Thursday, January 18, 2007

Randy Old Goats

So, sometime ago, I posted about the news that the MHRA had ordered Gillian McKeith to stop making claims about a product that contained an extract of Horny Goat Weed. Now, over at badscience, some discussion is going on about other products with similar claims.

For example, from
The perfect aphrodisiac and vitalising tonic for both men and women. Can support or improve any of the following: • Libido/Sexual Desire • Energy & Stamina • Erectile Dysfunction • Sperm Production • Sensory Stimulation
So, this has got me wondering, what is the evidence for the effect of horny goat weed?

Just as a background to the research, I stopped off at the wikipedia article on Horny Goat Weed (I wanted to know the latin name). The wikipedia article claims that an extract, known as icariin, promotes the formation of nitric oxide and that it also acts as an inhibitor of an enzyme, PDE5. It also gets something rather wrong, where the article claims:

Viagra, a popular pharmaceutical, works by blocking the production of the PDE-5 enzyme.

It actually works by blocking the activity of PDE-5, not the production of the enzyme. Both inhibition of Nitric Oxide synthesis and PDE5 activity are viable mechanisms for inducing erections in men, but does it work?

Going to pubmed and searching for "Epimedium" returns eight pages of results, limiting these to English language returns four pages. Scanning through these results there are two articles that may be of interest.

First, one study demonstrated that doses ranging from 300 microgrammes to 10,000 microgrammes, injected directly into the region of the penis known as the corpus cavernosum, resulted in an increase in blood pressure in the rat penis (Chen and Chiu, 2006).

Second, a study demonstrated that, when incubated with extract, strips of rabbit penile tissue relaxed (Chiu et al, 2006).

So, some evidence that it may work through the mechanism claimed, but no evidence that it produces an effect by oral dosing, even in animals. Next up, the wikipedia article claims that the active component is a compound known as icariin. So, what is the evidence there? A couple of articles report icariin having an in vitro IC50 of around 1 micromolar against PDE5 (Ning et al, 2006 and Xin et al, 2003) which is a reasonable potency.

However, there is nothing on the compounds pharmacokinetics in humans or even rodents. So, all this in vitro work, interesting though it is, may be of no use if the compound is not absorbed by the intestine.

So, final word? There is no evidence to support the use of Horny Goat Weed at the moment. It has a plausible mechanism though, and further research may determine its efficacy.

Chen KK, Chiu JH.
Effect of Epimedium brevicornum Maxim extract on elicitation of
penile erection in the rat.
Urology. 2006 Mar;67(3):631-5.
PMID: 16527595
Chiu JH, Chen KK, Chien TM, Chiou WF, Chen CC, Wang JY, Lui WY, Wu CW.Epimedium brevicornum Maxim extract relaxes rabbit corpus cavernosum through multitargets on nitric oxide/cyclic guanosine monophosphate signaling pathway.
Int J Impot Res. 2006 Jul-Aug;18(4):335-42. Epub 2006 Jan 5.
PMID: 16395327

Ning, H., Xin, Z., Lin, G., Banie, L., Lue, T.F., Lin, C., et al.
Effects of icariin on phosphodiesterase-5 activity in vitro and cyclic guanosine monophosphate level in cavernous smooth muscle cells.
Urology,(2006) 68(6), 1350-4.
PMID: 17169663 Xin, Z.C., Kim, E.K., Lin, C.S., Liu, W.J., Tian, L., Yuan, Y.M., et al. Effects of icariin on cGMP-specific PDE5 and cAMP-specific PDE4 activities. Asian journal of andrology,(2003) 5(1), 15-8.PMID: 12646997

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