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.
A condition that results when serum calcium decreases, tetany is characterized by very spastic contractions of the muscle and muscle pain. This can also be caused by an increase in serum phosphorous, which decreases serum calcium.
A loss of bone mineral causing a thinning of the bone; although osteoporosis usually occurs in the elderly and postmenopausal women, people of any age can be affected if their intake of calcium is too low.
Conditions of the Parathyroid Gland
The parathyroid hormone, produced by the parathyroid gland, is involved in the control of plasma calcium levels. When the plasma calcium level drops, the parathyroid gland reacts to restore normal levels. This is achieved three ways: 1) a stimulation in the mucosa of the intestine to absorb more calcium; 2) rapid mobilization of calcium form the bone; 3) renal excretion of phosphate. This last method means that parathyroid hormone also controls phosphorous levels.
The Recommended Dietary Allowance (RDA) of Calcium, set by the Food and Nutrition Board of the National Academy of Sciences, varies between 800 and 1200 mg for adults. Pregnant and lactating women require 1200 mg/day. Infants require 300-500 mg/day and 800 mg/day is recommended for children.
Dairy products are the main sources of calcium in the diet. Calcium is also obtained in smaller amounts from eggs, green leafy vegetables, broccoli, legumes, nuts, and whole grains.
The recommended dosage for supplemental calcium is the RDA. Some forms of calcium supplements may contain high levels of lead and should be avoided. These include calcium in bone meal, dolomite, and unrefined calcium carbonate from limestone or oyster shells.
Dosages below 2000 mg/day are well tolerated. It has been suggested that higher dosages may increase the risk for kidney stones or soft-tissue calcification. However, calcium intake by diet or supplements is not usually the cause of kidney stones. High urine calcium levels due to renal leakage is the usual culprit.