A nine-country study of the protein content and amino acid composition of mature human milk
Sammanfattning
Background: Numerous studies have evaluated protein and amino acid levels in human milk. However, research
in this area has been limited by small sample sizes and study populations with little ethnic or racial diversity.
Objective: Evaluate the protein and amino acid composition of mature (]30 days) human milk samples collected
from a large, multinational study using highly standardized methods for sample collection, storage, and analysis.
Design: Using a single, centralized laboratory, human milk samples from 220 women (30188 days
postpartum) from nine countries were analyzed for amino acid composition using Waters AccQ-Tag highperformance
liquid chromatography and total nitrogen content using the LECO FP-528 nitrogen analyzer.
Total protein was calculated as total nitrogen6.25. True protein, which includes protein, free amino acids,
and peptides, was calculated from the total amino acids.
Results: Mean total protein from individual countries (standard deviation [SD]) ranged from 1,133 (125.5) to
1,366 (341.4) mg/dL; the mean across all countries (SD) was 1,192 (200.9) mg/dL. Total protein, true protein,
and amino acid composition were not significantly different across countries except Chile, which had higher
total and true protein. Amino acid profiles (percent of total amino acids) did not differ across countries. Total
and true protein concentrations and 16 of 18 amino acid concentrations declined with the stage of lactation.
Conclusions: Total protein, true protein, and individual amino acid concentrations in human milk steadily
decline from 30 to 151 days of lactation, and are significantly higher in the second month of lactation
compared with the following 4 months. There is a high level of consistency in the protein content and amino
acid composition of human milk across geographic locations. The size and diversity of the study population
and highly standardized procedures for the collection, storage, and analysis of human milk support the
validity and broad application of these findings.