What are the causes of calcium malabsorption?

Causes of Calcium Malabsorption

Calcium malabsorption results from disruption of normal absorption mechanisms in the proximal small intestine (duodenum and jejunum), vitamin D deficiency, or conditions that impair the acidic environment needed for calcium salt dissolution.

Anatomical and Surgical Causes

Bariatric Surgery and Intestinal Resection

• Malabsorptive bariatric procedures (biliopancreatic diversion, Roux-en-Y gastric bypass, duodenal switch) are major causes of calcium malabsorption, with vitamin D deficiency occurring in 63% of patients by the fourth postoperative year and hypocalcemia increasing from 15% to 48% over the same period 1.
• Small bowel resection, particularly involving the duodenum and proximal jejunum where calcium is primarily absorbed, leads to severe malabsorption that may require extremely high doses of supplementation (up to 3500 mg calcium citrate three times daily) 2, 3.
• Partial gastrectomy causes achlorhydria (loss of stomach acid), which prevents dissolution of calcium carbonate salts, making calcium citrate the preferred formulation as its absorption is independent of stomach acidity 2, 3.
• Short bowel syndrome from multiple intestinal resections creates chronic intestinal failure with severe malabsorption of all nutrients including calcium 2.

Inflammatory Bowel Disease

• Crohn's disease causes calcium malabsorption through multiple mechanisms: active intestinal inflammation, surgical resections, and corticosteroid-induced bone loss 2.
• Bile acid malabsorption after ileal resection or inflammation indirectly affects calcium absorption by causing fat malabsorption, which binds calcium in the intestinal lumen and prevents its absorption 2.
• The correlation between calcium malabsorption and enteric protein loss is stronger than associations with fat malabsorption or bacterial overgrowth 4.

Vitamin D Deficiency

Primary Mechanism

• Calcium absorption is fundamentally dependent on adequate vitamin D concentrations, as vitamin D regulates active transcellular calcium transport in the intestine 2.
• More than 50% of patients with inflammatory bowel disease have vitamin D deficiency, which directly impairs calcium absorption 5.
• The accepted adequate vitamin D concentration is >30 ng/mL; levels below this threshold compromise calcium absorption regardless of calcium intake 2.

Fat Malabsorption Impact

• Fat-soluble vitamin deficiencies (including vitamin D) occur progressively after malabsorptive procedures, with 63% of patients developing vitamin D deficiency by four years post-surgery 1.
• Conditions causing steatorrhea (fat malabsorption) prevent absorption of vitamin D, creating a cascade effect on calcium absorption 2.

Gastrointestinal Disorders

Achlorhydria and Hypochlorhydria

• Loss of gastric acid production from proton pump inhibitors, H2-blockers, or gastric surgery prevents dissolution of calcium carbonate, the most common supplemental form 3.
• Calcium citrate must be used instead of calcium carbonate in achlorhydric states, as demonstrated by a patient whose calcium levels could not be maintained above 7 mg/dL on calcium carbonate but normalized on calcium citrate 3.

Small Intestinal Bacterial Overgrowth (SIBO)

• SIBO can impair nutrient absorption including calcium, and should be considered when oral supplementation fails despite adequate dosing 2, 5.
• Bacterial overgrowth may consume nutrients and damage the intestinal mucosa, reducing absorptive capacity 2.

Celiac Disease and Other Enteropathies

• Mucosal damage from celiac disease, tropical sprue, or other enteropathies reduces the absorptive surface area in the duodenum and jejunum where calcium is absorbed 6.
• These conditions often present with subtle manifestations like osteoporosis or anemia rather than overt gastrointestinal symptoms 6.

Medication-Related Causes

Corticosteroids

• Chronic corticosteroid use in inflammatory bowel disease patients causes calcium malabsorption and accelerates bone loss, necessitating calcium and vitamin D supplementation in all steroid-treated patients 2.
• Corticosteroids increase bone resorption and impair calcium absorption through multiple mechanisms 2.

Other Medications

• Cholestyramine (bile acid sequestrant) can worsen fat malabsorption in severe cases, indirectly affecting calcium absorption 2.
• Medications that reduce gastric acid production chronically impair calcium carbonate absorption 3.

Secondary Hyperparathyroidism

Mechanism

• Chronic calcium malabsorption combined with vitamin D deficiency leads to secondary hyperparathyroidism, with 69% of post-bariatric surgery patients developing elevated PTH by the fourth postoperative year 1.
• However, calcium malabsorption alone does not cause secondary hyperparathyroidism; the age-related rise in PTH is primarily due to declining vitamin D levels and renal function rather than malabsorption per se 7.
• Secondary hyperparathyroidism increases bone resorption, creating a vicious cycle of calcium depletion and bone loss 1.

Clinical Consequences

Bone Disease

• The long-term result of calcium or vitamin D deficiency is loss of bone mass and bone metabolism diseases, with bariatric patients at risk for fractures and osteoporosis occurring anywhere from 8 weeks to 32 years post-surgery 2.
• Osteopenia and osteoporosis develop from the combination of rapid weight loss, absorption changes, and nutritional deficiencies 2.

Electrolyte Abnormalities

• Severe hypocalcemia can cause QT prolongation on EKG and ventricular arrhythmias 5.
• Hypomagnesemia frequently coexists with calcium malabsorption and must be corrected simultaneously, as hypocalcemia is often refractory to treatment until magnesium is repleted 8, 5.

Key Clinical Pitfalls

• Never assume calcium carbonate will be absorbed in patients with gastric surgery, PPI use, or achlorhydria—always use calcium citrate in these populations 2, 3.
• Comprehensive assessment of calcium status requires monitoring alkaline phosphatase, vitamin D, parathyroid hormone, and urinary calcium—not just serum calcium alone 2.
• Calcium supplementation alone without correcting vitamin D deficiency will fail, as vitamin D is essential for calcium absorption 2.
• In malabsorptive states, extremely high doses of calcium (1200-2400 mg elemental calcium daily) are often necessary, far exceeding normal dietary recommendations 2.