This work reports on the muscle loss and recovery after 17 wk of continuous bed rest and 8 wk of reambulation in eight normal male volunteers. Muscle changes were assessed by urinary levels of 3-methylhistidine (3-MeH), nitrogen balance, dual-photon absorptiometry (DPA), magnetic resonance imaging (MRI), and isokinetic muscle performance. The total body lean tissue loss during bed rest calculated from nitrogen balance was 3.9 +/- 2.1 (SD) kg (P < 0.05). Although the total loss is minimal, DPA scans showed that nearly all of the lean tissue loss occurred in the lower limbs. Similarly, MRI muscle volume measurements showed greater percent loss in the limbs relative to the back muscles. MRI, DPA, and nitrogen balance suggest that muscle atrophy continued throughout bed rest with rapid recovery after reambulation. Isokinetic muscle strength decreased significantly (P < 0.05) in the thigh and calf with no loss in the arms and with rapid recovery during reambulation. We conclude that there is great variability in the degree and location of muscle loss in bed rest and that the lower limb muscles are primarily affected.

The purpose of this work was to determine the rate and extent of bone loss and recovery from long-term disuse and in particular from disuse after exposure to weightlessness. For this purpose, bed rest is used to simulate the reduced stress and strain on the skeleton. This study reports on the bone loss and recovery after 17 weeks of continuous bed rest and 6 months of reambulation in six normal male volunteers. Bone regions measured were the lumbar spine, hip, tibia, forearm, calcaneus, total body, and segmental regions from the total-body scan. The total body, lumbar spine, femoral neck, trochanter, tibia, and calcaneus demonstrated significant loss, p less than 0.05. Expressed as the percentage change from baseline, these were 1.4, 3.9, 3.6, 4.6, 2.2, and 10.4, respectively. Although several areas showed positive slopes during reambulation, only the calcaneus was significant (p less than 0.05), with nearly 100% recovery. Segmental analysis of the total-body scans showed significant loss (p less than 0.05) in the lumbar spine, total spine, pelvis, trunk, and legs. During reambulation, the majority of the regions demonstrated positive slopes, although only the pelvis and trunk were significant (p less than 0.05). Potential redistribution of bone mineral was observed: during bed rest the bone mineral increased in the skull of all subjects. The change in total BMD and calcium from calcium balance were significantly (p less than 0.05) correlated, R = 0.88.

No abstract

To evaluate the role of glucagon in the pathogenesis of diabetic ketoacidosis in man, we studied the effect of suppression of glucagon secretion by somatostatin on changes in plasma beta-hydroxybutyrate and glucose concentrations (as well as changes in their precursors) after acute withdrawal of insulin from seven patients with juvenile-type diabetes. Suppression of glucagon secretion prevented the development of ketoacidosis for 18 hours after acute insulin withdrawal, whereas in control studies mild ketoacidosis occurred 10 hours after insulin was stopped. Plasma beta-hydroxybutyrate, glucose, free fatty acid, and glycerol levels were all markedly lower during suppression of glucagon secretion (p smaller than 0.001), whereas plasma alanine levels were higher (p smaller than 0.001). These studies indicate that insulin lack per se does not lead to fulminant diabetic ketoacidosis in man and that glucagon, by means of its gluconeogenic, ketogenic, and lipolytic actions, is a prerequisite to the development of this condition.

The abnormal physiology that manifests itself in healthy humans during their adaptation to the microgravity of space has all the features of accelerated aging. The mechano-skeletal and vestibulo-neuromuscular stimuli which are below threshold in space, result in an overall greater than 10-fold more rapid onset and time course of muscle and bone atrophy in space and the development of balance and coordination problems on return to Earth than occur with aging. Similarly, the loss of functional capacity of the cardiovascular system that results in space and continuous bed rest is over 10 times faster than in the course of aging. Deconditioning in space from gravity deprivation has brought attention to the medical hazards of deconditioning on Earth from gravity withdrawal as in sedentary aging. Though seemingly reversible after periods of 6 months in space or its ground analog of bed rest, it remains to be seen whether that will be so after longer exposures. Both adaptation to space and aging do not merely parallel but converge as disorders of mechanotransduction. Like spaceflight, its analog bed rest telescopes the changes observed with aging and serves as a useful clinical model for the study of age-related deconditioning. The convergence of the disciplines of aging, along with gravitational and space physiology is advancing the understanding and prevention of modern lifestyle medical disorders.

Nine male volunteers participated in a 10 week metabolic study in which subjects underwent 5 weeks of ambulatory control and 5 weeks of complete horizontal bed rest. Bed rest is a model commonly used to simulate space flight. The changes in muscle area and strength of the calf dorsiflexors and plantar flexors were measured before and after bed rest using magnetic resonance imaging (MRI) and a Cybex II dynamometer. The muscle area of the plantar flexors (gastrocnemius and soleus) decreased 12%, whereas the muscle area of the dorsiflexors was not significantly decreased. The maximal muscle strength of the plantar flexors decreased 26%; the muscle strength of the dorsiflexors was not significantly decreased. These results, which demonstrate differential muscle atrophy and a larger loss in strength relative to muscle area, have important implications in the development of exercise counter-measures to be implemented during space flight. The results also have implications for patients who have severe orthopaedic disorders and must be bed rested for long periods of time, and for persons who are voluntarily inactive (a large number of the elderly).

No abstract

A group of 68 premenopausal women participated in a controlled 12 month exercise program. Two groups were matched according to age, body size (body mass index), and typical activity level. Data collection included bone mineral density (BMD) of the lumbar spine with dual-photon absorptiometry and of the os calcis with single-photon absorptiometry, lean body mass, urinary calcium/creatinine, and urinary gamma-carboxyglutamic acid (Gla). Subjects were given a daily 500 mg supplement of elemental calcium. There was no significant difference between groups in terms of diet, in urinary calcium/creatinine or Gla, or in lean body mass. The weight lifting group had a nonsignificant increase in mean lumbar BMD of 0.81% and the control group exhibited a nonsignificant decrease of 0.5%. However, a paired t-test revealed a significant change in the means in either group or as matched pairs. The relatively small change seen as a result of this modified Nautilus exercise program may prevent moderate weight lifting from being a practical answer for osteoporosis, even in a highly motivated population.