A small portion of circulating insulin-like growth factor I (IGF-I) is detected in the free or readily dissociable state, which is thought to be the metabolically active form. The amount of free/dissociable IGF-I in serum is dependent on a complex interplay between the production rate and the concentrations of IGF-I and IGF-binding proteins (IGFBPs). IGF availability is also influenced by posttranslational changes in IGFBPs that affect the affinity of IGFBPs for IGF-I. In the present study, we examined whether a short term fast (similar to 12 h) alters the serum concentration of free/dissociable IGF-I, and whether these changes are associated with alterations in IGFBP-1 and the proteolysis status of IGFBP-3. Circulating free/dissociable IGF-I concentrations, as assessed by a two-site immunoradiometric assay, did not differ between fasting and 4 h after a morning meal (1.48 +/- 0.07 vs. 1.50 +/- 0.07 mu g/L, respectively). Likewise, free/dissociable IGF-I levels measured by RIA after separation by centrifugal ultrafiltration were not different between the two groups (1.43 +/- 0.14 vs. 1.38 +/- 0.18 mu g/L, respectively). IGF-I bioactivity, as measured by thymidine incorporation by fibroblasts, did not differ in fasting and 4-h postprandial sera. There was no difference In IGFBP-3 and total acid-ethanol-extractable IGF-I concentrations in serum from fasted and fed subjects. In contrast, the concentration of IGFBP-1 in the serum was increased similar to 5-fold in the fasted state compared to fed values. IGFBP-1 existed in a highly phosphorylated form under fasting conditions. There was no change in IGFBP-3 proteolysis assessed either in vivo or in vitro between the fasting and fed states. The results indicate that a physiologically relevant short term overnight fast does not alter the circulating levels of free/dissociable IGF-I despite a marked elevation in IGFBP-1.