As the fifth most abundant element found on Earth, it is not surprising that calcium is the most prevalent mineral found in the human body. Calcium is chemically classified as an alkaline earth metal, a substance that forms ions, which are atoms with a measurable electric charge. Ions bind with free electrons in these metals to form metallic bonds, created when the electrons are shared between the atoms of the metal. The metallic bond formed by such metals tends to make these elements strong, capable of being formed or shaped, as well as being a good conductor of both heat and electricity.

Calcium is not often found in its pure state in nature, as its atomic structure lends calcium to the ready formation of compound substances, such as calcium carbonate, or calcite, the essential component of limestone. Minerals are substances defined as elements that are mined from the earth; although not extracted in its pure form, calcium is defined as a mineral for the purposes of understanding its role in the function of the body.

Between 2.2 lb (1 kg) and 3 lb (1.4 kg) of calcium, usually as a compound, are contained in the body of an average healthy adult person. Approximately 99% of the calcium store is located in the bones and the teeth, in the form of calcium phosphate. Calcium is also essential to the sophisticated physical processes of muscle contraction, blood clotting, and the regulation of heartbeat and the transmission of impulses through the central nervous system.

Given the prominence of calcium in this variety of functions, it is imperative that sufficient quantities of calcium be consumed daily. This fact is even more profoundly important to athletes, who are placing regular and intense stress upon their skeletal structure and other systems. The stated recommended daily allowance (RDA) of calcium for a typical adult is 1,200 mg (milligrams); calcium is absorbed into the body through the action of vitamin D, which assists in the transport of calcium in food into the body. Vitamin D is a fat-soluble hormone, long identified as one of the essential vitamins to healthy human function. Vitamin D is most commonly produced through the human skin by way of exposure to ultraviolet light; it is also an additive in dairy products.

Calcium is available in a variety of foods as well as through mineral and athletic supplements. Foods that are rich in calcium include dairy products, sardines, salmon, green, leafy vegetables such as lettuce and spinach, and tofu.

Healthy bone development, maintenance, and repair are directly related to optimum levels of calcium. Bones are constructed of calcium phosphate and collagen, a protein that gives the bone a measure of flexibility. The bone components form a mesh of the very hard calcium phosphate and the softer collagen. Bone formation is a dynamic process, as cells known as osteoblasts (bone-formation cells) and osteocytes (bone-lining cells) continually process calcium for bone construction. The body has a built-in fail-safe mechanism regarding bone structure: when the calcium level in the body falls below the amount required for the other processes, such as muscle contraction, calcium will be removed from the processes of bone formation and directed to the needs of the other process. For this reason, a calcium deficiency tends to impact on the skeleton more than on any other part of the body. In a similar fashion, too little vitamin D will inhibit calcium utilization, which leads to a similar consequence.

A calcium deficiency is a potentially serious and progressive structural problem, the effects of which are cumulative in nature. If calcium intake is below what the body requires, bones tend to become thin and more brittle, a condition known as osteoporosis. This condition typically occurs in persons over the age of 45, and more commonly in women than men.

A calcium deficiency in the diet of an athlete will create a number of interrelated consequences. Acts of resistance in training or competition, where forces are applied to the skeletal structure, tend to stimulate the growth of denser bones. When the calcium is not present as a store to be drawn upon by the skeletal formation process, the athlete's bones will not be as strong or as supportive. As athletic activities usually require maximum muscle response in performance, a calcium deficiency has been shown to affect the reliability of the transmission of the body' response signals, impacting upon the reaction time in given situations.

Certain athletic supplements contain calcium. Calcium can be as readily ingested through a supplement as through the food diet. Excess intake of calcium leads to the risk of the formation of calcium oxalate, a compound that forms in the kidneys and is the chief component of kidney stones. The stones, which can vary in size, are a potential blockage of the passage from the kidney to the urinary tract, creating a usually painful impairment of kidney function. Kidney stones are caused by a combination of too much calcium, dehydration or poor hydration, and excess vitamins C or D.

SEE ALSO Bone, ligaments, tendons; Minerals; Musculoskeletal injuries.