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.

1 mmol calcium = 40 mg calcium

Recommendations by life stage and gender

Infants

Age AI
0-6 months 210 mg/day
7-12 months 270 mg/day

Rationale: The AI for 0-6 months was set by multiplying together the average intake of breast milk (0.78 L/day) and the average concentration of calcium in breast milk (264 mg/L) from 10 studies reviewed by Atkinson et al (1995), and rounding. Formula-fed babies require additional intakes in the vicinity of 350 mg/day as calcium is less bioavailable in formula. The AI for infants 7-12 months was set by adding an estimate for calcium from breast milk at this age, to an estimate of intake from supplementary foods. A breast milk volume of 0.60 L/day was assumed at older ages (Dewey et al 1984). The concentration of calcium in breast milk at this age averages 210 mg/L (Atkinson et al 1995). This gives a contribution of 126 mg/day from breast milk that is added to 140 mg/day from complementary foods (Abrams et al 1997, Specker et al 1997) and rounded, giving an AI of 270 mg/day.

Children & adolescents

Age EAR RDI
All
1-3 yr 360 mg/day 500 mg/day
4-8 yr 520 mg/day 700 mg/day
Boys
9-11 yr 800 mg/day 1,000 mg/day
12-13 yr 1,050 mg/day 1,300 mg/day
14-18 yr 1,050 mg/day 1,300 mg/day
Girls
9-11 yr 800 mg/day 1,000 mg/day
12-13 yr 1,050 mg/day 1,300 mg/day
14-18 yr 1,050 mg/day 1,300 mg/day

Rationale: The EAR for children 1-8 years was set by modelling the components of calcium requirements, including a component for skeletal growth (FAO:WHO 2001). Requirements were estimated from data on accumulation of whole-body calcium, which was converted to a daily rate of calcium accretion. This, together with consideration of urinary calcium losses, dermal losses and daily skeletal increments, gives an estimate of daily net absorbed calcium needs. For children 1-8 years, this results in a figure of about 220 mg. EARs were set for this age band based on the estimated amounts needed - 440 mg/day on average - to provide this level of absorbed calcium, assuming absorption rates of 1 SD above those of adults. A lower figure of 360 mg/day was applied to the younger age band as their requirements will be less and 520 mg/day to the older group, on an approximate body weight basis. The RDI was set assuming a CV of 15% for the EAR (as variation in the needs of children and adolescents are likely to be greater than for adults) and rounding, giving an RDI of 500 mg/day for 1-3 year-olds and 700 mg/day for 4-8 year-olds.

From 9-11 years of age, calcium accretion rates are similar to those in younger children with EARs being 800 mg/day, assuming absorption at 1 SD above that for adults. There is a striking increase in the rate of skeletal calcium accretion from 12 to 18 years of age (FAO:WHO 2001). For this age group, net absorbed calcium needs to be 440 mg. Assuming high calcium absorption (+2 SDs above that for adults) this requires an EAR of 1,046 mg/day. Assuming a CV of 15% for the EAR, this gives an RDI of 1,300 mg in the older adolescents. For children aged 9-11 years who have physically matured much earlier than average, the recommendations for 12-18 year-olds may be more appropriate.

Adults

Age EAR RDI
Men
19-30 yr 840 mg/day 1,000 mg/day
31-50 yr 840 mg/day 1,000 mg/day
51-70 yr 840 mg/day 1,000 mg/day
>70 yr 1,100 mg/day 1,300 mg/day
Women
19-30 yr 840 mg/day 1,000 mg/day
31-50 yr 840 mg/day 1,000 mg/day
51-70 yr 1,100 mg/day 1,300 mg/day
>70 yr 1,100 mg/day 1,300 mg/day

Rationale: The EAR for adults was set by calculating calcium requirement as the intake at which excreted calcium equals net absorbed calcium, based on the results of 210 balance studies on 81 subjects (FAO:WHO 2001). This occurs at an intake of 520 mg/day to which losses through sweat must be added. Insensible losses of calcium have been estimated at 60 mg/day (Charles et al 1983, Hasling et al 1990). Taking the low absorption that occurs at about 500 mg/day into account, an additional intake of 320 mg is required to cover these losses, increasing the EAR to 840mg. At menopause, an additional 30 mg is lost in urine (Nordin et al 1999) and absorption probably decreases (Heaney et al 1989, Nordin 1997) raising the EAR to 1,100 mg. This gives an RDI of 1,000 mg/day for men and premenopausal women, and 1,300 mg for postmenopausal women (EAR+2SD = RDI), assuming a CV of 10% for the EAR.

