Background:The purpose of the current study was to determine the degree to which milk-borne insulin-like growth factor-I (IGF-I) is absorbed.Methods:Cesarean-derived piglets were fitted with umbilical arterial and venous catheters within 2 h of birth and were administered formula containing 21.7 ± 1.8 μCi of iodinated recombinant human IGF-1 (125I-rhIGF-I) by orgogastric gavage. Blood samples were taken before administration of the125I-rhIGF-I (t0) and for 4 h postgavage. Plasma was obtained by centrifugation and total and trichloroacetic acid precipitable radioacitivity were determined. Immunoreactive125I-rhIGF-I was assessed using a polyclonal antibody to human IGF-I. Four hours after feeding, intestines were removed, divided into 13 segments, and flushed with saline. Radioactivity within the small intestinal lumen and wall were measured.Results:Radioactivity in portal blood was higher than t0 at all times points (p < 0.05), whereas arterial radioactivity did not differ from t0 until 30 min postgavage. On average 18-20% of total radioactivity in both portal and arterial blood was acid-precipitable, with the proportion decreasing over time (p < 0.001). Immunoprecipitable radioactivity averaged 3-5% of the total radioactivity and was higher in portal than arterial blood (p < 0.05). Based on a plasma volume of 0.062 ± 0.005 L and a baseline plasma IGF-I concentration of 1.81 ± 0.56 nmol/L, absorbed125I-rhIGF-I represented 0.205% of the total plasma IFG-I pool, whereas 14% of the dose was associated with the lining of the intestine.Conclusions:Absorption of orally administered IGF-I does not contribute significantly to circulating IGF-I.