A Case of Fanconi Syndrome with Biopsy-Proven Acute Proximal Tubular Injury Induced by a Dietary Supplement, Benikoji

Introduction: Fanconi syndrome is a common consequence of drug-induced nephrotoxicity characterized by diffuse injury of the proximal tubules that results in the malabsorption of various electrolytes and substances. However, cases of dietary supplement-induced Fanconi syndrome are rare and there is no report on renal histology in these patients. Herein, we present a case of dietary supplement-induced Fanconi syndrome confirmed by renal biopsy. Case Description: A 49-year-old woman who had been taking a lipid-lowering dietary supplement, Benikoji, was referred to our hospital due to anorexia and nausea over the past 13 days. Blood examinations revealed renal dysfunction, hypokalemia, hypophosphatemia, hypouricemia, and metabolic acidosis. Urinalysis showed proteinuria, glycosuria, hematuria, aminoaciduria, increased excretion of potassium, phosphorus, and uric acid levels, which were not explained by plasma concentration, along with high levels of urinary β2-microglobulin and N-acetyl-beta-D-glucosaminidase. A drug-induced lymphocyte stimulation test of her serum showed positive for the dietary supplement. A percutaneous kidney biopsy was performed on the second day after admission. Light microscopy analysis showed significant tubular changes, with 50% of the tubular epithelial cells showing simplification, vacuolization, and shedding, indicating acute tubular necrosis. Immunohistochemical staining with an anti-megalin antibody, a proximal tubule marker, was positive for the damaged tubules. Acute proximal tubular injury due to the intake of the dietary supplement was found based on her medical history, laboratory examination, and pathological findings, which led to diagnosis of Fanconi syndrome. Her general condition and clinical laboratory data markedly improved after discontinuation of the dietary supplement and correction of dehydration and electrolyte imbalance. She was discharged 14 days after admission. Discussion: Clinicians should consider dietary supplement-induced Fanconi syndrome as a differential diagnosis if patients are taking dietary supplements. Kidney biopsy is useful for diagnosing and determining the pathogenesis of dietary supplement-induced Fanconi syndrome. However, further reports are warranted to investigate the exact mechanism of dietary supplement-induced Fanconi syndrome.