Calcium absorption is decreased by which of the following?

• A. Diet deficient in calcium or high in phosphates
• B. Low levels of vitamin D
• C. Malabsorption of the GI tract
• D. All of the above

Answer: (D)

Calcium absorption in the gut relies on vitamin D–driven active transport in the small intestine and is influenced by dietary calcium and phosphate balance as well as overall intestinal health. When vitamin D is deficient, the enterocytes reduce the expression of calcium transport proteins, so less calcium is absorbed. A diet low in calcium lowers the substrate available to absorb, and a high phosphate intake can bind calcium in the gut to form insoluble calcium phosphate, reducing uptake. If the GI tract is malabsorptive, the absorptive surface or transport mechanisms are impaired, directly decreasing calcium absorption. Because each of these scenarios can lower the amount of calcium that enters the bloodstream, all of the above conditions decrease calcium absorption.