Patterns of agerelated bone loss

The pattern of age-related bone loss varies between genders. Women undergo two phases of bone loss — an early, accelerated transient phase followed by a slow continuous phase — whereas men undergo only a slow continuous phase (Riggs et al 1998). The accelerated bone loss in the early postmenopausal phase decreases

TABLE 1 Lifetime risk of major fractures due to osteoporosis for white men and women at age 50

Women (%)

Men (%)

Hip fracture

17.5

6.0

Vertebral fracture

15.6

5.0

Forearm fracture

16.0

2.5

Any of the three

39.7

13.1

exponentially over 5—10 years to merge asymptotically with the subsequent slow phase that continues indefinitely. This early phase involves predominantly cancellous bone loss; it accounts for losses of 20—30% of cancellous bone but only of 5—10% of cortical bone. It is associated with high bone turnover and the increase in bone resorption is greater than the increase in bone formation. The reason for the slowing and eventual cessation of the rapid phase of bone loss is unclear. However, it is likely that with the rapid depletion of the cancellous bone, biomechanical forces act to limit further loss.

The late slow phase in women involves equal losses (* 20—25% each) of cortical and cancellous bone over life. The slow continuous phase in men is similar to the late slow phase in postmenopausal women, both with respect to its rate and time course and with respect to the type and amount of bone loss. The slow phases of bone loss in both genders are associated with high bone turnover. These patterns are shown schematically in Fig. 1.

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