Calcium is the most abundant mineral in the body. Calcium's primary function is to build and maintain skeletal tissue. Ninety-nine percent of the body's calcium supply resides in the bone and teeth. Osteoblasts are bone-forming cells in which calcium phosphate is deposited. Ameleoblasts are the tooth- forming cells that deposit calcium to form teeth.

Calcium has many other important functions in the body even though only 1% of the body's calcium performs these functions.

  • Calcium is involved in maintaining the stability of fibrin, which allows blood to clot.
  • Calcium is required for the transmission of nerve impulses.
  • Calcium controls the flow of fluid through cell membranes.

Finally, Calcium also has a vital role in muscle contraction and relaxation. In muscle fibers, calcium ions are released from a tubular reticulum and activate the chemical reaction between myosin and actin that releases energy from adenosine triphosphate (ATP) and causes the muscle to contract. The calcium ions then quickly bind back to the reticulum and the muscle relaxes. This function is particularly important for heart muscle contraction and relaxation.

Absorption and Excretion

Approximately 10-30% of dietary calcium is absorbed into the small intestine, mainly in the duodenum. Vitamin D is responsible for synthesizing the calcium-binding protein carrier that transports calcium into mucosal cells and blood.

There are many factors that increase calcium absorption.

  • Calcium is absorbed more readily during periods of great need, such as growth and pregnancy.
  • More calcium is absorbed with a diet high in protein, but such a diet also causes increased excretion by the kidneys and subsequently a negative calcium balance.
  • Lactose, the sugar contained in dairy products, helps the body absorb more calcium by producing lactic acid, which lowers the pH in the intestine.
  • Calcium is more soluble in an environment with a low pH, or high acidity.

There are also dietary factors that decrease the absorption of calcium.

  • A deficiency in vitamin D reduces the amount of calcium that can be absorbed.
  • Malabsorption of fat inhibits calcium absorption and results in the formation of insoluble calcium soaps. These soaps are excreted and the calcium is lost.
  • Dietary components that bind calcium and hinder its absorption include oxalic acid and phytic acid. Oxalic acid is the most potent inhibitor and is found in some green leafy vegetables, particularly spinach and rhubarb. Phytic acid is the storage form of phosphorous in seeds and is contained in the hulls of cereal grains.
  • Fiber is also thought to interfere with calcium absorption, although it may be the phytate in high fiber foods that is the culprit.