RAREFYING DISEASE OF THE SKELETON 437 



short time, perhaps one or two years, generally at some interval 

 between ages 50 and 65. Recent writers contend that polysaccharide 

 is lost, that collagen is degraded, and that the disorder is basically 

 a degenerative process. The morphology and biochemistry of the 

 changes in bone associated with osteoporosis resemble in most 

 respects abnormalities attributable to the physiologic process of 

 aging. The disappearance of osteccytes and the occurrence of 

 patches of necrotic bone are striking. 



It appears that in the aged, and more so in the osteoporotic, the 

 movement of calcium from the gut to the blood plasma is slower 

 than normal, movement of calcium from bone to extracellular fluid 

 is accelerated, and movement of calcium from plasma to feces and 

 urine is normal or high. Emphasis is often placed upon the low 

 calcium intake and insufficient peptides (from protein) in the diet 

 in osteoporotic women. Our view is that negative calcium and 

 nitrogen balance mav reflect the process of resorption of dead bone 

 and mav be the result rather than the cause of osteoporosis. Bone 

 mineral is among the products of breakdown of structural bone, and 

 this is the source of the excess calcium excreted in both the urine 

 and the feces. A low level of osteogenetic activity is apparent during 

 the later stages of the process of osteon formation, when reactive 

 new bone is normallv converted to structural bone. The movement 

 of calcium from gut to plasma to bone therefore declines in rate, 

 and significantlv greater than normal amounts of calcium move 

 from the plasma to the urine and feces. In young adults, the daily 

 net absorption of calcium, regardless of the total amount (800 mg) 

 in the diet of an adult, is approximately 150 mg (Dent, 1956). 

 Though radioisotope kinetic studies reveal the internal movement 

 of Ca++ and a high rate of turnover of bone in the acute stage of 

 osteoporosis, the low dailv net absorption of calcium remains to be 

 accuratelv measured in the chronic stages. Indeed, in the aged 

 osteoporotic, to judge from the total amount of calcium in 24-hour 

 samples of urine, this may be as little as 50 mg. 



Twenty-five years ago, the emphasis in research on osteoporosis 

 was on the endocrine control of calcium metabolism. The occur- 

 rence of the disorder in patients with acromegaly, hyperthyroidism, 

 diabetes, and hypogonadism was attributed to endocrine imbalance. 