Little known about calcium metabolism in the elderly, but absorption is known to decrease with age in both sexes (Ebeling et al 1994, Morris et al 1991, Need et al 1998). Data for increased need at menopause are strong but those for older men are not. As a precaution, an additional average requirement of 250 mg/day is recommended, translating to an additional 300 mg for the RDI.

Pregnancy

Age EAR RDI
14-18 yr 1,050 mg/day 1,300 mg/day
19-30 yr 840 mg/day 1,000 mg/day
31-50 yr 840 mg/day 1,000 mg/day

Rationale: The EAR and RDI for pregnancy were based on the needs of the mother plus any additional allowance for the foetus and products of conception. The foetus retains about 25-30 g, mostly in the third trimester of pregnancy, but there is evidence that pregnancy is associated with increased calcium absorption (Cross et al 1995a, Heaney & Skillman 1971, Kent et al 1991, Kumar et al 1979). Significant increases in maternal calcium accretion, bone turnover and intestinal absorption early in pregnancy before foetal bone mineralisation have also been shown (Heaney & Skillman 1971, Purdie et al 1988).

Dietary calcium intake does not appear to influence changes in maternal bone mass in pregnancy (Raman et al 1978) and there is no relationship between the number of previous pregnancies and bone mineral density (Alderman et al 1986, Koetting & Warlaw 1988, Kreiger et al 19832, Walker & Linkswiler 1972, Wasnich et al 1983) or fracture risk (Johansson et al 1993). Indeed, some studies show a positive correlation between number of children born and radial bone mineral density or total body calcium (Aloia et al 1983) as well as reduction in the risk of hip fracture (Hoffman et al 1993).

These findings support the concept that maternal skeleton is not used for foetal calcium needs. The work of Prentice (2003) also confirms no additional need for calcium in pregnancy. The available information thus does not support the need for additional dietary intake in pregnancy as maternal adaptive mechanisms including enhanced efficiency of absorption more than meet the additional needs in the last trimester. The implication is that normal calcium intake is sufficient to meet the calcium requirement in the pregnant state.

Lactation

Age EAR RDI
14-18 yr 1,050 mg/day 1,300 mg/day
19-30 yr 840 mg/day 1,000 mg/day
31-50 yr 840 mg/day 1,000 mg/day

Rationale: During pregnancy, 210 mg calcium/day on average is secreted in milk. The primary source of this calcium appears to be from increased maternal bone resorption (Affinato et al 1996, Dobnig et al 1995, Kent et al 1990) which is independent of calcium intake (Cross et al 1995b, Sowers et al 1995, Specker et al 1994). This bone loss is replaced after weaning. There is no evidence that the calcium intake of lactating women should be increased above that of non-lactating women.

Upper Level of Intake

Age UL
Infants
0-12 months Not possible to establish
Children and adolescents
1-3 yr 2,500 mg/day
4-8 yr 2,500 mg/day
9-13 yr 2,500 mg/day
14-18 yr 2,500 mg/day
Adults 19+ yr
Men 2,500 mg/day
Women 2,500 mg/day
Pregnancy
14-18 yr 2,500 mg/day
19-50 yr 2,500 mg/day
Lactation
14-18 yr 2,500 mg/day
19-50 yr 2,500 mg/day

Rationale: Because of the inverse relationship between fractional calcium absorption and calcium intake, an additional intake of 1,000 mg added to a typical western diet would only increase calcium in urine by about 60 mg. Urinary calcium rises slowly with intake and risk of developing kidney stones (nephrolithiasis) from calcium supplements is therefore negligible. Toxic effects of calcium have only been seen when calcium is given in high doses as the carbonate as an antacid. The result is hypercalcaemia with renal calcification and renal failure and is known as the milk alkali syndrome or MAS (Burnett et al 1949).

