Strict phosphorus-restricted diet causes hypophosphatemic osteomalacia in a patient with chronic kidney disease

Abstract

Patients with chronic kidney disease (CKD) need to follow phosphorus-restricted diets because impaired phosphorus excretion induces hyperphosphatemia in CKD, which increases mineral and bone disorders, cardiovascular diseases, and mortality; however, hypophosphatemic osteomalacia caused by phosphorus-restricted diets in CKD patients has not been described. We report here a patient with CKD in whom hypophosphatemia resulted in osteomalacia because of a phosphorus deficiency from a strict phosphorus-restricted diet. The patient (CKD stage G3b; estimated glomerular filtration rate, 31.5 mL/min/1.73 m2) presented with bone pain, muscle weakness, and multiple fragility fractures. She had been diagnosed with severe osteoporosis and treated with risedronate sodiumand 1a-hydroxyvitamin D3 (alfacalcidol). Laboratory tests showed hypophosphatemia (1.7 mg/dL) with aphosphaturia; serum fibroblast growth factor 23 (FGF23) was < 3 pg/mL. Radiological studies showed multiple fractures, and bone histology and morphometry of the iliac crest demonstrated a large amount of osteoid, with marked increases in all osteoid-associated parameters. Despite continuous administration of alfacalcidol, hypophosphatemic osteomalacia did not improve. The patient maintained a strict phosphorus-restricted diet, and laboratory examination indicated osteomalacia accompanied by phosphorus deficiency. Phosphorus supplementation improved her symptoms and pathological fractures in parallel with improved bone metabolic markers. Additionally, bone mineral density of the lumbar spine increased, suggesting calcification of unmineralized bone tissue. We conclude that this is a unique case of hypophosphatemic osteomalacia induced by a strict phosphorus-restricted diet and subsequent phosphorus deficiency in a patient with CKD. Histomorphometry revealed a marked increase in osteoid and phosphorus supplementation improved symptoms, fractures, and bone metabolic markers.