(1) Background: Premature infants require mothers' milk fortification to meet nutrition needs, but breast milk composition may be variable, leading to the risk of inadequate nutrition. We aimed at determining the factors influencing mothers' milk macronutrients. (2) Methods: Milk samples were analyzed for the first five weeks after premature delivery by infrared spectroscopy. Mothers' nutritional intake data were obtained during standardized interviews with dieticians, and then analyzed with reference software. (3) Results: The composition of 367 milk samples from 81 mothers was (median (range) g/100 mL): carbohydrates 6.8 (4.4⁻7.3), lipids 3.4 (1.3⁻6.4), proteins 1.3 (0.1⁻3.1). There was a relationship between milk composition and mothers' carbohydrates intake only (r = 0.164; p < 0.01). Postnatal age was correlated with milk proteins (r = -0.505; p < 0.001) and carbohydrates (r = +0.202, p < 0.001). Multiple linear regression analyses showed (coefficient) a relationship between milk proteins r = 0.547 and postnatal age (-0.028), carbohydrate intake (+0.449), and the absence of maturation (-0.066); associations were also found among milk lipids r = 0.295, carbohydrate intake (+1.279), and smoking (-0.557). Finally, there was a relationship among the concentration of milk carbohydrates r = 0.266, postnatal age (+0.012), and smoking (-0.167). (4) Conclusions: The variability of mothers' milk composition is differentially associated for each macronutrient with maternal carbohydrate intake, antenatal steroids, smoking, and postnatal age. Improvement in milk composition could be achieved by the modification of these related factors.
Keywords: breast milk; maternal nutrition; milk composition; premature delivery.