Tryptophan supplements were once used, mostly to aid sleep, by around 30 million consumers, but they were removed from the UK and US markets in 1989, after one batch was linked to an outbreak of Eosinophilia-Myalgia Syndrome (EMS), a rare autoimmune disease, in more than 1500 people.
It is now believed that the epidemic was solely the result of L-tryptophan containing a contaminant - 'peak E' - resulting from the production process at one manufacturer, Showa Denko.
However the supplement market remains considerably reduced. A law introduced in England in 1990 banned the amino acid from being added to foods, although it was modified to allow those classified under the EU regulation for PARNUTs (foods for particular nutritional use) and for foods consumed under supervision of healthcare professionals.
But following a request from the Institute for Optimum Nutrition (ION) in 2002, the UK's Food Standards Agency (FSA) is now planning to add a new exemption to the law so that laevorotatory tryptophan (L-tryptophan) can be marketed in supplement form if the ingredient meets the purity criteria laid out in European Pharmacopeia.
Supplement labels will also be required to advise a maximum daily dose of 220mg, according to the FSA proposal, open for consultation until 25 May.
ION nutritionist and founder Patrick Holford argues however that this maximum level is unfounded.
Prior to the ban, tryptophan supplements were sold in doses of 500mg or 1000mg, he told NutraIngredients.com. In Belgium, which along with Holland has allowed tryptophan supplements for several years, consumers can buy products in doses of up to 6g per day.
"We already have a logical precedent for setting the upper safe level, which is to look at the lowest level of a nutrient for which there is evidence of an adverse effect. The UK's EVM report was based on this practice."
"But the committee on medicines, whose remit is not to advise on USLs, has gone on to recommend that the tryptophan USL should be a tenth of the average therapeutic dose. This is totally illogical and contradicts the basis of working out USLs previously established by FSA," said Holford.
The ION is in the process of preparing a document to submit to FSA's consultation requesting that the dosage requirements are changed.
Whether at low dosages or not, Holford believes the amino acid, the only one not available in supplement form in the UK, will return to the market and regain its prior popularity.
Although research has dried up since the early 1990s, there is good evidence of its benefit in people with insomnia and depression.
Manufacturers of the ingredient are not so sure however. The global food industry currently only uses around 100-150 tons of the amino acid annually (most is used in animal feed), and Kyowa Hakko, one of two main manufacturers, is only selling around 2 tons a year into Italy and Spain.
Ennio Squara, executive vice president of Kyowa Hakko's southern Europe division, said: "The market has been stable for many years. And there is a reason why it is stable and will have problems growing.
"The dosages used are very small. Everyone is now very careful when using this product."
There is also an additional concern for the marketplace.
"A few months ago tryptophan made in China appeared on the market so we have a question mark over how it is manufactured," warned Squara.Both Kyowa Hakko and Ajinomoto make the product through fermentation, said to have a lower risk of the impurities thought to be responsible for the ESM outbreak in the late 80s, than biosynthesis.
New applications, such as the growing weight-loss market, could however drive new growth.
Unilever recently commissioned food research organisation NIZO to manufacture food-grade tryptophan, through an alternative method to the usual fermentation process, for use in a diet beverage.
It is thought that low intake of the amino acid may be responsible for mood swings that make it difficult for some people to stick to a weight-loss regime.
It is also being used in a mixture of amino acids to boost energy levels in sports products, according to Squara.