Although the effectiveness was not demonstrated, a subgroup of women treated for breast cancer can probably be supported by the program. The results of the present study are a starting point for further development and use of the program.

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Background/Aims: Tryptophan is the precursor of serotonin and niacin (vitamin B₃). The latter is critical for normal cellular metabolism. Tryptophan and niacin can be deficient in patients with serotonin-producing neuroendocrine tumors (NETs). Niacin deficiency may lead to severe symptoms including pellagra. In patients with serotonin-producing NET, data on niacin status are scarce and niacin supplementation hardly receives attention. We aimed to assess the niacin status before and after supplementation in these patients. Methods: We identified serotonin-producing NET patients who had received oral niacin supplementation (mean dose 144 mg daily) for tryptophan deficiency and/or pellagra-associated symptoms. Presupplementation plasma tryptophan levels and niacin status based on the urinary niacin metabolite N¹-methylnicotinamide (N¹-MN) before (n = 42) and after the start of the supplementation (in 34 paired samples) were assessed. Reference values for urinary N¹-MN levels were established in 133 healthy individuals. Results: The mean presupplementation plasma tryptophan level was 31.8 ± 9.7 µmol/l (reference value 40.0-70.0). Presupplementation urinary N¹-MN levels were lower in patients (median 17.9 µmol/24 h, range 2.6-70.3) compared to healthy controls (median 43.7 µmol/24 h, range 9.5-169.3, p < 0.0001) and below normal in 45% of the patients. Niacin supplementation increased urinary N¹-MN levels to high normal levels (median 55.5 µmol/24 h, range 7.4-489.0) in 86% of the niacin-deficient patients. Conclusion: In serotonin-producing NET patients, niacin deficiency is prevalent. Therefore, urinary N¹-MN deserves to be included in their standard biochemical evaluation. Niacin supplementation normalizes the niacin status in most niacin-deficient serotonin-producing NET patients. A prospective study is warranted.