Protein and Free Amino Acid Composition of Preterm Human Milk: A Systematic Review and Meta-analysis

The composition of total protein and free amino acids (FAA) in preterm human milk (HM) is crucial for optimizing infant growth and development. The objective of this systematic review and meta-analysis is to quantify the amount of true, crude, and unspecified protein and FAA in preterm HM. EBSCO, PubMed, and Scopus databases were searched up to July 2023 measuring total protein and FAA in preterm HM. Two reviewers, working independently, screened all titles and abstracts using Covidence software to identify studies meeting inclusion criteria [preterm <37 wk; Human Development Index >0.8; cross-sectional, case-controlled (n > 1), prospective cohort, and randomized clinical trials; English language]. Random-effects models were used to estimate mean protein and FAA content across studies. Data were aggregated for studies reporting multiple estimates (e.g. across time). Heterogeneity was estimated using I² and publication bias using Kendall tau rank correlation coefficient. Of the 884 articles identified, a total of 66 original studies were included for the meta-analysis comprising an estimated 30,421 preterm HM samples. Preterm colostrum (<4 d) contained the highest mean (95% confidence interval) true protein at 2. 32 (1. 96, 2. 68) g/100 mL, followed by transition preterm HM (5-14 d) mean true protein of 1. 77 (1. 60, 1.93) g/100 mL. Mature (>14 d) preterm HM had the lowest mean true protein content at 1.46 (1.34, 1.59) g/100 mL. Glutamate was the most prevalent FAA reported. This systematic review provides updated estimates of protein and FAA concentrations in preterm HM. There was significant variability in the quality of studies, completeness of the reported results, and analytical methodologies across studies. This trial was registered at PROSPERO as CRD42023445191.