I over simplified my comments. L-Tryptophan easily converts to serotonin in the gut but this serotonin does not cross the blood brain barrier. The L-Tryptophan that crosses the blood brain barrier must be processed into 5-HTP by an enzyme (tryptophan hydroxylase) that is strictly controlled on the brain side of the blood brain barrier. This 'metering' system regulates the amount of serotonin available to the brain. There is some discussion about whether this tryptophan hydroxylase system malfunctions causing a deficient amount of serotonin to be made available.

5-HTP, on the other hand, readily converts to serotonin without this metering system creating a risk of serotonin syndrome when 5-HTP is combined with an SSRI.

Basically, the amount of serotonin available from L-Tryptophan on the brain side of the blood brain barrier is limited. Some even suggest that if serotonin is deficient, adding L-Tryptophan does not correct this deficiency. These people suggest taking 5-HTP to bypass this broken metering system. It is something I have considered trying.

mrsD, I'd love to hear what you know about this L-Trypt vs 5-HTP issue. There are a wide variety of positions presented on this issue.