Calcium Intake and Skeletal Integrity: Is there a Critical Relationship?

Abstract

The emergence of osteoporosis as a major health con cern has placed dietary calcium on center stage of pub lic nutritional awareness. Government and profes sional organizations involved with bone health urge proper attention to dietary calcium throughout life (1). Pharmaceutical firms promote calcium supplementa tion for women of all ages. Antacids are advertised for their calcium content as much as for their acid-neu tralizing capacity, and soft-drink producers now add small amounts of calcium to their products. Recent advances in the noninvasive measurement of bone den sity have led to studies directly relating calcium intake to bone mass. Though this body of work remains small and tentative, it has been widely publicized, even in such venerable journals as Science (2). Therefore it seems reasonable and timely to examine the pertinent evi dence and to ask whether current popular enthusiasm for calcium is justified. I restrict this examination to the relationship of calcium intake to bone health. In my opinion, recent interest in dietary calcium stems from three landmark publications. The first of these is the seminal contribution of Heaney et al. (3), which showed that middle-aged women are generally in neg ative calcium balance when tested on self-selected cal cium intakes. Second is the report of Matkovic et al. (4) showing a relationship between calcium intake, cor tical bone density and hip fracture prevalence in rural Yugoslavia. Third are the Health and Nutrition Eval uation Survey (HANES) (5, 6) figures that showed that, beginning at age 10, calcium intake in most American women fails to meet RDA standards. Taken together, these data suggest that calcium in take is critically important to the maintenance of skel etal integrity and the avoidance of fracture. In this re gard, I consider the following four questions. How important is dietary calcium during years of growth to the development of peak bone mass? Does dietary cal cium during young adult life influence age-related bone loss? Does a high calcium intake at menopause prevent accelerated bone loss? How effective is calcium sup plementation as therapy for the elderly? Although a considerable number of studies relevant to these ques tions have been reported or are in progress, in no case does sufficient evidence exist to permit a definitive answer. If agreement exists on any point, it is that bone is lost with age. Little if any decrease in mineral density of the cortical, appendicular skeleton occurs before the fifth decade of life, but current data suggest that loss of axial trabecular bone begins early, perhaps in the third decade (7, 8). The basis of age-related bone loss is remodeling, a continuous and inherently unbalanced process of coupled breakdown (rA©sorption)and renewal (formation). By unbalanced, I mean that the amount of bone formed during a remodeling cycle does not com pletely replenish that which has been removed by rA© sorption. Alterations in remodeling activity are the means by which mechanical, hormonal and dietary fac tors modulate the rate of bone loss.