Lactose, a disaccharide that includes the monosaccharides glucose and galactose, is the main carbohydrate found exclusively in mammalian milk. Lactase is found in the intestinal mucosa and is located at the ends of the villi, while it is a factor of maximum clinical importance in milk tolerance and in the occurrence of diarrheal disease. Developmental lactase deficiency is defined as the relative lactase deficiency observed in premature newborns less than 34 weeks of gestation. In the immature gastrointestinal tract, lactase and other disaccharidas are deficient until at least 34 weeks of gestation. One study in premature babies reported a benefit from the use of lactase-supplemented or lactose-reduced formulas, while the use of lactose-containing formulas and human milk did not appear to have short- or long-term harmful effects on infants and infants. Up to 20% of dietary lactose can reach the colon in newborns and young infants. Due to the inadequate functional development of lactase, premature infants may not digest and absorb the main source of carbohydrate energy, lactose. The high osmotic load associated with indigestible lactose is one of the many possible causes of diarrhea and food intolerance in premature babies. As a result of diarrhea stools and small bowel damage, the already low functional activity of lactase will be further reduced affecting weight gain, while it takes up to 2 weeks for lactase activity to be restored. Carlson et al showed that the addition of lactase to premature formula reduces the amount of lactose by 70%, with a negligible effect on osmolarity. Previous studies have found that premature infants fed a reduced lactose formula had a better weight gain rate than those who took 100% lactose formula. In two of these studies, weight gain improved despite eating fewer calories. In the present study we intend to test whether the use of lactase to hydrolysis lactose in premature milk would result in better weight gain and improved dietary tolerance by taking the same calorie intake. This is a prospective, double-blind, randomized study to evaluate tolerance and weight gain in premature infants who received either (1) human milk or premature formula enriched with Delictase ® drops (lactase group) or (2) unfortified human milk or premature formula (control group). The study will be carried out at the Second Neonatal Clinic and Neonatal Intensive Care Unit of the Aristotle University of Thessaloniki, in premature newborns (gestational age \[GA\] 28-34 weeks) who will be hospitalized in the Neonatal Intensive Care Unit. The study will enroll newborns (1) with a gestational duration of 28 to 34 weeks, (2) who receive ≥75% of their energy needs intestinally, (3) with the absence of severe congenital malformations or gastrointestinal diseases, including necrotizing enterocolitis (NEC) (4) without taking postnatal steroids or diuretics. Small, suitable and large for newborns of gestational age will be eligible for the study. Exclusion criteria will be neonates whose guardians refuse to participate in the study, neonates with congenital malformations or gastrointestinal diseases, and neonates receiving postnatal steroids or diuretics. The participation of newborns in the study will take place after written consent of the parents after information.