Amino Acids: Tryptophan is an essential amino acid because it cannot be produced by the body and therefore must be consumed in the diet. Tryptophan, like most amino acids, is used to make proteins, but tryptophan has many other important functions outside of its role in proteins. Tryptophan acts as a precursor to serotonin, melatonin and niacin. Tryptophan was a popular dietary supplement because as a precursor to serotonin and melatonin it tended to act as a sleep aid. Tryptophan was removed from the market as a dietary supplement for a time due to some concerns over its manufacture. It is again available as is its metabolite 5-hydroxytryptophan. One possible concern with the supplementation of tryptophan and 5-hydrxytroptophan is the formation of serotonin outside of the brain. Serotonin has been shown to cause pathology of the heart valves like that seen with the popular weight loss combination Phen/Fen. 5-HTP is sometimes used in combination with carbidopa to prevent this side effect.