We have previously demonstrated that IGF-binding protein-1 (IGFBP-1) levels rise steadily during fasting, following an inverse relationship with insulin. The function of the IGFBP-1 rise is unknown, but it has been hypothesized that IGFBP-1 serves as a glucose counterregulatory hormone during fasting and hypoglycemia by binding free IGFs, thus inhibiting IGF interactions with IGF receptors. Our objective in this study was to determine levels of free and total IGFs during fasting together with their interrelationships with simultaneous IGFBP-1, insulin, and glucose levels. Our patient population consisted of 22 children, aged 6 months to 15 yr, who underwent diagnostic fasting studies in the General Clinical Research Center. Blood was sampled at baseline and at 6-h intervals for glucose, IGFBP-1, free and total IGF-I, and insulin. The fasting studies lasted 14-40 h and were terminated at a glucose concentration of less than 50 mg/dl (n = 11) or for the completion of the allotted fasting duration (n = 11). Of the children studied, 11 had ketotic hypoglycemia, 8 had no disorder, 2 had steroid-induced adrenal suppression, and 1 had recovered transient hyperinsulinism. During fasting, IGFBP-1 levels rose above mean initial levels of 27.1 +/- 13.4 ng/ml to a mean of 318.4 +/- 29.9 ng/ml at the end point (P < 0.001). Insulin levels declined from a mean initial level of 7.4 +/- 1.3 mU/ml to a mean level of 1.4 +/- 0.4 mU/ml at the end point (P < 0.001). Concomitantly, free and total IGF-I levels declined from initial levels of 0.48 +/- 0.08 and 180.3+/- 27 ng/ml, respectively, to mean levels of 0.10 +/- 0.02 ng/ml (P < 0.001) and 119.3 +/- 22 ng/ml (P = 0.001), respectively, at the end point. Levels of free IGF-I were inversely associated with IGFBP-1 over the course of fasting (P = 0.002). Similarly, total IGF-I was negatively associated with IGFBP-1 (P = 0.01). We conclude that free and total IGF-I levels decline steadily over the course of fasting. This decline in free IGF-I appears to be the result of the steady rise in IGFBP-1 that occurs as insulin declines. We speculate that the decline in IGF levels, controlled by the rise in IGFBP-1, serves to protect against possible insulin-like activity of the IGFs during fasting.