Tryptophan is one the 22 amino acids and is an essential amino acid, meaning we must eat it in our diet, we cannot make it ourselves. Tryptophan is also known to be a precursor used by the body to make serotonin. Serotonin is the chemical in the brain that almost all antidepressants act on to increase its activity in the brain. So it is logical that increasing tryptophan in the diet may increase serotonin and possibly help treat depression. And indeed, this has been looked at going at least as far back as 1976.
Since that time multiple small studies have been done showing that tryptophan levels are lower in depressed patients and tryptophan supplementation does help alleviate symptoms of depression. In 1976 there was a study by Dr. Farkas of 16 patients. After that there were multiple studies in the 1990s and early 2000s showing that depressed patients have lower levels of tryptophan in the blood. Shaw K et. al. from Australia in 2002 did a good review of all clinical trials that had been done to that date and found of the studies that were done almost all pointed to tryptophan being better than placebo for depression. However, it was noted that these studies were mostly small and of poor quality making it difficult to draw conclusions.
And now, 37 years after the original studies, we are still without compelling evidence to prove or disprove the effectiveness of tryptophan. So we have a totally benign substance that has good pharmacological reasons why it would work for depression and multiple studies pointing in the direction that it does work, and still we have no studies sufficiently large enough to recommend for or against its use.
Currently it is estimated that 11 percent of all American adults are taking medications for depression. All those antidepressants cost Americans $11 BILLION (that is billion with a B) in 2010 alone.
If tryptophan was found to work as well or better than these drugs we could instead be using tryptophan which costs about 15 dollars a bottle or better yet we could just get more tryptophan in our diet.
Tryptophan can be found in all kinds of foods, especially protein based foods. Soybeans, cheese, sesame or sunflower seeds, meats, milk, and many other foods are all good sources. One great source is Spirulina which is a microscopic cyanobacterium found in tropical lakes. It was eaten in the past by many ancient American civilizations such as the Aztecs. It is a complete protein containing all of the essential amino acids which includes tryptophan. Diets replacing 60% of all protein intake with Spirulina showed no adverse side effects. It does however contain large amounts of vitamin K which has to be avoided by patients on blood thinning (anticoagulant) medications.
Nearly 40 years is far too long to wait for such a potentially beneficial and safe treatment for depression to be evaluated.