Population-based reference values for IGF-I and IGF-binding protein-1: relations with metabolic and anthropometric variables

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Abstract
Population-based reference values for IGF-I and IGF-binding protein-1 (IGFBP-1) have been established. One hundred and one women and the same number of men, 20-70 years old, were randomly selected from the population registry in the community of Linköping. Participation rate was 67%. Venous blood was drawn in the fasting state. Serum IGF-I was measured by RIA after acid-ethanol extraction and IGFBP-1 was determined by ELISA. IGF-I levels did not differ between genders and the decline with age was similar in men and women (men: Y = 366-3.28 x age (years), r = -0.61, P < 0.0001; women: Y = 386-3.49 x age, r = -0.57, P < 0.0001, P = 0.4 for difference in slope). There were negative correlations between IGF-I and plasma lipids and blood pressure in both genders, but none was independent of age. Serum angiotensin-converting enzyme activity correlated positively with IGF-I in men independently from age (r = 0.21, P = 0.01). The distribution of IGFBP-1 was positively skewed and it was higher in women than in men (5.9 +/- 4.8 micrograms/l and 4.0 +/- 3.3 micrograms/l respectively; Mann-Whitney, P = 0.002). In men and in the women not taking oestrogen, IGFBP-1 correlated positively with age (Spearman rank correlation (Spearman: men: r = 0.32, P = 0.002; women: r = 0.24, P = 0.03). C-peptide correlated negatively (Spearman: men: r = -0.38, P = 0.002; women: r = -0.49, P < 0.000) and sex hormone binding globulin positively with IGFBP-1 (Spearman: men: r = 0.50, P < 0.0001; women: r = 0.55, P < 0.0001). IGF-I declined with age while IGFBP-1, which is considered to modulate the free bioactive fraction of IGF-I, increased. This suggests that IGF-I activity might be even lower in elderly subjects than is accounted for by the low total IGF-I.

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