Conversion of 5-HTP into serotonin in the gut is normal. However, it could very well be that a sick gut might result in excessive serotonin conversion. That concern is what brought me here to this forum (the information here is invaluable. Thank you all!).
Serotonin is necessary for the proper function of several organs, aside from the brain. One of the most common manifestations of serotonin deficiency in the brain is depression. However, it's not at all uncommon for people to have loads of serotonin circulating in their bodies, but very little as a neurotransmitter in the brain.
AllyB infers that serotonin crosses the blood-brain barrier. Not true! Serotonin cannot cross the BBB. However, serotonin precursors, such as 5-HTP and tryptophan can cross the BBB, where it will then be converted to serotonin.
Sadly, many have experienced the dismay of taking 5-HTP supplements for depression, only to discover that it has no benefit. This is likely because the 5-HTP is being converted into serotonin in the gut, and therefore never reaching the brain. An unhealthy digestive tract is the most likely culprit. One possible solution is to take 5-HTP in sublingual form.
Tryptophan is a precursor of 5-HTP. If you've tried 5-HTP for Depression and are displeased with the results, try tryptophan. It readily crosses the BBB, where it is converted first to 5-HTP and then into serotonin. As is the case with all such amino acids, it's best to take it on an empty stomach so that it has less competition in the bloodstream and can more readily cross the BBB.
Tryptophan is an essential amino acid, derived from protein (e.g. raw milk, meat, eggs). However, few of us get enough tryptophan in our diet, and Depression is more commonplace now than ever, as a result.
The FDA, being the valiant protectors of our health that they are, banned tryptophan in 1990. No doubt just a coincidence, the ban took place a mere four days prior to the FDA's official public announcement of its approval of Prozac.
The good news is that the FDA took tryptophan off its ban list a couple years ago. Few people seem aware of this, and only a few supplement-makers are again making it. It can also be quite expensive. However, I did find it reasonably priced at http://iherb.com
Let me please highly recommend a book that I just got done reading by Julia Ross called The Mood Cure http://moodcure.com She gives some excellent recommendations on beating Depression through diet and amino acid supplements. My only complaint about the book is that she fails to make any mention of how a sick digestive system is likely to interfere with the proper assimilation of her diet recommendations.