intensive research at present,35 but it is not yet possible to intervene to improve osteoblast function. Consequently, attention is directed to the role that calcium, vitamin D, and exercise play in bone loss.
The dietary requirement for calcium is relatively high because of the obligatory fecal and urinary losses of 150 to 250 milligrams per day (mg/day).41 When the amount of calcium absorbed from the diet is insufficient to offset these losses, calcium must be withdrawn from bone, which contains 99 percent of the total body stores. Moreover, active intestinal absorption of calcium decreases with aging, particularly after age 70, in response to a decrease in biologically active vitamin D.41 This phenomenon, in turn, may be due to age-related loss of kidney tissue with its enzyme that converts vitamin D to the active form or to a decrease in vitamin D receptors in intestinal cells.
The level of calcium intake required to overcome these problems is controversial, and population studies generally have not demonstrated a strong relationship between calcium intake and bone loss.19 The recommended daily allowance (RDA) for calcium has not been rigorously established but is currently set at 800 mg/day for adults.35 The efficacy of this recommendation is uncertain, however. Recent trials have generally found that calcium supplementation shortly after the menopause has little effect on spinal bone loss and only a modest effect on bone loss from the limbs.41 It is conceivable that calcium would be more effective after the estrogen-dependent phase of accelerated bone loss has ended, but data regarding this hypothesis are conflicting as well. A prospective study that measured rates of bone loss from the vertebrae and radius by serial bone density measurements found no relationship between calcium intake and bone loss.42 In contrast, a cohort study of elderly men and women living in a retirement community in California showed that those who had hip fractures within 14 years after initial evaluation had had a lower calcium intake at the baseline examination.13 There are no randomized controlled clinical trials that show directly that calcium supplementation prevents fractures. Nonetheless, the mean dietary calcium intake reported for the general population barely approximates the RDA for calcium in American men and is only 550 mg/day in middle-aged and elderly women.35 Consequently, it seems prudent to ensure at least the minimum recommended levels of calcium intake at all ages. Detailed guidelines are available.32
Similarly, it is reasonable to ensure an adequate vitamin D intake. Such assurance is important because there is an age-related decrease in the ability of the skin to synthesize vitamin D and in the ability of the intestine to absorb it.41 Thus, although most people get