Fanconi syndrome is a disorder of the proximal tubule that leads to malabsorption of various electrolytes and substances and is a common consequence of drug-induced nephrotoxicity. However, cases of dietary supplement-induced Fanconi syndrome are rare, and detailed reports on the evaluation of renal histology in patients with this syndrome are lacking. We present two cases of dietary supplement-induced Fanconi syndrome that was confirmed by kidney biopsy. Based on their medical history and laboratory and histological findings, both cases were diagnosed as acute proximal tubular injury caused by ingestion of a lipid-lowering dietary supplement containing beni-koji leading to Fanconi syndrome. After discontinuation of the dietary supplement and correction of dehydration and electrolyte imbalance, renal function completely recovered in one case but progressed to chronic kidney disease in the other. Clinicians should consider dietary supplement-induced Fanconi syndrome as a differential diagnosis in patients who become ill while taking a dietary supplement. Kidney biopsy is useful for diagnosing acute tubular injury with Fanconi syndrome and investigating its pathogenesis. Patients who have developed dietary supplement-induced Fanconi syndrome require long-term monitoring to detect and prevent progression to chronic kidney disease.

Fanconi syndrome with acute proximal tubular injury induced by a dietary supplement containing beni-koji: a case series report