Objective: Muscle receptors and selected anabolic effects have been identified for both insulin-like growth factor I (IGF-I) and 1,25-dihydroxyvitamin D (1,25(OH)2D3). The aim of the present study was to test the hypothesis that the decreasing concentrations of these endocrine factors might be involved in the decline in muscle function that characterizes normal human ageing.
Design: Cross-sectional study.
Study participants: A community-based sample of 245 healthy elderly women aged 70-90 years. Exclusion criteria were diseases or medications known to affect muscle function or the somatotrophic axis.
Measurements: Knee extension strength was evaluated using an isokinetic dynamometer. A standardized questionnaire was used to assess habitual physical activity. IGF-I and 1,25(OH)2D3 were measured by radioimmunoassay. Vitamin D binding protein (DBP) was measured by single radial immunodiffusion and the free 1,25(OH)2D3 concentration calculated as the molar ratio of total 1,25(OH)2D3 to DBP.
Results: The differences in isometric and isokinetic strength over the age range were equivalent to losses of 0.9-2.4% per year. However, no relationship was found between the somatotrophic axis or vitamin D status and knee extension strength, despite markedly decreasing concentrations of circulating IGF-I and free 1,25(OH)2D3 with age.
Conclusion: Levels of circulating IGF-I and free 1,25(OH)2D3 appear not to be involved in the loss of muscle function that characterizes normal human ageing.