Acute tryptophan depletion balances altered resting-state functional connectivity of the salience network in female patients recovered from anorexia nervosa

Recent Advances in Current Uptake Situation, Metabolic and Nutritional Characteristics, Health, and Safety of Dietary Tryptophan

Tryptophan (Trp) is an essential amino acid which is unable to be synthesized in the body. Main sources of Trp are uptake of foods such as oats and bananas. In this review, we describe the status of current dietary consumption, metabolic pathways and nutritional characteristics of Trp, as well as its ingestion and downstream metabolites for maintaining body health and safety. This review also summarizes the recent advances in Trp metabolism, particularly the 5-HT, KYN, and AhR activation pathways, revealing that its endogenous host metabolites are not only differentially affected in the body but also are closely linked to health. More attention should be paid to targeting its specific metabolic pathways and utilizing food molecules and probiotics for manipulating Trp metabolism. However, the complexity of microbiota-host interactions requires further exploration to precisely refine targets for innovating the gut microbiota-targeted diagnostic approaches and informing subsequent studies and targeted treatments of diseases.

New Perspective on Anorexia Nervosa: Tryptophan-Kynurenine Pathway Hypothesis

Anorexia nervosa (AN), affecting up to 4% of all females and 0.3% of all males globally, remains the neuropsychiatric disorder with the highest mortality rate. However, the response to the current therapeutic options is rarely satisfactory. Considering the devastating prognosis of survival among patients with AN, further research aimed at developing novel, more effective therapies for AN is essential. Brain and serum tryptophan is mostly converted along the kynurenine pathway into multiple neuroactive derivatives, whereas only 1-2% is used for the synthesis of serotonin. This narrative review provides an update on the experimental and clinical research data concerning the metabolism of tryptophan along the kynurenine pathway in anorexia nervosa based on the available literature. We propose that in AN, lower levels of L-kynurenine and kynurenic acid result in diminished stimulation of the aryl hydrocarbon receptor, which could contribute to abnormally low body weight. The impact of L-kynurenine supplementation on anorexia in animal models and the effects of changes in tryptophan and downstream kynurenines on the clinical progression of AN require further investigation. Moreover, prospective clinical studies on larger cohorts of restrictive and binge-eating/purging AN patients and assessing the potential benefit of L-kynurenine as an add-on therapeutic agent, should follow.