Using MAS as the critically defined endpoint, a LOAEL of about 5 g can be identified for adults from 16 studies involving 26 subjects (FNB:IOM 1997).

A UF of 2 takes into account the potential for increased risk of high calcium intake, given the relatively common occurrence of kidney stones in Australia and New Zealand, the fact that hypercalciuria in people with renal stones has been shown to occur at intakes as low as 1,700 mg /day in men and 870 mg in women (Burtis et al 1974) and concern that calcium will interfere with absorption of other minerals such as zinc and iron in vulnerable populations. The UL is therefore set conservatively at 2,500 mg/day.

As there is little evidence for other age and physiological groups, this figure is used for all age and gender groups and physiological states, particularly in relation to the need to prevent interference with zinc and iron absorption.

References

Abrams SA, Wen J, Stuff JE. Absorption of calcium, zinc and iron from breast milk by 5- to 7-month-old infants. Pediatr Res 1997;41:1-7.

Affinito ZP, Tommaselli GA, DiCarlo C, Guida F, Nappi C. Changes in bone mineral density and calcium metabolism in breast-feeding women: A one year follow-up study. J Clin Endocrinol Metab 1996;81:2314-8.

Alderman BW, Weiss NS, Daling JR, Ure CL, Ballard JH. Reproductive history and postmenopausal risk of hip and forearm fracture. Am J Epidemiol 1986;124:262-7.

Aloia JF, Vaswani AN, Yeh JK, Ross P, Ellis K, Cohen S. Determinants of bone mass in postmenopausal women. Arch Intern Med 1983;143:1700-4.

Atkinson SA, Alston-Mills BZP, Lonnerdal B, Neville MC, Thomson MP Major minerals and ionic constituents of human and bovine milk. In: Jensen RJ, ed. Handbook of milk composition. California: Academic Press, 1995. Pp 93-619.

Burnett CH, Commons RM, Albright F, Howard JE. Hypercalcaemia without hypercalciuria or hypophosphatemia, calcinosis and renal insufficiency. A syndrome following prolonged intake of milk and alkali. N Engl J Med 1949;240:787-94.

Burtis WJ, Gay L, Insogna KL, Ellison A, Broadus AE. Dietary hypercalciuria in patients with calcium oxalate kidney stones. Am J Clin Nutr 1974;60:424-9.

Charles ZP. Jensen FT, Mosekilde L, Hansen HH. Calcium metabolism evaluated by 47Ca Kinetics: Estimation of dermal calcium losses. Clin Sci 1983;65:415-22.

Cooper C, Atkinson EJ, Hensrud DD, Wahner HW, O'Fallon WM, Riggs BL, Melton LJ 3rd. Dietary protein intake and bone mass in women. Calcif Tissue Int 1996;58:320-5.

Cross NA, Hillman LS, Allen SH, Krause GF, Vieira NE. Calcium homeostasis and bone metabolism during pregnancy, lactation and postweaning: a longitudinal study. Am J Clin Nut.1995a;61:514-23.

Cross NA, Hillman LS, Allen SH, Krause GF. Changes in bone mineral density and markers of bone remodelling during lactation and postweaning in women consuming high amounts of calcium. J Bone Miner Res 1995b;10:1312-20.

Cumming RG, Nevitt MC. Calcium for prevention of osteoporotic fractures in postmenopausal women. J Bone Miner Res 1997;12:1321-9.

Dawson-Hughes B, Dallal GE, Krall EA, Sadowski L, Sahyoun N, Tannenbaum S. A controlled trial of the effect of calcium supplementation on bone density in postmenopausal women. N Engl J Med 1990;323:878-83.

Delmas PD. Clinical use of biochemical markers of bone remodelling in osteoporosis. Bone 1992;13:S17-S21.

Devine A, Criddle RA, Dick IM, Kerr DA, Prince RL. A longitudinal study of the effect of sodium and calcium intakes on regional bone density in postmenopausal women. Am J Clin Nutr 1995;62:740-5.

