Dynamic Profiles of Total Protein, Amino Acids, and Representative Bioactive Proteins in Human Milk across Lactation Stages: A Systematic Review and Meta-Analysis

Proteins and amino acids are indispensable for infant growth and development; however, a systematic and quantitative synthesis of their global concentrations remains limited. This study conducted a systematic review, integrating data from 71 studies and 113,829 human milk samples. Meta-regression and subgroup analyses were used to quantify variations attributable to the lactation stage. Pooled mean concentrations were determined for total protein, total amino acids, and representative bioactive proteins, including lactoferrin and osteopontin. All components declined over lactation except osteopontin, which uniquely increased. Notably, the relative proportions of indispensable amino acids remained highly stable. Overall, these data could establish a consolidated, stage-resolved benchmark for the protein and amino acid composition of human milk.