Human milk (HM) is the optimal food source for the nutrition, growth, and development of newborns. The protein fraction of HM plays a crucial role in the healthy development of infants. HM contains a wide variety of minor whey proteins and peptides with important bioactive functions, many of which are still unknown. Proteomics allows for the study of biological samples with inherently complex protein mixtures. Proteins are essential for the development of living organisms, both in quantitative and qualitative terms. The combination of proteomic techniques currently enables the study of protein variability and minor peptides in HM across different lactation stages and allows for differential quantification according to gestational age and birth weight. However, studies on the human milk serum proteome during these stages are limited. The aim is to explore the minor whey proteins and peptides in human milk through a longitudinal analysis of five groups of breastfeeding mothers (with 30 extremely low birth weight newborns, 30 very low birth weight newborns, 30 low birth weight newborns, 30 adequate birth weight newborns, and 30 high birth weight newborns). Gestational age will also be considered to ensure homogeneous group distribution according to this condition. HM samples will be collected from each mother during three lactation periods after birth: within the first 48 hours (colostrum), at 5-14 days (transitional milk), and at 100-120 days (mature milk) for the five birth weight groups. In these neonatal/infant groups, minor proteins from whey fraction and peptides will be separated, quantified, and identified using label-free proteomic techniques. This study aims to expand our understanding of the minor proteins and peptides in human milk and their bioactive roles in neonatal health. By examining these components across different birth weight groups and lactation stages, the research will offer insights into how protein and peptide profiles vary by gestational age and birth weight, potentially influencing neonatal development. The findings from this proteomic analysis could not only demonstrate the complexity of human milk composition but also contribute to targeted nutritional support for preterm or low-birth-weight infants, customizing protein supplementation in HM banks and therefore enhancing their growth and developmental outcomes.

Inclusion Criteria:

• Healthy mothers
• With monitored pregnancies within the care area of the Reina Sofía University Hospital in Córdoba
• With newborns expected to be exclusively breastfed until 4 months

Exclusion Criteria:

• Mothers whose newborns have any of the following conditions: congenital malformation, chromosomal abnormality, hypoxia-ischemia, gastroschisis, polycythemia, hypoglycemia, sepsis, blood incompatibility
• Pathological pregnancy, pregnancy by in vitro fertilization, or multiple pregnancies
• With no plan to exclusively breastfeed until 4 months
• Under medical treatment
• Have a drug addiction
• Refuse informed consent
• Have had previous breast surgery
• Live outside the metropolitan area

Five groups of breastfeeding mothers: Group 1 (n=30) - with neonates of extremely low birth weight; Group 2 (n=30) - with neonates of very low birth weight; Group 3 (n=30) - with neonates of low birth weight; Group 4 (n=30) - with neonates of adequate birth weight; Group 5 (n=30) - with neonates of high birth weight. Additionally, gestational age will be considered to achieve a homogeneous distribution of the groups according to this condition.

The datasets generated and analyzed in this study are not publicly available due to data regulations and ethical considerations. Participants consented to the use of their data exclusively by the original team of investigators, ensuring their privacy and adherence to consent agreements.