We are asked this question a lot, so I wanted to take some time to review what these two compounds are and how the body/brain uses them so it becomes clear why one is better than the other to raise serotonin levels in the brain.

Both 5-hydroxytryptophan (5-HTP) and tryptophan are amino acid precursors of serotonin. Both cross the blood-brain-barrier and both can be used to increase serotonin levels in the brain. However, as you will soon see, 5-HTP is better for this purpose.

Tryptophan

As an amino acid, tryptophan is a precursor of serotonin. Tryptophan is first converted into 5-hydroxytryptophan (5-HTP) which is then converted into serotonin in the brain. Serotonin cannot cross the blood-brain-barrier; however, tryptophan and 5-HTP can cross the blood-brain-barrier. Thus, serotonin levels in the brain are dependent on enough tryptophan or 5-HTP reaching the brain so this conversion can take place.

Tryptophan is one of the essential amino acids found in the human diet. Tryptophan is found in most dietary proteins, but it is especially abundant in meat, poultry, milk, yogurt, eggs, fish, spirulina, sesame/sunflower/pumpkin seeds and oats. However, tryptophan is the least abundant essential amino acid in foods, making up only about 1% of the amino acids available in dietary proteins. This is important since many amino acids compete with tryptophan to enter the brain (via the blood-brain-barrier). Due to the  relative scarcity of tryptophan in the diet and stiff competition with other, more prevalent amino acids leads to very little tryptophan uptake into the brain. Because of this, studies have shown that food sources of tryptophan cannot dramatically increase brain levels of serotonin.

However, tryptophan is available as a dietary supplement. As a supplement, tryptophan is available in two isomeric forms: D- and L-tryptophan. The L-isomer is more common and it is the form of tryptophan that is used by the body to make serotonin. The use of L-tryptophan as a dietary supplement was banned for most of the 1990s because of bacterial contamination left in the final product by one of the major suppliers of the amino acid. These bacterial contaminants caused several cases of eosinophilia-myalgia syndrome (EMS). However, L-tryptophan is now commonly available as a dietary supplement and is considered safe.

5-HTP

5-HTP is a naturally occurring amino acid and chemical precursor to serotonin. It is derived from tryptophan and is converted into serotonin by the aromatic-L-amino acid decarboxylase enzyme with the help of vitamin B6.

5-HTP can readily cross the blood-brain-barrier, whereas serotonin cannot.

There are no food sources of 5-HTP, although it can be derived from the metabolism of tryptophan. As a supplement, 5-HTP is obtained from the seeds of Giffonia simplicifolia, which is a plant native to Africa.

5-HTP does not compete with other amino acids to cross the blood-brain-barrier. However, 5-HTP and tryptophan can also be converted to serotonin in the liver and other tissues outside the brain. In fact, more serotonin is produced and stored outside the brain than inside. However, serotonin cannot cross the blood-brain-barrier and only the 5-HTP or tryptophan that is taken up into the brain can increase serotonin levels in the central nervous system.

5-HTP vs. Tryptophan

Although tryptophan and 5-HTP are both precursors to serotonin, they do have some important differences when it comes to efficiency and effectiveness for increasing serotonin levels in the central nervous system/brain.

Efficiency

5-HTP is the direct precursor of serotonin (and an intermediary precursor in the synthesis of melatonin) while tryptophan is the direct precursor of 5-HTP. However, tryptophan can also be used to synthesize other compounds, including niacin, proteins and enzymes. Thus 5-HTP is more efficient than tryptophan and will more consistently increase serotonin levels.

In addition, the conversion of tryptophan into 5-HTP is the rate-limiting step in the production of serotonin, meaning that this is the slowest part of the reaction needed to make serotonin. Using 5-HTP directly can bypass this step and lead to a faster conversion into serotonin.

Once more, tryptophan competes with several other amino acids to cross the blood-brain-barrier; 5-HTP does not compete with any other compounds to enter the brain. Thus, using 5-HTP is a more efficient method to increase brain levels of serotonin.

Effectiveness

Both 5-HTP and tryptophan have been shown to increase serotonin levels in the brain and exhibit pharmacological effects. However, tryptophan is more likely to be converted into serotonin in the liver than 5-HTP. This occurs because 5-HTP is more readily taken across the blood brain barrier while tryptophan must compete with several other amino acids to enter the brain. While tryptophan remains outside the brain, it is more likely to be converted into serotonin in the liver (or into other compounds such as niacin, proteins or enzymes).

Taking into account all these differences, the data shows that 5-HTP is more efficient and effective as a way to increase serotonin levels in the brain and central nervous system than using tryptophan.