Have you read the newest white paper? If not, check it out here. There's even more great news about 5-HTP:
Tryptophan is an essential amino acid, meaning it cannot be made by the body and therefore must be obtained from dietary sources. A typical diet provides about 1000mg of tryptophan, but that may not be enough for all the roles tryptophan plays within the body. For example, tryptophan can be used by the liver the create vitamin B3 (niacin). However it takes 60mg of tryptophan just to make 1mg of B3. Whatever tryptophan is left over still needs to attach to a transport molecule to get into the brain, but other amino acids are looking for a ride on those same molecules. As explained in the white paper, insulin will remove those competing amino acids.
Even with a healthy diet very little tryptophan makes it to the brain, and that amount decreases with age or during certain disease states such as chronic inflammation. This provides at least one explanation of the increased depression, insomnia, and weight gain that is associated with aging.
5-HTP avoids the hurdles of tryptophan. As an intermediary in the conversion of tryptophan to serotonin, 5-HTP does not need to attach to a transport molecule and it can not be used for anything else so the brain is free to convert it to serotonin as needed.
(source: http://www.lifeextension.com/Magazine/2008/4/Why-Aging-People-Become-Depressed-Fatigued-And-Overweight/Page-01 )