Calcium and Osteoporosis
Special Issue: Calcium
Calcium and Osteoporosis
April 2000; Volume 2; 25-27
By Bess Dawson-Hughes, MD
Osteoporosis is a prevalent condition that involves low bone mass and architectural deterioration of bone. Heredity and lifestyle contribute both to the development of peak bone mass and the preservation of bone mass in adults. Women are at higher risk for osteoporosis than men because they have a lower peak bone mass and lose bone mass more rapidly. Calcium and its companion, vitamin D, are essential to develop optimal peak bone mass and to minimize the loss and architectural deterioration of the skeleton. However, an adequate intake of calcium and vitamin D is not sufficient to prevent bone loss in everyone. An inadequate intake of calcium (and/or vitamin D) results in less absorbed calcium and a subtle lowering of the blood calcium concentration. The drop of calcium triggers the release of parathyroid hormone, a compound that stimulates the resorption of bone; this returns the blood calcium concentration to normal. In the process, bone loss occurs and this loss contributes directly to fracture risk. For instance, a bone density loss of about 10% from the hip increases risk of hip fracture 2.5-fold.1 This article will consider how calcium and vitamin D affect the skeleton and review the evidence that calcium promotes bone health in pre- and postmenopausal women.
Calcium and Bone Density
The strongest evidence that calcium affects bone density comes from randomized, controlled clinical trials. Few such trials have been conducted in healthy premenopausal women because their bone density is fairly stable. One available study, however, examined the effect of increasing calcium intake by an average of 610 mg/d by consuming more dairy products.2 The control group maintained its usual average calcium intake of about 900 mg/d. Over the three-year study period, women taking the dairy products had stable bone density at the spine whereas the control group lost bone density from this site.
Many calcium intervention trials have been performed in postmenopausal women, with the general result that calcium reduces bone loss at one or more skeletal sites. Among women in the first five years after menopause (who were not taking estrogen), supplemental calcium reduced bone loss to some extent,3,4 but it did not prevent all loss. This is in contrast to estrogen replacement, which can slow or prevent bone loss in most women.
Among older postmenopausal women, added calcium significantly improves bone density at the spine, hip, and forearm, and of the total skeleton.3,5,6 These studies evaluated calcium supplements; another compared two calcium sources, each supplying 1,000 mg, with control.7 Both supplements had similarly favorable effects on reducing bone loss from the ankle and hip.7 Postmenopausal women with lower dietary calcium intakes tend to benefit most from added calcium,3 although significant benefits have also been observed in women with initial calcium intakes as high as 750-800 mg/d.5-7
Calcium and Fracture Rates
In the last few years, information about the effect of calcium supplementation on the incidence of fractures has begun to emerge. Four calcium intervention trials have reported fracture incidence.5,8-10 Although these studies were small, three out of four found calcium reduced fracture rates.5,8,9 In the largest study, calcium supplements significantly lowered the spine fracture incidence in women with a prior spine fracture but not in women without a prior spine fracture.9 None of these studies was large enough to establish the efficacy of calcium supplements in preventing fractures. In contrast, a very large placebo-controlled trial examined the effect of combined calcium and vitamin D supplementation on fracture incidence in elderly French women (mean age 84 years) who resided in nursing homes.11 Daily supplementation with 1,200 mg of calcium and 800 IU of vitamin D lowered hip fracture incidence by 25%. Fractures at other sites also were reduced by a similar percentage. These women had a low average dietary calcium intake of about 500 mg/d and low 25-hydroxyvitamin D levels, indicating vitamin D insufficiency. In another placebo-controlled trial in men and women, age 65 and older, treatment with 500 mg of calcium and 700 IU of vitamin D daily for three years not only reduced bone loss from every measured site (the spine, hip, and total body) but also significantly lowered the incidence of clinical fractures.6 The men and women in this study had an average dietary calcium intake of 750 mg/d and a vitamin D intake of 200 IU/d. From these studies, it appears that calcium provides the most consistent benefit when taken with vitamin D and that the benefit from calcium is not limited to those with low usual calcium intakes.
Calcium, Exercise, and Estrogen
Exercise and estrogen have well-recognized favorable effects on the skeleton. Recent studies suggest that calcium intake may influence the impact of exercise and estrogen on bone density. In a meta-analysis of controlled exercise-intervention studies, Specker et al noted that greater bone density gains occurred when the study participants had calcium intakes greater than 1,000 mg/d than when they consumed less calcium.12 In a different meta-analysis, Nieves et al assessed the impact of calcium intake on hormone replacement therapy and bone density in early postmenopausal women.13 Of 31 estrogen intervention trials in the meta-analysis, supplemental calcium was provided to both the estrogen and control groups in 20 trials (bringing total average calcium intake to 1,183 mg) but not in the other 11 trials (mean calcium intake 563 mg/d). The authors found that the average gains in bone density at the spine, hip, and forearm were significantly greater in the women on estrogen who took added calcium compared with those who did not.
