The study was approved by the medical ethical committee of the Radboud University Nijmegen Medical Centre. Written informed consent was obtained from the participants.

Five female outpatients (age mean 24.0, SD 3.5 year, range 19 to 29 year) who met the US Center for Disease Control and Prevention criteria for CFS, were recruited through the outpatient clinic of the Department of General Internal Medicine of the Radboud University Nijmegen Medical Centre. A structured Clinical interview for DSM-IV (SCID-I) was performed by the researcher, to exclude patients with current psychiatric co-morbidity. Exclusion criteria were: vegetarians, nursing women, pregnant women, use of psychotropic medication in the last month, previous or current engagement in CFS or psychiatric research. Patients were tested in the follicular phase.

We used ATD mixtures and placebo mixtures as described by Sobczak et al. 2002[13]. There were two test days, one week apart. On each test day, patients were assessed after an overnight fast. 24 hours before the test days patients had a tryptophan low diet[12]. Each participant received placebo and ATD. Baseline assessments (T0) took place at 8.30 a.m and five hours after ingestion of the mixtures the second assessment (T1) took place. The hospital pharmacy prepared the mixtures and the order of treatment was balanced over two the test days. The participants, investigators and laboratory technicians were blinded to the treatment condition.

To evaluate the efficacy of the ATD procedure, at T0 and T1, total plasma Tryptophan and LNAAs (phenylalanine, tyrosine, isoleucine, leucine and valine) were measured simultaneously after completion of the study with high-performance liquid chromatography as described elsewhere[14].

All patients drank the ATD- and placebo mixtures within 15 minutes. Besides a little discomfort during and just after drinking the mixtures, the patients could tolerate the mixtures quite well.

Different lines of evidence support the hypothesis of an increased serotonergic state in CFS. In this pilot study we evaluated the effect of an overall serotonin depletion in a cross-over design in CFS-patients. Serotonin depletion was achieved by acute depletion of the serotonin precursor tryptophan.

There is some evidence that there is a threshold for tryptophan depletion that needs to be exceeded before behavioral effects occur[18]. Although ATD resulted in a significant (96%) reduction of the tryptophan/LNAA ratio, no improvements were found in CFS-related symptoms. Since ATD very probably resulted in a transient serotonin depletion for a few hours, one could hypothesize that a more prolonged 5-HT depletion would be necessary to achieve neurobiological 5-HT changes and changes in fatigue severity in a chronic condition like CFS. A prolonged overall increased serotonergic synaptic neurotransmission could lead to a chronic downregulation of 5-HT1a receptors and a reduced binding potential[7]. Only a short and transient serotonin depletion would probably not lead to structural changes at the level of serotonin receptors. Furthermore, the most consistent finding of ATD is a mood-lowering response in patients recovered from depression[19]. To our knowledge, studies have not yet been performed investigating ATD effects on chronic fatigue in CFS patients; it is possible that ATD has a smaller impact in these patients. On the other hand, ATD studies have reported fast and significant effects on mood patients recovered from depression, particularly in those taking serotonergic antidepressants[20] and in patients with social anxiety disorder[20, 21]. Assessment took place 5 hours after ingestion of the mixtures. Plasma tryptophan reaches its minimum level between 5 to 7 hours after ingestion of the ATD mixture. It can be understood that there is a delay from the time plasma tryptophan reaches the point of maximal depletion to the time of maximum serotonergic depletion in the brain. In this study it is not exactly known when maximum effects occur. The study was performed in a homogeneous group of medication-free female CFS-patients. To avoid confounding effects of current psychiatric conditions that can affect brain serotonergic function we recruited CFS patients without current axis-1 psychiatric comorbidity. There is a group of subjects who are vulnerable to the effects of ATD. Patients who had previous episodes of depression or people with a family history of depression are vulnerable[20]. In our study we did not screen in detail the patients history or the family history. However any undetected vulnerability for depression would have increased the chance for finding ATD would lead to lowering of mood. In fact, in the study we did not observe a mood lowering effect of ATD. A major limitation of this study is the small sample size. Although there was no change in any of the outcome measures and robust improvements at the individual level were not detected either, further research in larger sample sizes is needed to allow a more definite answer, regarding the effect of ATD in CFS. In conclusion, our earlier treatment study with a selective serotonin blocking agent as well as the present study with alimentary induction of significant ATD showed no positive results in any of the outcome measures in the treatment of CFS. Administering of ATD in a much larger sample size is needed to draw final conclusions on the hypothesis of an increased serotonergic state in the pathophysiology of CFS.