Excretion, on the other hand, is more complex. In 1974, a scientist by the name of H. Eberhardt published an article entitled "The Regulation of Sebum Excretion in Man." In the abstract, he explains that a series of tests he conducted demonstrates that excretion is regulated by surface tension. I was unable to locate the full-text article through the online databases to which I subscribe, but here is a link to the abstract. You can purchase the complete article here, as well.
http://www.springerlink.com/content/pv373302mt827067/A similar study was conducted in 1982 by Millns and Maibach of the Department of Dermatology at the University of California Medical School. The study yielded similar results. I was unable to access the list of references they used in their article, but I can almost guarantee that they built upon the work of Eberhardt.
http://www.springerlink.com/content/ut32254520970266/For some long threads I started several years ago in which I discussed the scientific evidence against the "feedback theory" (the idea that washing sebum off the skin stimulates it to make more oil), here are the names of those threads, along with the direct links to them:
http://www.acne.org/messageboard/myth-skin...ebu-t35818.html"The myth of skin washing and sebum production"
http://www.acne.org/messageboard/evidence-...-th-t71047.html"Still more evidence against the "feedback theory"
http://www.acne.org/messageboard/FINALLY-d...est-t81548.html"FINALLY: a more direct test of the 'feedback theory' ".
I haven't read the study you cited above by Eberhardt, who claims (rather oddly) that sebum excretion is somehow regulated by "surface tension"; in fact, Kligman and Shelley (the two doctors who wrote the famous study in 1958 disproving the "feedback theory") did some experiments in their study which would appear to make such a weird idea seem implausible; they covered the skin of human volunteers with heavy artificial oil, and found that they
still produced sebum at about the same rate; and they taped small glass cups to the foreheads of other volunteers for a considerable length of time (one month), and found that they
still continued to excrete sebum at that location: "In each case a fatty, horny mass accumulated, having a queer verrucous appearance. Neither the sebaceous gland nor the epidermis showed any sign of shutting down." The obvious implication, of course, is that in areas of high sebum production, the stuff tends to flow away or be wiped away, even though it continues to be produced and excreted at about the same rate all the time.
And really, this makes a lot of sense when you think about the nature of facial sebum. If hormones were the only cause of oily skin, people with high levels of androgens or androgen receptors would have oil dripping off of their faces all the time.
Not necessarily. In a later study also done by Kligman, he found that giving a rather surprising amount of extra androgen every day to male human volunteers (up to 300 mg of methyl testosterone) had no effect at all on their sebum production; it had a variable effect on women. His explanation for that lack of effect in men was that even their
normal levels of testosterone production were sufficient to "maximize" the stimulation of their sebaceous glands, and giving them extra androgens had no extra effect.
But that's not how it works. Even people who tend to have really oily skin reach a point during the day where their skin has reached its oiliest point. Now this is not to say that hormones don't play a role in excretion, but the point is that there's more to the mechanism.
I think there's more to it in the sense that other hormones besides just the sex hormones (androgens and estrogens) can also have some effect on sebaceous gland activity, but not these other weird issues of "surface tension" and washing. (Ambient temperature does appear to play a part in how quickly sebum is excreted, too.)
Edited by bryan, 16 March 2011 - 05:40 AM.