Je. Zerwekh et al., THE EFFECTS OF 12 WEEKS OF BED REST ON BONE-HISTOLOGY, BIOCHEMICAL MARKERS OF BONE TURNOVER, AND CALCIUM HOMEOSTASIS IN 11 NORMAL SUBJECTS, Journal of bone and mineral research, 13(10), 1998, pp. 1594-1601
This study was undertaken to examine the effects of 12 weeks of skelet
al unloading on parameters of calcium homeostasis, calcitropic hormone
s, bone histology, and biochemical markers of bone turnover in 11 norm
al subjects (9 men, 2 women; 34 +/- 11 years of age). Following an amb
ulatory control evaluation, all subjects underwent 12 weeks of bed res
t. An additional metabolic evaluation was performed after 12 days of r
eambulation. Bone mineral density declined at the spine (-2.9%, p = 0.
092) and at the hip (-3.8%, p = 0.002 for the trochanter). Bed rest pr
ompted a rapid, sustained, significant increase in urinary calcium and
phosphorus as well as a significant increase in serum calcium. Urinar
y calcium increased from a pre-bed rest value of 5.3 mmol/day to value
s as high as 7.3 mmol/day during bed rest. Immunoreactive parathyroid
hormone and serum 1,25-dihydroxyvitamin D declined significantly durin
g bed rest, although the mean values remained within normal limits. Si
gnificant changes in bone histology included a suppression of osteobla
stic surface for cancellous bone (3.1 +/- 1.3% to 1,9 +/- 1.5%, p = 0.
0142) and increased bone resorption for both cancellous and cortical b
one. Cortical eroded surface increased from 3.5 +/- 1.1% to 7.3 +/- 4.
0% (p = 0.018) as did active osteoclastic surface (0.2 +/- 8.3% to 0.7
+/- 0.7%, p = 0.021). Cancellous eroded surface increased from 2.1 +/
- 1.1% to 4.7 +/- 2.2% (p = 0.002), while mean active osteoclastic sur
face doubled (0.2 +/- 0.2% to 0.4 +/- 0.3%, p = 0.020). Serum biochemi
cal markers of bone formation (osteocalcin, bone-specific alkaline pho
sphatase, and type I procollagen extension peptide) did not change sig
nificantly during bed rest. Urinary biochemical markers of bone resorp
tion (hydroxyproline, deoxypyridinoline, and N-telopeptide of type I c
ollagen) as well as a serum marker of bone resorption (type I collagen
carboxytelopeptide) all demonstrated significant increases during bed
rest which declined toward normal during reambulation. Thus, under th
e conditions of this study, the human skeleton appears to respond to u
nloading by a rapid Land sustained increase in bone resorption and a m
ore subtle decrease in bone formation.