Dewey KG, Finley DA, Lonnerdal B. Breast milk volume and composition during late lactation (7-20 months). J Pediatr Gastroenterol Nutr 1984;3:713-20.

Dobnig H, Kainer F, Stepan V, Winter R, Lipp R, Schaffer M, Kahr A, Nocnik S, Patterer G, Leb G. Elevated parathyroid hormone-related peptide levels after human gestation: relationship to changes in bone and mineral metabolism. J Clin Endocrinol Metab 1995;80:3699-707.

Ebeling PR, Yegey AL, Vieira NE, Burritt MF, O'Fallon WM, Kumar R, Riggs BL Influence of age on effects on endogenous 1,25-dihydroxy-vitamin D on calcium absorption in normal women. Calcif Tissue Int 1994;55:330-4.

Elders PJM, Lips P, Netelenbos JC, van Ginkel FC, Khoe E, van der Vijgh WJF, van der Stelt PF. Long-term effect of calcium supplementation on bone loss in perimenopausal women. J Bone Miner Res 1994;9:963-70.

Food and Agricultural Organization of the United Nations: World Health Organization. Human vitamin and mineral requirements. Report of a joint FAO:WHO expert consultation, Bangkok, Thailand. Rome: Food and Agricultural Organization of the United Nations, 2001.

Food and Nutrition Board: Institute of Medicine. Dietary Reference Intakes for calcium, phosphorus, magnesium, vitamin D and fluoride. Washington DC: National Academy Press, 1997.

Geinoz G, Rapin CH, Rizzoli R, Kraemer R, Buchs B, Slosman D, Michel JP, Bonjour JP. Relationship between bone mineral density and dietary intakes in the elderly. Osteoporos Int 1993;3:242-8.

Grant AM, Avenell A, Campbell MK, McDonald AM, MacLennan GS, McPherson GC, Anderson FH, Cooper C, Francis RM, Donaldson C, Gillespie WJ, Robinson CM, Torgerson DJ, Wallace WA; RECORD Trial Group. Oral vitamin D3 and calcium for secondary prevention of low-trauma fractures in elderly people (Randomised Evaluation of Calcium Or vitamin D, RECORD): a randomised placebo-controlled trial. Lancet 2005;365:1621-8.

Hannan MT, Tucker KL, Dawson-Hughes B, Cupples LA, Felson DT, Kiel DP. Effect of dietary protein on bone loss in elderly men and women: the Framingham Osteoporosis Study. J Bone Miner Res 2000;15:2504-12.

Hasling C, Charles P, Jensen FT, Mosekilde L. Calcium metabolism in post-menopausal osteoporosis: the influence of dietary calcium and net absorbed calcium. J Bone Mineral Res 1990;5:939-46.

Heaney RP. Calcium, bone health and osteoporosis. In: Peck WA ed. Bone and Mineral Research, Annual 4: A yearly survey of developments in the field of bone and mineral metabolism. New York: Elsevier, 1986. Pp 255-301.

Heaney RP, Skillman TG. Calcium metabolism in normal human pregnancy. J Clin Endocrinol Metab 1971;33:661-70.

Heaney RP, Saville PD, Recker RR. Calcium absorption as a function of calcium intake. J Lab Clin Med 1975;85:881-90.

Heaney RP, Recker RR, Hinders SM. Variability of calcium absorption. Am J Clin Nutr 1988;47:262-4.

Heaney RP, Recker RR, Stegman RR, Moy AJ. Calcium absorption in women: relationships to calcium intake, estrogen status and age. J Bone Miner Res1989;4:469-75.

Heaney RP, Recker RR, Weaver CM. Absorbability of calcium sources: the limited role of solubility. Calcif Tissue Int1990;46:300-4.

Hoffman S, Grisso JA, Kelsey JL, Gammon MD, O'Brien LA. Parity, lactation and hip fracture. Osteopor Int 1993;3:171-6.

Johansson C, Mellstrom D, Milsom I. Reproductive factors as predictors of bone density and fractures in women at the age of 70. Maturitas 1993;17:39-50.

