Calcium

BACKGROUND

Calcium is required for the normal development and maintenance of the skeleton as well as for the proper functioning of neuromuscular and cardiac function. It is stored in the teeth and bones where it provides structure and strength. Low intakes of calcium have been associated with a condition of low bone density called osteoporosis which is quite common in western cultures and which often results in bone fracture. It is one of the major causes of morbidity amongst older Australians and New Zealanders, particularly postmenopausal women. Calcium intake throughout life is a major factor affecting the incidence of osteoporosis, however other factors, notably adequate vitamin D status and exercise, also play a role.

Bone mass increases by about sevenfold from birth to puberty and a further threefold during adolescence (Peacock 1991) and then remains stable until about age 50 in men and until the menopause in women. During the adolescent growth spurt, the required calcium retention is two to three times higher than that required for the development of peak bone mass which occurs at the same time as maximum height (Nordin et al 1979).

For approximately 5–10 years both during and after the climacteric and menopause (Heaney 1986), women lose bone more rapidly than men (2%–3% per year). Thereafter, the age-related loss in both sexes is about 0.5 to 1.0% per annum. All of the body's calcium reserve is stored in the skeleton. The size of the reserve is directly affected by the body's external calcium balance which depends on the relation between calcium intake and absorption on the one hand and losses of calcium through the skin, kidney and bowel on the other.

Until recently, the amount of dietary calcium needed to replace losses through sweat had not been included in estimates of calcium requirements. This omission accounts to a large extent for an apparent increase in calcium intake recommendations seen in the recent revisions of the FAO:WHO (2001) and US:Canadian (FNB:IOM 1997) recommendations and in the current revision of the Australian/New Zealand recommendations.

Calcium balance deteriorates at menopause when there is a decline in intestinal calcium absorption and/ or an increase in urinary calcium excretion. In post menopausal women, there is evidence that a high calcium intake will slow the rate of bone loss and may reduce the risk of fracture (Cumming & Nevitt 1997, Dawson-Hughes et al 1990, Elders et al 1994, Nordin 1997, Prince et al 1995, Reid et al 1993, 1995) but it has been suggested that the improvement may attenuate over time (Mackerras & Lumley 1997). A systematic review was also undertaken by Cumming & Nevitt (1997) of 14 studies of calcium supplements (including 4 RCTs), 18 studies of dietary calcium and hip fracture (no RCTs), and 5 studies of dietary calcium and other fracture sites (no RCTs). The 4 RCTs of calcium supplements (mean calcium dose 1,050 mg) found relative risk (RR) reductions of between 25% and 70%. Cochrane reviews by Shea et al (2003, 2004) also concluded that calcium supplementation had a small positive effect on bone density and a trend towards reduction in vertebral fractures but concluded that it was unclear if calcium reduces the incidence of non-vertebral fractures. However, one recent large intervention trial in 5,292 previously ambulatory elderly people who had already experienced a fracture showed no effect on the occurrence of further fractures of calcium and/or vitamin D supplements at levels of 1,000 mg calcium or 20 µg daily oral vitamin D3 alone or in combination (Grant et al 2005).

Calcium is found predominantly in milk and milk-based foods, with smaller amounts in bony fish, legumes and certain nuts, fortified soy beverages and breakfast cereals. Consumption of vegetarian diets may influence calcium needs because of their relatively high oxalate and phytate content, however, on balance, lacto-ovo-vegetarians appear to have similar calcium intakes to omnivores (Marsh et al 1980, Pedersen et al 1991, Reed et al 1994) and similar urinary excretion (Lloyd et al 1991, Tesar et al 1992).

Vegans have a lower calcium intake than vegetarians and omnivores (Larsson & Johansson 2002, New 2004), however one study by Kohlenberg-Mueller & Raschka (2003) has shown that both lactovegetarians and vegans can attain calcium balance. Intakes of calcium in adults in Australia and New Zealand average about 850 mg of which about 40% comes from non-milk sources.

For natural food sources of calcium, content is of equal or greater importance than bioavailability. The efficiency of calcium absorption varies across foods as calcium may be poorly absorbed from foods rich in oxalic acid (eg spinach, rhubarb, beans) or phytic acid (seeds, nuts, grains, certain raw beans and soy isolates). Absorption from soy milk can be, but is not always, as high as that from milk. Compared to milk, calcium absorption from dried beans is about 50% and from spinach, 10%.

Bioavailability from non-food sources (eg supplements) depends on the dosage and whether they are taken with a meal. In standardised studies of 250 mg calcium supplements given with a breakfast meal, absorption from supplements gave fractional absorption rates of 25–35% compared to a rate for calcium from milk of 29% (Heaney et al 1989, 1990, Miller et al 1988, Smith et al 1987). Efficiency of absorption of calcium from supplements is greatest at doses of 500 mg or less (Heaney et al 1975, 1988), but once the active transport mechanism is saturated, only 5–10% of additional calcium is absorbed.

Sodium intake can also affect calcium requirements as sodium and calcium excretion are linked in the kidney tubules (Nordin & Polley 1987, Matkovic et al 1995, O'Brien et al 1996, Devine et al 1995) – 2,300 mg of sodium takes out about 40 mg of calcium. The amount of protein in the diet can also affect calcium need. High intakes of protein increase urinary calcium excretion (Linkswiler et al 1981, Margen et al 1974) – each gram of protein takes out 1 mg of calcium. In contrast, diets that are particularly low in protein have also been shown to be of concern in terms of bone health, possibly due to lowered calcium absorption (Cooper et al 1996, Geinoz et al 1993, Hannan et al 2000, Kerstetter et al 2003a,b). The effect of protein on calcium retention is unclear (Delmas 1992, Walker & Linkswiler 1972).

Indicators that have been used to assess calcium requirements include balance studies, factorial estimates of requirements or assessment of changes in bone mineral density and bone mineral content. In setting the Australian and New Zealand recommendations, a balance approach used for the earlier Australian /New Zealand RDIs and used by FAO:WHO in their 2001 revision of Human Vitamin and Mineral Requirements (FAO:WHO 2001) was adopted. Other approaches, such as the various methods used by the US:Canadian DRI review (FNB:IOM 1997) give widely varying and inconsistent results, making interpretation problematic.

For adults, the results of 210 balance studies on normal individuals quoted in the FAO:WHO report were used to calculate calcium requirements. The estimate was based on the intake at which excreted calcium equals net absorbed calcium, adding an allowance for insensible losses. In postmenopausal women, allowance was made for an additional loss of calcium in urine.

The calcium requirements for other age/gender/physiological groups, for whom there were few balance studies, were estimated from the amount of calcium that each group must absorb in order to meet obligatory calcium losses, together with a consideration of their desirable calcium retention and then calculation of the intake required to provide this necessary rate of calcium absorption. The only exception to this was for infants in whom the concentration of calcium in breast milk formed the basis of recommendations.