The Effects of Old Age and Immobility on Protein Turnover in Human Subjects with Some Observations on the Possible Role of Hormones

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Abstract
Protein turnover (PT), synthesis (PS) and breakdown (PB) were studied using L-(l-14C)-leucine and primed continuous infusion of 14C-HCO3 over a 6-h study period in three groups: group 1 (n = 9) fit old; group 2 (n = 6) fit young; and group 3 (n = 13) immobile old. Plasma cortisol, growth hormone (GH) and insulin levels were measured during the studies. There were significant reductions in PT (p < 0.02), PB (p < 0.02) and PS (p < 0.01) in fit old compared to fit young subjects. Comparison between fit old versus immobile old subjects revealed that the immobile group (Group 3) had higher PT (p< 0.05), PB (p < 0.05) and PS (p < 0.01) rates. The rates of protein turnover and components in group 3 were similar to those in group 2. There was greater variance in protein balance in the immobile subjects. Significantly higher plasma cortisols were found in the immobile than in the healthy old subjects. The immobile group had higher GH levels in response to feeding but there was no difference in insulin response to feeding. These higher plasma cortisol and GH levels may be partly responsible for the changes in PT found in immobile elderly patients.