L-tryptophan is an essential amino acid and a metabolic precursor to serotonin. Serotonin acts as a chemical messenger, or neurotransmitter, in the central nervous system and appears to affect the sleep/wake state of arousal. Serotonin levels in the brain can be increased by the ingestion of L-tryptophan, and this has been shown to hasten the onset of sleep in humans.
L-tryptophan may also be of some benefit in the treatment of some psychiatric disorders. Taking their cues from these facts, promoters of "nutritional supplements" have pushed L-tryptophan as a safe and "natural" sleep aid and as a general mood enhancer.
But as far back as 1973, the U.S. Food and Drug Administration knew that this amino acid supplement was associated with significant health risks. In 1985, Canada restricted L-tryptophan's availability to that of a prescription drug, while the FDA did nothing about the promotion of the substance in this country.
In 1973, Canada reported only 11 cases of eosinophilia-myalgia syndrome (EMS), 10 of which were among victims who purchased the supplement in the United States. EMS is a serious and often fatal disorder linked to L-tryptophan use and, in one case recently, possibly to use of the amino acid supplement lysine. Finally, in 1990, after a 1989 outbreak of EMS in the U.S. in which at least 27 died and some 1500 were affected, the FDA pulled L-tryptophan off of store shelves.
Research is continuing into the nature and exact cause of EMS. Among the candidate chemical culprits being considered are various impurities associated with the preparation of L-tryptophan. One is chemically similar to a contaminant found in cooking oil that in 1981 was responsible for an outbreak of EMS in Spain. But chemical forms of L-tryptophan itself still have not been ruled out as being involved.
The lesson in this story is that, while L-tryptophan is an essential amino acid, it is not normally found in the human diet by itself. It' s certainly not ordinarily ingested in the amounts provided by supplements. Monosodium glutamate, or MSG, is also an amino acid, but it's well recognized that the use of this ingredient in cooking can be a problem for many people. Likewise, though the simple sugar glucose is indispensable to metabolism, a diet high in sugars is not a healthy one for most people.
It's also worth considering that the body normally keeps the circulating levels of most of the constituents of blood within fairly narrow ranges. We know that there can be harmful effects of chronically high or low levels of many of these substances. Higher levels of glucose, as experienced by diabetics, for example, are known to cause damage to vital organs. The hemoglobin (the red pigment in the blood) of diabetics is commonly checked by physicians for the arnount of sugar which becomes abnormally attached to it as a way of measuring how high the average glucose level has been.
What harm might there be in having chronically (or intermittently) increased levels of tryptophan in the blood? No one knows. But here, unlike the situation with respect to some other scientific propositions, the null hypothesis is that a substance is unsafe until it is proven safe. Otherwise, consumers become the guinea pigs of supplement promoters.
Just because a little is good doesn't mean that more is better. Therefore, before a drug or "supplement" is promoted as being safe and effective for a given purpose, there should be adequate evidence to support the claims made for it. And the burden of providing such evidence, of course, falls on those who make the claims.
The above FAQ Sheet was adapted from an article originally published in the September 1992 issue of the newsletter of the North Texas Skeptics.