Calcium: Safety and Recommendations
In the calcium intervention studies cited above, no serious adverse events were attributed to the calcium. The safety of dietary and supplemental calcium also has been examined in a large observational study. Curhan et al found that women with calcium intakes above 1,000 mg/d from food sources actually had a lower risk of developing a first kidney stone than did women with lower calcium diets.14 The same investigators identified about a 20% higher risk of first kidney stone among women taking more than 500 mg of calcium per day as supplements compared with women taking less than 100 mg of supplemental calcium per day.14 The National Academy of Sciences examined available safety data for calcium and set a safe upper limit for healthy adults at 2,500 mg/d.15 The safe upper limit of vitamin D is 2,000 IU/d.
In 1997, the National Academy of Sciences increased the recommended daily intake of calcium for adult women from 800 mg/d (the old Recommended Dietary Allowance or RDA) to 1,000 mg/d for premenopausal women and 1,200 mg/d for women (and men) age 51 and older.15 These recommendations acknowledge the evidence accrued in the last decade that higher calcium intakes will lower the risk of osteoporotic fractures in the elderly. Unfortunately, this knowledge appears to have had little impact on dietary calcium consumption. The 1994 USDA Continuing Survey revealed that the median calcium intake of women over age 50 is less than 50% of the amount now recommended.16
Conclusion
There is now solid evidence that meeting the age- specific calcium intake recommendations will promote bone development and reduce bone loss. In the presence of adequate vitamin D, calcium will lower fracture rates in the elderly. The skeletal benefits of exercise and hormone replacement therapy appear to be enhanced in women who meet the calcium requirement. Food sources of calcium are safe and effective; unfortunately, few women in the United States are meeting the calcium requirement through their diets. Changing dietary patterns to increase calcium intake should significantly reduce the burden of osteoporosis.
Dr. Dawson-Hughes is Professor of Medicine and Chief of the Calcium and Bone Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, Boston, MA.
References
1. Cummings SR, et al. Bone density at various sites for prediction of hip fractures. Lancet 1993;341:72-75.
2. Baran D, et al. Dietary modification with dairy products for preventing vertebral bone loss in premenopausal women: A three-year prospective study. J Clin Endocrinol Metab 1990;70:264-270.
3. Dawson-Hughes B, et al. A controlled trial of the effect of calcium supplementation on bone density in postmenopausal women. N Engl J Med l990;323:878-883.
4. Aloia JF, et al. Calcium supplementation with and without hormone replacement therapy to prevent postmen-opausal bone loss. Ann Intern Med 1994;120:97-103.
5. Reid IR, et al. 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.
6. Dawson-Hughes B, et al. Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older. N Engl J Med 1997;337:670-676.
7. Prince R, et al. The effects of calcium supplementation (milk powder or tablets) and exercise on bone density in postmenopausal women. J Bone Miner Res 1995;10:1068-1075.
8. Chevalley T, et al. Effects of calcium supplements on femoral bone mineral density and vertebral fracture rate in vitamin-D-replete elderly patients. Osteoporos Int 1994;4:245-252.
9. Recker RR, et al. Correcting calcium nutritional deficiency prevents spine fractures in elderly women. J Bone Miner Res 1996;11:1961-1966.
10. Riggs BL, et al. Long-term effects of calcium supplementation on serum parathyroid hormone level, bone turnover, and bone loss in elderly women. J Bone Miner Res 1998;13:168-174.
11. Chapuy M-C, et al. Effect of calcium and cholecalciferol treatment for three years on hip fractures in elderly women. BMJ 1994;308:1081-1082.
12. Specker BL. Evidence for an interaction between calcium intake and physical activity on changes in bone mineral density. J Bone Miner Res 1996;11:1539-1544.
13. Nieves JW, et al. Calcium potentiates the effect of estrogen and calcitonin on bone mass: Review and analysis. Am J Clin Nutr 1998;67:18-24.
14. Curhan GC, et al. Comparison of dietary calcium with supplemental calcium and other nutrients as factors affecting the risk for kidney stones in women. Ann Intern Med 1997;126:497-504.
15. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Institute of Medicine. Dietary Reference Intakes: Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press; 1997.
16. Nusser SM, et al. A semiparametric transformation approach to estimating usual daily intake distributions. J Am Stat Assoc 1996;91:1440-1449.
April 2000; Volume 2; 25-27
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