited on the skin.|
As for getting a dose of caffeine, I would highly doubt an effective dose of caffeine would permeate the skin from such a product. Though there are reviews that extol the virtues of caffeinated soaps and their ability to deliver a satisfactory "jolt", their limited testing does not confirm anything more than the possibility of a placebo effect: "The smell of the peppermint oil was strong enough to make my eyes open wide."
A search for scientific studies about the permeability of caffeine through mammalian skin yielded several webpages describing caffeine's poor ability to be absorbed transdermally. While one page did describe caffeine as having "good penetrating capability", this was in a study in which the skin was exposed for 4 hrs using radioactivly labelled caffeine. Even then, with a 1 mg or 9.3 mg caffeine per 100 cm^2 area of skin the absorbed dose was calculated to be only 17% and 1.6%.
Assuming the entire 250mg dose of caffeine suggested by the soap manufacturer made it to the skin and was held there for 4 hours, the above research would suggest that one would expect a dose between 4 and 42mg of caffeine. This dose is less than a typical cup of coffee.
Another major factor which is likely to reduce absorption of the "250 milligrams" of caffeine in each "dose" of the product is that the soap itself is likely to keep much of the caffeine from coming in direct contact with the skin by holding the chemical in suspension. The caffeine is diluted by the water and soap and generally soap is washed off the skin after less than a few minutes. As a comparison, most transdermal drug delivery is done with substances with active doses under 1-2 milligrams.
A letter from the Hawaiian Department of Health about the possibility of skin exposure from contaminated water sources (due to concerns from studies on frogs) was particularly helpful. While filled with scientific jargon, it was a wealth of information on this topic. Primarily, it describes and cites studies where caffeine was found to have rather low skin permeability. In addition, it addresses a physically measured quantity called the "octanol:water partitioning coefficient". This coefficient is a measure of how well a substance dissolves in octanol vs. water. This value is often useful in predicting the ability of pharmaceuticals to move into and/or through the body, as your cells are surrounded by a "lipid bi-layer," essentially a thin layer of fat.
Caffeine's octanol:water partitioning constant is quite low, indicating it is very water soluble and very fat insoluble. This is sufficient for oral consumption of caffeine, because to get into the bloodstream it only has one or two cell layers to get through. However with skin, you have a thick layer of dead cells before you even get to the lower, blood vessel-containing dermis. It is possible, although unlikely, that the presence of soap could increase the membrane/cell permeability of caffeine, but it seems far more likely that the soap and water will keep the caffeine from being in contact with the skin.
In conclusion, until I see a double-blind study using this product, I will remain highly dubious that the effects felt by using caffeinated soap are anthing more than placebo and subconscious attempt to justify buying overpriced soap.
[ Chemicals ]
[ Caffeine ]