Phosphate levels increase with which condition?

• A. High calcium diet
• B. Phosphate-binding antacids
• C. Low calcium diet
• D. Vitamin D sufficiency

Answer: (C)

When calcium intake is low, the body responds by increasing parathyroid hormone to raise serum calcium. That surge in PTH stimulates bone resorption to release calcium, and it also releases phosphate from bone into the bloodstream. Although PTH also promotes renal excretion of phosphate, the immediate effect of bone turnover can transiently raise circulating phosphate levels before the kidneys fully compensate. This makes phosphate levels rise in the setting of a low calcium intake. High calcium intake and phosphate-binding antacids work to lower phosphate by reducing absorption, while vitamin D sufficiency mainly increases intestinal absorption of both calcium and phosphate but does not drive the same acute bone-resorption–driven phosphate release seen with low calcium intake.