CSF-1 is a hematopoietic growth factor, which regulates the survival, proliferation and differentiation of mononuclear phagocytes. Like many other growth factors, CSF-1 is present in high concentrations in human breast milk(BM). In order to determine whether intact biologically-active CSF-1 is absorbed from BM via the GI tract, this study sought to measure CSF-1 levels in BM during the course of lactation and to compare the levels of circulating CSF-1 in babies who are breastfed (BF) with those who are formula-fed (FF). CSF-1 concentrations (mean±SEM; ng/mL) were measured by RIA at birth(cord), on days(D) of life 1-2, 3-4 and 10-25 in serum and at the latter 3 points in BM. CSF-1 concentrations were markedly elevated in BM compared to serum and seemed to decline with time (D1-2: 166.1±30.2[n=13]; D3-4: 123.5±44.0 [n=13];D10-25:66.6±20.5 [n=16,p=0.07 vs. D1-2;ANOVA & Tukey]). Results in serum were: Table Although circulating CSF-1 concentrations nearly doubled over the first days of life, serum levels did not differ between BF and FF babies and there was no correlation between BM and serum CSF-1 concentrations in individual babies. We conclude that milk-borne CSF-1 is not absorbed from the developing GI tract and speculate that it probably exerts its effects locally in that organ.

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