Calcium is an essential mineral element in the human diet, responsible for the execution of a wide variety of biological roles. Apart from its celebrated role as a primary mineral component of bones and teeth, calcium directly participates in a myriad of metabolic reactions, including energy production, muscle contraction, heartbeat, blood coagulation, and the functions of the immune, nervous, and endocrine systems. The human body derives its calcium entirely from the diet, and, not surprisingly, has evolved an intricate system for relevant levels of its absorption, distribution, storage, and maintenance. Without adequate intake of calcium (and the appropriate accessory nutrients), the health and well-being of several bodily systems would be compromised.
SOURCES AND FORMS
Calcium comes in a variety of forms: the naturally occurring components found in dietary foodstuffs (dairy products, dark green vegetables, and, to a lesser extent, meats, fish, and legumes) and, increasingly, through supplementation. The average American adult ingests between 500 to 800 mg of calcium per day, with men having higher average intakes than women. This average intake, however, is significantly less than the established Dietary Reference Intake (DRI) of 1000 mg/day for adults aged 19 to 50 and 1200 mg/day for those over 50. The disparity is likely attributed to a devolving Western diet (many high-calcium foods like broccoli, mustard greens, and sardines have fallen out of favor). Compounding factors, such as poor vitamin D and high-sugar intake, have been shown to create a deficiency of usable calcium in the body. Studies show poor calcium status, especially early in life, can have irreversible, detrimental effects as we age.
The calcium obtained through supplementation is available in different forms, usually as calcium salts (gluconate, citrate, carbonate, phosphate, among others). Though the merits of each solicit much debate (particularly in bioavailability), nearly all have proven to be good calcium sources under most conditions. Any differences between them are usually modest due to the complex, highly regulated system that humans use to absorb and use calcium.
Calcium is absorbed in the small intestine (and possibly the large intestine) both actively (by a transport system) and passively (by diffusion). Active and passive absorption allows the digestive system to rapidly respond to the body’s need for calcium; ensuring effective calcium absorption when dietary intake is low, while preventing over-absorption (which can stress the renal system) when dietary calcium levels are high.
Working in concert, the active and passive transport systems limit calcium absorption to about 1/4 to 1/3 of ingested calcium, assuming dietary calcium levels are adequate. Limiting calcium absorption appears to be an evolution stopgap measure guarding against over-absorption (hypercalcemia) carried over from prehistoric times. Modifying calcium dosage, by dividing total intake into small, well-spaced portions of 500 mg or less, or improving absorption conditions by increasing intestinal pH can modestly increase uptake. Different stages of human development also influence calcium requirements, which increase significantly during periods of rapid growth (infancy, puberty, pregnancy) and decline later in life.
In general, calcium is absorbed in its free (elemental) form, meaning it must be released (dissociated) from its bound salt or protein in order to be absorbed. Healthy individuals with sufficient calcium intake have similar dissociation rates of different calcium salts; hence, the bioavailability of these varying calcium forms are roughly the same and are determined by the limits of the active/passive transport systems. Bioavailability varies only under conditions of deficiency or compromised digestive function (i.e., achlorhydria or lack of sufficient stomach acidity to properly dissolve calcium carbonate). Once free calcium is absorbed, it can enter the circulatory system for distribution and use by the body (either as free calcium or bound to transport proteins) or stored in bones for later use.
CALCIUM IN BONES AND OTHER SYSTEMS
The skeleton is the reservoir for calcium, where it performs a structural role in the bone composite. And from the skeleton, calcium is released into the blood for other uses. Bone calcium is in the form of hydroxyapatite (Ca10(PO4)6(OH)2). This calcium phosphate mineral form, which comprises approximately 40% of the bone’s mass and 99% of the body’s total calcium store, is embedded into a matrix of collagen and polysaccharides (osteoid) to create the bone composite. Cells within the bone composite (osteoblasts and osteoclasts) control how the calcium is merged into and released from the bone matrix. The concerted effort between the two types of cells is important for both maintaining the skeleton’s structural integrity as well as the calcium demands by other body systems. If more calcium is released from bone than is replaced (i.e., insufficient dietary intake, hormonal imbalances following menopause, or aging), a net loss of bone calcium can occur, which can decrease the bone’s density.
Bone mineral density is one gauge of bone strength; decreases in bone mineral densities have been shown to increase the risk of fracture.
Another consideration to improving levels and building bone density is calcium’s interdependency with other nutrients. Not only is it important to ingest the amount appropriate for your age, but the body requires the proper amount of vitamin D (via diet, supplementation, or sunlight exposure) to mobilize the dietary calcium from the intestines to the systems where it is needed.
Calcium, although imperative, is not sufficient for bone growth or maintenance of bone mineral density. Proper calcium uptake 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 weight bearing exercise).
Our natural inclination to address a deficiency with rapid excess, however, isn’t the answer. Human metabolism functions most efficiently under a balance of dietary factors. Calcium can compete with other essential trace elements (particularly magnesium) for absorption, so a sudden increase of calcium may lead to deficiencies in other trace elements.
Avoid deficiency by selecting calcium-rich foods such as organic (when possible) dark leafy greens and dairy options while reducing foods such as caffeine and sugar that leach calcium from the system. Also consider calcium supplementation as well as the inclusion of those nutrients that work synergistically with calcium for proper digestion. Jarrow‘s Bone Up is an excellent bone-building formula that incorporates calcium from microcrystalline hydroxyapatite (MCHA) from Australian bovine bone, MK-7 (an advanced form of vitamin K2), methylcobalamin (Methyl B12), and glucosamine. Adding weight bearing forms of exercise to your routine will also help increase bone density.
With patience and application, your bones will have the strength of Hercules.
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