Kent GN, Price RI, Gutteridge DH, Smith M, Allen JR, Bhagat CI, Barnes MP, Hickiling CJ, Retallack RW, Wilson SJ, Devlin RD, Davies C, St John A. Human lactation: forearm trabecular bone loss, increased bone turnover and renal conservation of calcium and inorganic phosphate with recovery of bone mass following weaning. J Bone Miner Res 1990;5:361-9.

Kent GN, Proce RI, Gutteridge DH. The efficiency of intestinal calcium absorption is increased in late pregnancy but not in established lactation. Calcif Tissue Int 1991;48:293-5.

Kerstetter JE, O'Brien KO, Insogna KL. Dietary protein, calcium metabolism and skeletal homeostasis revisited. Am J Clin Nutr 2003a;78:584S-592S.

Kerstetter JE, O'Brien KO, Insogna KL. Low protein intake: the impact on calcium and bone homeostasis in humans. J Nutr 2003b;133:855S-861S.

Koetting CA, Wardlaw GM. Wrist, spine and hip bone density in women with variable histories of lactation. Am J Clin Nutr 1988;48:1479-81.

Kohlenberg-Mueller K, Raschka K. Calcium balance in young adults on a vegan and lactovegetarian diet. J Bone Miner Metab 2003;21:28-33.

Kreiger N, Kelsey JKL, Holford TR, O'Connor T. An epidemiologic study of hip fracture in postmenopausal women. Am J Epidemiol 1982;116:141-8.

Kumar R, Cohen WR, Silva P, Epstein FH. Elevated 1.25-dihydroxyvitamin D plasma levels in normal human pregnancy and lactation. J Clin Invest 1979;643:342-4.

Larsson C, Johansson G. Dietary intake and nutritional status of young vegans and omnivores in Sweden. Am J Clin Nutr 2002;76:100-6.

Linkswiler HM, Zemel MB, Hegsted M, Shuette S. Protein-induced hypercalciuria. Fed Proc 1981,490:2429-33.

Lloyd T, Schaeffer JM, Walker MA, Demers LM. Urinary hormonal concentrations and spinal bone densities of premenopausal vegetarian and non vegetarian women. Am J Clin Nutr 1991;54:1005-10.

Mackerras D, Lumley T. First and second year effects in trials of calcium supplementation on loss of bone density in postmenopausal women. Bone1997;21:527-33.

Margen S, Chu JY, Kaufmann NA, Calloway DH. Studies in calcium metabolism 1. The calciurietic effect of dietary protein. Am J Clin Nut 1974;27:584-9.

Marsh AG, Sanche TV, Mickelsen O, Keiser K, Mayor G. Cortical bone density in adult lacto-ovo-vegetarian and omnivorous women. J Am Diet Assoc 1980;76:148-51.

Matkovic V, Illich JZ, Andon MB, Hseih LC, Tzagournis MA, Lagger BJ, Goel PK. Urinary calcium, sodium, and bone mass of young females Am J Clin Nutr 1995;62:417-25.

Miller JZ, Smith DL, Flora L, Slenda C, Jiang X, Johnston CC. Calcium absorption from calcium carbonate and a new form of calcium in healthy male and female adolescents. Am J Clin Nutr 1988;138:225-36.

Morris HA, Need AG, Horowitz M, O'Loughlin PD, Nordin BEC. Calcium absorption in normal and osteoporotic postmenopausal women. Calcif Tissue Int 1991;49:240-3.

Need AG, Morris HA, Horowitz M, Scopasa F, Nordin BEC. Intestinal calcium absorption in men with spinal osteoporosis. Clin Endocrinol 1998;48:163-8.

New S. Do vegetarians have a normal bone mass? Osteoporos Int 2004;15:679-88.

Nordin BEC, Horseman A, Marshall DH, Simpson M, Waterhouse GM. Calcium requirement and calcium therapy. Clin Orthop 1979;140:216-46.

Nordin BEC. Calcium and osteoporosis. Nutrition 1997;13:664-86.

