Introduction
Calcium (Ca), an essential metallic element in human nutrition, fulfils a wide variety of biological roles. You are probably quite familiar with its celebrated structural role as a primary mineral component of bones and teeth, but did you know calcium also directly participates in a myriad of metabolic reactions, including those that control energy production, muscle contraction, heartbeat, blood coagulation, and the functions of the immune, nervous, and endocrine systems? The human body derives its entire pool of calcium from the diet, and not surprisingly, has evolved an intricate system for the absorption, distribution, storage, and maintenance of relevant calcium levels. So is it necessary to take a calcium supplement to support all these functions? For most people, absolutely!

Sources and Forms of Calcium
~Food~
We can consume calcium as naturally-occurring components of dietary foodstuffs (dairy products, dark green vegetables, and to a lesser extent meats, fish, and legumes). Sadly, the average American adult is estimated to intake between 500 to 800 mg of calcium per day, with men having higher average intakes than women. This average intake is significantly less than the established Dietary Reference Intake (DRI) of 1,000 mg/day for adults aged 19 to 50, and 1,200 mg/day for those over 50. The disparity is likely attributed to the evolving (or devolving) Western diet (in which many high-calcium foods like broccoli, mustard greens, and sardines have fallen out of favor) and our avoidance or intolerance of dairy products as we age. Compounding factors such as poor vitamin D status and carbonated soft-drink consumption have been shown to further decrease the amount of usable calcium in the body. Studies have shown that poor calcium status, especially early in life, can have lasting, irreversible, detrimental effects as we age.

~Supplements~
Increasingly, supplementation has been required. Currently, an estimated 25% of U.S. women take a calcium supplement. These supplements are available in a variety of different forms, usually as calcium salts (gluconate, citrate, carbonate, phosphate, among others). There has been considerable debate as to the merits of each (particularly in bioavailability), but it is important to note that nearly all have been demonstrated to serve as good calcium sources under most conditions, and any differences between them are usually modest. This is due to the complex, highly regulated system that the human uses to absorb and utilize calcium.

Calcium Absorption
Calcium is absorbed in the small intestine (and possibly the large intestine) both actively (by a transport system) and passively (by diffusion). The combination of active and passive absorption allows the digestive system to rapidly respond to the body's calcium requirements; ensuring calcium is efficiently absorbed when dietary intake is low, but preventing over-absorption (and undue stress on the renal system) when dietary calcium levels are high. Calcium absorption is limited to approximately 1/4 to 1/3 of ingested calcium, assuming dietary calcium levels are also adequate. These values are also altered during different points in human development, increasing significantly during periods of rapid growth (during infancy, puberty, pregnancy) and declining later in life (after menopause, intestinal calcium absorption decreases by about 0.2% annually)

In general, calcium is absorbed in its free (elemental) form. This means it must be released (dissociated) from its bound salt or protein in order to be absorbed. In healthy individuals with sufficient calcium intake, the rates of dissociation of different calcium salts are similar; hence, differences in bioavailability of different forms of calcium are modest under these circumstances.

Calcium in bones and other Systems
The skeleton is the reservoir for calcium, where it performs a structural role in the bone composite, but can also be released into the blood for use in other body systems. Bone calcium is in the form of hydroxyapatite (scientifically known as Ca10(PO4)6(OH)2,) a calcium phosphate mineral. If more calcium is released from bone than is replaced (which may occur if dietary calcium levels are insufficient, or as a consequence of imbalanced hormone levels following menopause), a net loss of bone calcium can occur, which can decrease the bone's density and increase the risk of fracture.

Mineral in Your body

Limitations
Calcium is of obvious importance in human health, but one must also consider its interdependency with other nutrients when choosing to improve calcium status. Calcium intake at a level relevant to one's age group is important, but equally as important is ensuring proper amounts of vitamin D (though diet, supplementation, or sunlight exposure) to mobilize the dietary calcium from the intestines to the systems where it is needed. Calcium, although required, is not sufficient for the growth of bone or the maintenance of bone mineral density. This is consistent with our knowledge of the complex chemistry of bone formation, and has been demonstrated in controlled clinical settings. The incorporation of calcium into bones depends on some controllable dietary and lifestyle factors, particularly sufficient trace minerals (like iron, silicon, boron, copper), vitamins (notably vitamin K), as well as factors which build a strong bone matrix (such as physical activity).

Bone-Up, from Jarrow, combines microcrystalline hydroxyapatite (MCHA), from free-range Australian calves, with essential vitamins and minerals for optimal calcium absorption and increased bone density and hardness. Vitamin K2, as MK-7 (a more bioavailable form of vitamin K) and 1000 IU of vitamin D3 help support the deposition of calcium into the bones as well as assist in building the organic bone matrix. Minerals such as magnesium, zinc, manganese, and boron are essential for optimal bone health. Also added is the nutrient potassium citrate, for optimal osteo support.