This degradation of tryptophan is expressed by the kynurenine/tryptophan ratio, which was first introduced by Fuchs et al. The enzyme indoleamine 2,3-dioxygenase, which is induced by proinflammatory cytokines (i.e., interferon-Оі, tumor necrosis factor-О±), converts tryptophan into kynurenine 1. It is also known as a precursor of the serotonin synthesis in the central nervous system. Tryptophan is an essential amino acid, which is utilized for protein biosynthesis and metabolized via the kynurenine pathway. In the last few years, the clinical interest in evaluating the tryptophan metabolism has increased. Further studies are needed to get better insight in the causality of these findings. This study showed little/moderate associations between haemoglobin, biomarkers of iron metabolism and tryptophan markers.

Six patients with anaemia of chronic disease were observed with the lowest serum tryptophan levels and the highest kynurenine/tryptophan ratio compared to 11 individuals with iron deficiency anaemia and 413 non-anaemic patients. Overall, 115 patients with iron deficiency showed lower tryptophan and kynurenic acid concentrations compared to 315 individuals without iron deficiency. The cubic regression model yielded the highest R-square values between haemoglobin and tryptophan markers. The strongest correlation was observed between tryptophan and haemoglobin (pвЂ‰<вЂ‰0.001, rвЂ‰=вЂ‰0.305). Indicators of tryptophan metabolism were positively correlated with haemoglobin and markers of iron metabolism (p-values: <0.001вЂ“0.038 r-values: 0.100вЂ“0.305).

Here, we evaluated potential associations of tryptophan, kynurenine, and kynurenic acid with indicators of iron metabolism (i.e., mean corpuscular volume, mean corpuscular haemoglobin, ferritin, transferrin saturation, serum iron, transferrin, soluble transferrin receptor, reticulocyte haemoglobin) and haemoglobin in 430 individuals grouped by the presence or absence of iron deficiency or anaemia. However, study cohorts are rather small and final conclusions are still lacking. Current literature proposes associations between tryptophan metabolism and anaemia.

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