Calcium is an essential macromineral, 99 per cent of which is found in bones and teeth in adult humans. It is extremely important because bone tissue is constantly being reformed throughout life, though to a much lesser extent in adults than in developing individuals. This mineral is also involved in vascular control, muscle contraction, nerve transmission and glandular secretion.
      The absorption of calcium at low and moderate levels of intake is primarily dependant on the presence of vitamin D.

      The recommended dietary consumption of calcium in the following table takes into account many of the factors that effect the normal absorption of this mineral, though specific requirements can vary considerably depending upon life stage, and individual circumstances (see Food Sources and Deficiencies below).

      The primary sources of calcium in North American diets are milk and other dairy products, grains, broccoli, kale and other green, leafy vegetables, some fruits, and calcium fortified foods and beverages such as orange juice. The calcium in some food, such as beans, sweet potatoes, rhubarb, spinach, seeds, nuts, and some grains, may be poorly absorbed due to high concentrations of other, inhibiting compounds (oxalic acid and phytic acid).
      Although calcium intake is closely related to bone development and maintenance, adequate dietary intake alone is not sufficient to ensure bone health. There is ample evidence to suggest that inadequate exercise can contribute to deficient bone development and health, even in the presence of the recommended levels of dietary intake of calcium.

      Calcium deficiency is relatively common, though the causes are diverse and impact individuals in considerably different ways. A number of the more common interactions known to decrease calcium levels are reviewed in the following paragraphs.
      At moderate to high levels of calcium intake, it has been shown that high dietary salt intake (as sodium chloride) causes a higher excretion rate of calcium in the urine, resulting in lower available calcium levels. While not drastic enough to have a reported impact on bone structure or fracture rate, it is worth noting when considering calcium supplements.
      A similar calcium lowering effect has also been reported for individuals who consume large amounts of caffeine, and those on high protein diets. In the latter case, low protein diets have been reported to cause poor recovery rates from certain forms of bone fracture caused by osteoporosis. While this appears contradictory, both conditions could be valid.
      While any of these factors alone are not sufficient to warrant a specific change in calcium intake, they point out all too well how important a well balanced diet and exercise regimen is for good, overall health. If yours is a high salt, traditional, so-called “meat-and-potatoes” diet, and you also consume large amounts of coffee or tea, and you also have lower than average dietary calcium consumption, it probably wouldn’t hurt to boost your calcium intake a bit.
      Strict vegetarians may have limited calcium intake as a result of their selection of foods, the largest amount of the mineral being derived from protein sources. Legumes and other vegetables rich in calcium would be an important dietary item for such individuals.
      A number of conditions resulting in decreased estrogen levels in women have long been known to affect calcium balance, interfere with bone development, or cause outright bone loss. Among the most prominent are amenorrhea (premature cessation of menstruation), whether induced by anorexia or excessive physical exercise, and post-menopausal decreases in the hormone resulting in progressive osteoporosis (bone loss).
      Another common cause of decreased calcium is the omission of rich dietary sources of this mineral such as milk and other dairy products, most common among lactose intolerant individuals who purposely steer away from such foods.
      Deficiency in vitamin D also decreases the absorption of calcium at low and moderate levels of intake of the mineral. While it is true that a natural, internal regulation exists that allows more calcium to be absorbed when intake levels are significantly lowered, this mechanism does not completely make up for the deficient intake, and gradually declines with age.
      The result of prolonged calcium deficiency, from whatever cause, is reduced bone mass and osteoporosis, leading to bone fragility and increased risk of fracture. The best remedy for this condition, in most cases, is increased consumption of calcium from either dietary or supplemental sources. It should also be said that there is evidence that supplemental calcium may not be the best, or only choice for post-menopausal women deficient in calcium (see Current Issues below).

      Calcium is widely available and widely consumed in supplement form. As mentioned in Food Sources and Deficiencies above, there are many nutrient-to-nutrient and other interactions that affect calcium absorption and excretion that would warrant consideration of calcium supplementation.
      Various reports have suggested that supplemental calcium is not absorbed as readily as equivalent amounts of the mineral from dietary sources, that individual doses of 500 mg or less are absorbed more efficiently than larger doses, and that absorption is greater when supplements are consumed after a meal rather than on an empty stomach.
      Also quite important is the form in which supplements are taken: calcium supplements are not all created equal! Calcium carbonate contains the highest concentration of calcium, as does oyster shell (essentially calcium carbonate), with much lower levels in calcium lactate, dicalcium phosphate, and calcium gluconate. And don’t forget that adequate (but not excessive!) amounts of vitamin D are essential for proper calcium absorption.

      The adverse health effects associated with excessive calcium intake (hypercalcemia) have generally been associated with the use of supplements. As a result of the complex biochemical interaction between calcium and other essential minerals, and to a lesser degree, other nutrients, numerous problems have been reported with disruptions at the cellular level.
      The formation of kidney stones has often been associated with excessive calcium intake. While it would appear that this is a contributing factor, it is not yet clear how much the disruption of other minerals from the large amounts of calcium present in the system adds to the problem. As a separate issue, insufficient kidney function has also been reported from excessive levels of calcium, a condition if left unremedied can lead to kidney failure and ultimately death.
      It is because of the complexity of calcium biochemistry and the numerous adverse interactions that could potentially result from excessive intake of calcium, that the most recent National Academy of Sciences (NAS) study on Dietary Reference Intakes (see Reference Values for Calcium) has recommended a tolerable upper intake level (UL) for both adolescents and adults of 2500 mg/day for calcium.

      It has long been believed that increased dietary and supplemental calcium can reduce the risk of osteoporosis, and the increased risk of fracture associated with loss of bone mass, particularly in post-menopausal and elderly women. There is mounting evidence that suggests this may not be the case, or that the effects of supplemental calcium may have considerably less value than previously believed. It is our intent to present this subject in more detail in the future.
      The potential role of calcium in reducing the risk of high blood pressure has been reported in a number of studies, and in particular, the reduction of high blood pressure resulting from pregnancy.
      Similarly, a number of reports have suggested a potential risk reduction factor in colon cancer, though these results have been much more inconsistent.