The Role of Dietary Tryptophan on Aryl Hydrocarbon Receptor Activation

Not Applicable

Completed

Brief Summary: This study evaluates the role of dietary L-tryptophan, an essential amino acid, in the activation of a specific cellular component: the aryl hydrocarbon receptor.

Detailed Description: The Aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor implicated in a range of key cellular events. In the gut, AHR is crucial for maintaining intestinal barrier immune homeostasis. The physiology of the AHR, however, is not completely understood; its precise gut luminal activators and functional consequences are unknown.

Some AHR ligands originate from the diet. Commensals play crucial roles in metabolizing tryptophan and other amino acids such as tyrosine, with the subsequent production of tryptophan metabolites. Previous studies show that inflammatory bowel disease (IBD) patients have impaired production of AHR agonists by the microbiota. Furthermore, dietary supplementation with tryptophan ameliorates clinical parameters of colitis in rodent models. Whether these findings translate into human pathophysiology has not been explored.

In the present study, the investigators will evaluate the effect of high- versus low-tryptophan diet on AHR activation in healthy participants. Briefly, participants will be instructed to follow a standardized low-tryptophan diet and will be randomized to a 3-week L-tryptophan supplement or placebo. Later, after a 2-week washout period, participants will crossover to the other arm. In addition, the effect of tryptophan and microbiota-derived metabolites on AHR activation will be analyzed.

Recruitment & Eligibility

Inclusion Criteria

Exclusion Criteria

Arm && Interventions

Group	Intervention	Description
Low-tryptophan diet and placebo	Placebo	Standardized low-tryptophan diet (500-1000 mg of L-tryptophan and 1800 kcal) + placebo.
Low-tryptophan diet and L-tryptophan.	L-tryptophan	Standardized low-tryptophan diet (500-1000 mg of L-tryptophan and 1800 kcal) + L-tryptophan supplements (3 g/day).

Primary Outcome Measures

Name	Time	Method
AHR activation levels in stool and duodenal content.	three weeks	Changes in AHR activation levels will be assessed in stool and duodenal samples before and after the intervention (high- and low-tryptophan diets) using an AHR cell-reporter line.

Secondary Outcome Measures

Name	Time	Method
Tryptophan metabolites levels, including host and bacterial catabolites, in blood, urine and stool.	Three weeks	Changes in tryptophan metabolites leves will be compared before and after the intervention, in blood, urine and stool samples.
mRNA levels in duodenal and rectum/sigmoid biopsies.	three weeks	Changes in mRNA levels in duodenal and rectum/sigmoid biopsies will be assessed before and after the intervention.
Bacterial and fungal microbiota composition in stool, duodenum and rectum/sigmoid biopsies.	Three weeks	Changes in bacterial and fungal microbiota composition will be assessed before and after the intervention in stool samples, duodenum and rectum biopsies.
Cytokines in serum.	three weeks.	Changes in cytokines in the serum (IL-22, IL-6, IL-2, IL-10, IL-12p70, IL-23p19, IFNγ, TNFα and CRP will be measured by ELISA in cell culture supernatants after stimulation with LPS, curdlan and ConA ) will be measured before and after the intervention and patients will be grouped into two categories for each measurement: high vs. low, according to the cutoff reference test value for each of the cytokines.
Gastrointestinal symptoms	three weeks.	Changes in gastrointestinal symptoms before and after the intervention will be assessed using a validated questionnaire (The Gastrointestinal Symptoms Rating Scale)
Mood	three weeks	Changes in mood before and after the intervention will be assessed using a validated questionnaire (Hospital anxiety and depression scale)