Anorexia nervosa is a recognized risk factor for pellagra owing to the restricted intake of niacin and tryptophan, which are precursors of nicotinamide adenine dinucleotide synthesis [10, 11]. The presence of Casal’s necklace in this case provided a key diagnostic clue consistent with classic pellagra. Nicotinamide adenine dinucleotide is essential for repairing UV-induced DNA damage; thus, pellagra-associated lesions predominantly occur on sun-exposed areas, including the face, neck, chest, upper back, forearms, and dorsal hands, and may progress to leathery hyperpigmentation [15]. Photosensitivity in pellagra is attributed to an increased kynurenic acid concentration and a decreased urocanic acid concentration. Niacin deficiency enhances kynurenic acid synthesis via the tryptophan–kynurenine–nicotinic acid pathway, while reducing urocanic acid in the stratum corneum, thereby diminishing UVB protection [5, 16]. The differential diagnoses include photosensitive dermatoses such as drug-induced photosensitivity, porphyria, lupus erythematosus, and dermatomyositis. Cutaneous findings in anorexia nervosa arise not only from malnutrition but also from behavioral and psychiatric comorbidities. Russell’s sign consists of knuckle calluses from repeated self-induced vomiting and is indicative of purging-type anorexia nervosa. The deceased in our case showed several skin findings associated with anorexia nervosa, including Russell’s sign, toe perniosis, interdigital intertrigo, paronychia, and delayed wound healing (Fig. 2a–c) [17]. The histopathological features of pellagra vary with disease progression and may resemble psoriasis [18, 19]. In our case, the characteristic findings of pellagra included epidermal hyperplasia with hyperkeratosis, parakeratosis, loss of the granular layer, and irregular elongation of the rete ridges (Fig. 4b). Furthermore, the lack of Munro’s microabscesses supported the differentiation from true psoriasis. Individuals with pellagra may also have subepidermal blisters resembling sunburn injury [20].

Amino acid profiling revealed severe tryptophan depletion, providing biochemical evidence of pellagra. Prior studies report that cases of alcoholic pellagra show similar reductions in amino acids, including decreased concentrations of tryptophan (36.5 ± 39.4 nmol/mL vs. 59.4 ± 166 nmol/mL in healthy controls), leucine, valine, lysine, and methionine [21]. Our findings in this case were compared with population-based data from healthy Japanese women (median age, 52 years; range, 24–80 years) [14]. Amino acid concentrations are significantly influenced by sex and age, while short-term lifestyle factors (e.g., fasting duration, time of day of blood sampling, season, physical activity, NSAID use, smoking, and temporal alcohol consumption) have minimal effects [22]; therefore, these indicators have low individual variability and are reliable for assessing nutritional conditions.

The postmortem biochemical changes in amino acid concentrations remain unexplored, and these effects must be considered before their application in forensic practice. In this case, most amino acid concentrations were markedly elevated compared with the concentrations in living individuals, likely due to a combination of hemolysis, tissue lysis, and potential bacterial biosynthesis (Fig. 5). A previous study revealed that leaving blood samples at room temperature for 6 h after collection increases the ornithine and glutamic acid concentrations, and decreases the arginine and cystine concentrations [23]; however, the tryptophan concentration shows only a mild increase of 7.5% [23]. In contrast, the deceased in our case had a markedly decreased tryptophan concentration, which cannot be explained by known pre-analytical variables alone. Further studies are needed to elucidate the postmortem alterations and improve the reliability of biochemical assessments in amino acid profiling as part of forensic evaluation.