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Reduced secretion of GH and production of insulin-like growth factor I (IGF-I) contribute to altered body composition in human aging. IGF-binding proteins (IGFBPs) are important modulators of IGF action, yet little is known regarding their role and regulation in aging. Accordingly, we measured levels of IGFBP-1, an important short term modulator of IGF bioavailability that is suppressed by insulin, and levels of IGFBP-3, the major circulating IGF carrier protein, and examined their relationships to insulin, glucose, IGF, and dehydroepiandrosterone sulfate levels and anthropometric measures in old (63-89 yr) and young (23-39 yr) men. Serum levels of IGFBP-1 were increased 3-fold in old vs. young men despite high insulin levels in elders. Nevertheless, IGFBP-1 and insulin levels correlated in old and young men (r = - 0.49; P < 0.002 and r = -0.42; P < 0.025), suggesting that insulin continues to play an important role in the regulation of IGFBP-1 in aging. Glucose levels also were significantly inversely related to IGFBP-1 in old and young men (r = 0.37; P = 0.02 and r = -0.49; P < 0.01), and this relationship was not accounted for by the effect of insulin. IGF-I levels were reduced by 33% in elders (P < 0.001) and correlated with IGFBP-1 levels among old (r = -0.40; P < 0.01), but not young, men, indicating that low GH secretion and/or IGF-I production may contribute to the elevation of IGFBP-1 levels in aging. IGFBP-3 levels were reduced among elders, but not to the same extent as IGF-I, resulting in a relative excess of IGFBP-3 in elders (IGFBP-3/IGF-I ratio, 20.1 +/- 0.9 vs. 15.4 +/- 1.0; P < 0.001). The IGFBP-3/IGF-I ratio correlated with IGF-I levels in young and old men (r = -0.79; P < 0.001 and r = -0.82; P < 0.001), indicating that diminished GH secretion also may contribute to a relative excess of IGFBP-3 among elders. Dehydroepiandrosterone sulfate levels were low in elders, but did not correlate with IGF, IGFBP, insulin, or glucose levels in either age group. Serum levels of IGFBP-1 (but not IGF-I or -II or IGFBP-3) correlated with body mass index and upper arm fat and muscle areas in elders. These relationships were accounted for by the effects of insulin, suggesting that regulation of IGFBP-1 by insulin may play a role in determining body composition in aging. We conclude that insulin remains an important determinant of IGFBP-1 levels in elders, that the fasting glucose level is also a significant determinant of IGFBP-1 in both old and young subjects, and that reduced secretion of GH may contribute to impaired anabolism in aging through multiple mechanisms, including reduced production of IGF-I and alterations in circulating levels of both IGFBP-1 and -3. These findings are consistent with the concept that alterations in IGFBP levels may contribute to changes in IGF bioavailability and body composition in aging.
Benbassat et al. (Thu,) studied this question.