How is calcium absorbed in the body?

Gastrointestinal absorption of calcium Calcium is absorbed in the small intestine predominantly in the duodenum and jejunum. The gastrointestinal (GI) absorption of calcium is by various means:

• Passive (paracellular)—5%

• Active (transcellular)—20–70%

It is controlled by 1,25(OH)2D. 1,25(OH)2D induces calbindin which pro- motes active transport of calcium across the intestine. The GI absorption requires the presence of gastric acid. So, whenever calcium carbonate tablets are prescribed, they are advised to be taken with meals. In achlorhydric subjects, it is preferable to use calcium citrate tablets instead. Renal absorption of calcium PCT (65%) Passive cTAL (20%) Paracellular Inhibits by ↑ serum Ca/Mg via the CaSR directly DCT (10%) Transcellular Thiazides lower urinary Ca excretion Loop diuretics/saline infusion ↓ DCT Ca reabsorption Increased calcium reabsorption is promoted by PTH and 1,25(OH)2D. Patient’s serum phosphorus is 3.6 mg/dL. 25(OH)D level is 36 ng/mL and serum PTH is 50 pg/mL. She is immediately started on IV fluids (Normal saline) with furosemide 20 mg 8 hourly. Calcitonin SC is also given for 2 days. Zoledronate intravenous infusion 5 mg single dose is also given. Serum calcium levels are monitored daily—subsequent values are 14 mg/dL, 11.5 mg/dL, 10.6 mg/dL, 8.8 mg/dL. Serum creatinine value comes down to 1.0 mg/dL.