Objective: Formula-fed premature babies have a higher incidence of developing necrotizing 
enterocolitis (NEC) than breast-fed babies which may be caused by breast milk induced gut 
maturation. The effect of breast milk on maturation of the gut has been widely studied in animal 
models and recently in humans. The aim of this study: is to evaluate the effects of breast-feeding on 
maturation of the intestine in premature babies by measuring the postnatal values of a specific 
enterocyte marker which is urinary intestinal fatty acid binding protein (I-FABP). Methods: 
Maturation of the gut was studied in 60 premature babies (<37 weeks of gestation) without 
gastrointestinal morbidity. 30 of them were exclusively breast-fed and the other 30 were formula-fed.
Urinary I-FABP levels as the measure of gut maturation were measured at 7th, 12th, and 22nd postnatal days. Results: In breast-fed babies, there was a statistically significant increase in urinary IFABP levels between 7th
 and 12th
 days after birth compared with formula-fed babies (p< 0.01). 
Conclusions: The pattern of postnatal changes in urinary I-FABP levels suggests a delayed 
physiological response causing significantly delayed gut maturation in formula-fed babies compared 
with breast-fed ones.