How Does The Body Absorb Calcium?

Your body contains roughly 2.2 pounds of calcium, more than any other mineral. Your diet and supplements are the exclusive sources of body calcium, an essential mineral for bone, muscle, nervous system and heart health. Your intestines have two mechanisms to absorb dietary calcium; the predominant mechanism depends on the concentration of calcium in the foods you consume.

Transcellular Absorption

Cells in the first portion of your small intestine, the duodenum, actively absorb calcium. Specialized pores called TRP channels on the surface of duodenal intestinal cells take up calcium from digested food. A protein called calbindin transports absorbed calcium to pumps that export the mineral into your bloodstream. This mechanism, known as transcellular absorption, is the primary means of calcium absorption if there is a low concentration of calcium in the food undergoing digestion.

Paracellular Absorption

When you eat meals containing a high concentration of calcium, a second absorption system kicks in to absorb calcium not taken up by the transcellular mechanism. The paracellular calcium absorption system operates throughout your small and large intestines. The cells lining your intestines are shaped similar to microscopic bricks. The interface between the cells is called a tight junction. TJs are minute gaps between intestinal cells through which calcium and other small molecules can pass. When the calcium concentration is higher in your intestines than in your blood, the mineral crosses through the TJs and enters your bloodstream.

Vitamin D

Vitamin D regulates the transcellular calcium absorption system. Calbindin is required for transcellular absorption and production of this protein depends on adequate levels of vitamin D. If you have a vitamin D deficiency, you lose much of the calcium from your diet in your stool because the transcellular absorption system is impaired.

Dietary Inhibitors

Plant-derived foods that contain the chemicals oxalic acid or phytic acid inhibit calcium absorption by binding the mineral in your intestines. Bound calcium cannot be absorbed. Foods that contain a high concentration of oxalic acid include spinach, collards, radishes, beet greens, amaranth, carrots, lettuce and sweet potatoes. Concentrated amounts of phytic acid are found in nuts, seeds, beans and whole-grain products.

Age

The efficiency of calcium absorption from your diet changes with age. During infancy and early childhood, as much as 60 percent of dietary calcium is absorbed from your intestines to support rapid bone growth. Intestinal calcium absorption decreases progressively with age throughout adulthood and averages roughly 15 to 20 percent. The recommended daily allowance for calcium increases after age 50 for women and age 70 for men to compensate for reduced intestinal absorption.