enough vitamin D through sunlight exposure and their normal diet, housebound elderly persons are prone to vitamin D deficiency,32 particularly when they do not take supplementary vitamins and are consuming a diet marginal in vitamin D. The full extent of this problem is unknown; however, it is correctable with appropriate vitamin D supplementation.

Finally, it has been suggested that age-related bone loss could result, in part, from a decline in activity and fitness.37 Certainly it is true that bone responds to changes in loading: activity may cause an increase in bone mass, whereas disuse can result in dramatic bone loss.46 One study21 of hospitalized adults who required therapeutic bed rest showed that the bone mineral content in the lumbar spine decreased about 0.9 percent per week (equivalent to one year's worth at normal rates), although the rate of loss usually declines over time until a new steady state is reached. The exact pathophysiology of disuse osteoporosis is not known, nor are there specific approaches to remedy the situation except through restored activity. Nonetheless, this type of osteoporosis is a particular concern with regard to the rehabilitation of elderly people after an injury or other serious insult.

On a more positive note, it is thought that increased activity might help to retard bone loss and prevent osteoporosis.35 A number of short-term trials, mostly uncontrolled, indicate that exercise programs may augment bone mass,25 but it is not known whether these gains are maintained or whether they lead to a reduction in fractures. Increased activity throughout life appears to be generally beneficial, however, and should be encouraged.32 Specific recommendations regarding increased activity for older individuals are provided in Chapter 13.

Secondary Osteoporosis

Bone loss may be exacerbated by certain medical diseases, surgical procedures, and medications (Figure 6-3). Although such conditions may be a major cause of osteoporosis in some individuals, in general the bone loss from these factors is additive to the age-related slow bone loss that occurs universally and to the accelerated bone loss that occurs postmenopausally in women.43 Relatively common causes of this additional bone loss are corticosteroid and anticonvulsant use, gastrectomy, hyperthyroidism, and chronic obstructive lung disease. Less common are such conditions as Cushing's disease, acromegaly, hypopituitarism, and multiple myeloma. Altogether, one or more of these factors may be present in as many as 20 percent

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