Nordin BEC, Need AG, Morris HA, Horowitz M. Biochemical variables in pre-and postmenopausal women: reconciling the calcium and estrogen hypotheses. Osteoporos Int 1999;9:351-7.

Nordin BEC, Polley KJ. Metabolic consequences of the menopause. A cross-sectional, longitudinal and intervention study on 557 normal postmenopausal women. Calcif Tissue Int 1987;41:S1-S59.

O'Brien KO, Abrams SA, Stuff JE, Liang LK, Welch TR. Variables related to urinary calcium excretion in young girls. J Paediat Gastroenterol Nutr 1996;23:8-12.

Peacock M. Calcium absorption efficiency and calcium requirements in children and adolescents. Am J Clin Nutr 1991;54(Suppl):261S-265S.

Pedersen AB, Bartholomew MJ, Dolence IA, Aljadir LP, Netteburg KL, Lloyd T. Menstrual differences due to vegetarian and nonvegetarian diets. Am J Clin Nutr 1991;53:879-85.

Prentice A. Micronutrients and the bone mineral content of the mother, fetus and newborn. J Nutr 2003;133:1693S-1699S.

Prince R, Devine A, Dick I, Criddle A, Kerr D, Kent N, Price R, Randel A. The effects of calcium supplementation (milk powder or tablets) and exercise on bone density in postmenopausal women. J Bone Miner Res 1995;10:1068-75.

Purdie DW, Aaron JE, Selby PL. Bone histology and mineral homeostasis in human pregnancy. Br J Obstet Gynecol 1988;95:849-54.

Raman L, Rajalakshmi K, Krishnamachari KA, Sastry JG. Effect of calcium supplementation to undernourished mothers during pregnancy on the bone density of neonates. Am J Clin Nutr 1978;31:466-9.

Reed JA, Anderson JJ, Tylavsky FA, Gallagher PN. Comparative changes in radial bone density of elderly female lacto-ovo-vegetarians and omnivores. Am J Clin Nutr 1994:59:1197S-1202S.

Reid IR, Ames RW, Evans MC, Gamble GD, Sharpe SJ. Effect of calcium supplementation on bone loss in postmenopausal women. N Engl J Med 1993;328:460-4.

Reid IR, Ames RW, Evans MC, Gamble GD, Sharpe SJ. Long-term effects of calcium supplementation on bone loss and fractures in postmenopausal women: a randomized controlled trial. Am J Med 1995;98:331-335.

Shea B, Wells G, Cranney A, Zytaruk N, Robinson V, Griffith L, Hamel C, Ortiz Z, Peterson J, Adachi J, Tugwell P, Guyatt G. Calcium supplementation on bone loss in postmenopausal women. Cochrane Database Syst Rev. 2003;(4): CD004526. Updated Cochrane Database Syst Rev. 2004;(1):CD004526

Smith KT, Heaney RP, Flora L, Hinders SM. Calcium absorption from a new calcium delivery system. Calcif Tissue Int 1987;41:351-2.

Sowers M, Randolf J, Shapiro B, Jannausch M. A prospective study of bone density and pregnancy after an extended period of lactation with bone loss. Obstet Gynecol 1995;85;285-9.

Specker BL, Vieira NE, O'Brien K, Ho ML, Huebi JE, Abrams SA, Yergey AL. Calcium kinetics in lactating women with low and high calcium intakes. Am J Clin Nutr 1994;59:593-9.

Specker BL, Beck A, Kalkwarf H, Ho M. Randomised trial of varying mineral intake on total body bone mineral accretion during the first year of life. Pediatrics 1997;99:E12.

Tesar R, Notelowitz M, Shim E, Kauwell G, Brown J. Axial and peripheral bone density and nutrient intakes of postmenopausal vegetarian and omnivorous women. Am J Clin Nutr 1992;56:69-704.

Walker RM, Linkswiler HM. Calcium retention in the adult human male as affected by protein intake. J Nutr 1972;102:1297-302.

Wasnich R, Yano K, Vogel J. Postmenopausal bone loss at multiple skeletal sites: relationship to estrogen use. J Chronic Dis 1983;36:781-90.