This placebo cross-over trial aims to study the effect of the oral intake of an essential amino-acid 5-OH tryptophan, the precursor of serotonin, on the fatigue scores in IBD patients in deep clinical and biological remission.

Inclusion Criteria:

• The subject is male or female and aged 18 to 60 yrs (inclusive)
• The subject has a documented Crohn's disease or ulcerative colitis
• The subject is, in the opinion of the investigator, capable of understanding and complying protocol requirements
• The subject is in clinical remission over last 3 months (based on physician global assessment)
• The subject is in clinical remission at day 0 based on validated scores (SCCAI ≤ 2 for ulcerative colitis or Harvey Bradshaw index ≤ 4 for Crohn's disease).
• The subject reports fatigue on a quantified scale (visual analogue scale 0 - 10) of 5 or more
• The subject is treated with biologicals and/or immunosuppressives since at least 6 months with stable dose over last 3 months
• The subject is in biologic remission at day 0: CRP < 10 mg/l and faecal calprotectin value < 250 mg/kg

Exclusion Criteria:

• The subject has a clinical validated depression
• The subject is taking antidepressives or neuroleptics
• The subject has a psychiatric comorbidity
• The subject has important comorbidities: cardiovascular disease, obstructive lung pathology, neoplasia or other
• The subject has a documented Anemia (based on lab results including Hb < 12-13 g/dl respectively for saturation index < 20%, Vit B12 < 148 pmol/L or folic acid < 6 nmol/L)
• The subject has a documented hypothyreoidea (documented by a recent lab result including TSH)
• The subject reports an infection within 2 weeks before inclusion
• The subject reports any change in IBD medication (biologicals and immunosuppressives) in the last 12 weeks before inclusion
• The subject was treated with oral corticosteroids during the last 8 weeks before enrolment
• The subject reports an ongoing pregnancy or breastfeeding
• The subject has a history of lymphoproliferative disease or cancer other than basocellular skin cancer
• The subject underwent surgery in the past 12 weeks prior to the screening visit
• The subject reports a history of clinically significant drug abuse (defined as any illicit drug use) or alcohol abuse within 1 year prior to inclusion