Inclusion Criteria:  Preterm infants born <1000 g of birth weight that are fed with breast milk or donated milk. Written informed consent signed by the mother, father or legal guardian. To tolerate enteral feeding, at least 100mL / kg / day.  Exclusion Criteria:  Non-premature or premature patients weighing ≥ 1000 gr. Patients with major malformations. Patients with diagnosed chromosomal diseases or of high diagnostic suspicion. Patients with short bowel syndrome or any surgery on the gastrointestinal tract.