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Context: Many aspects of hormonal regulation and mechanisms of normal infancy growth are poorly understood., Objective: The objective of this study was to establish the determinants of serum growth factor levels in infancy and their association with growth., Design: A prospective, longitudinal, population-based birth cohort between 1997-2001 was studied., Participants: Study participants were 942 healthy appropriate weight for gestational age (AGA) infants (538 boys and 404 girls) and 49 small for gestational age (SGA) children (29 boys and 20 girls)., Interventions: INTERVENTIONS were anthropometrical measurements (0, 3, 18, and 36 months) and serum samples (3 months)., Main Outcome Measures: Height, weight, and serum IGF-I and IGF-binding protein-3 (IGFBP-3) were the main outcome measures., Results: IGF-I levels showed no gender difference [boys, 92 ng/ml (confidence interval, 49, 162); girls, 91 ng/ml (47, 149); P = 0.50]. IGFBP-3 levels were significantly higher in females [2174 ng/ml (1295, 3330)] than in males [2103 ng/ml (1266, 3143); P = 0.04]. Infants receiving breast milk had lower IGF-I levels [90 ng/ml (48, 154)] than infants receiving formula [n = 62; 97 ng/ml (58, 165)] or both [n = 123; 94 ng/ml (48, 169); P < 0.001]. IGF-I and IGFBP-3 levels were positively associated with weight gain and height gain from birth to 3 months of age in AGA, but not in SGA, children. SGA children had significantly lower IGF-I [88.0 ng/ml (28, 145); P = 0.05] and IGFBP-3 [1835 ng/ml (1180, 2793); P < 0.001] levels than AGA children., Conclusion: We found a significant, but weak, association between IGF-I and IGFBP-3 levels at 3 months and postnatal growth in AGA, but not SGA, children. Factors other than IGF-I must contribute to the regulation of normal postnatal growth, and these may differ between AGA and SGA children. IGFBP-3, but not IGF-I, showed a gender difference, which may reflect an influence of the postnatal activation of the pituitary-gonadal axis on binding protein levels.

A high protein intake results in increased kidney growth and glomerular filtration rate in human adults and young rats. It is unknown whether kidney size in young infants is influenced by increased protein intake in formula-fed compared with breast-fed infants. We investigated the effect of formula versus breast feeding on kidney growth in a cohort of 631 healthy children examined at birth, and at 3 and 18 months of age. Kidney size was determined by ultrasonography and related to gender, age, body size, and feeding category (fully breast fed, partially breast fed, or fully formula fed at 3 months). Serum urea nitrogen, serum creatinine, and estimated creatinine clearance were measured at 3 months of age. Kidney growth and serum urea nitrogen were significantly increased in partially or fully formula-fed 3-month-old infants. This effect was more pronounced in boys than in girls. The changes in relative kidney size were temporary, as they did not persist at 18 months of age, when all children received a normal mixed diet. The immediate renal effects of formula feeding should be taken into consideration for recommendations concerning infant feeding. Whether there are any long-term effects of early increased protein intake on later kidney function remains to be seen.