Abstract: Aim: To comparatively study the protein content and amino acid composition of breast milk at different lactation stages (0–12 months) from northern (Beijing) and southern (Shenzhen) urban Chinese mothers. Methods: Totally 30 lactating mothers of full-term infants were enrolled in Beijing and Shenzhen separately, and breast milk samples were collected between day 3 and 5 (colostrum), between day 13 and 15 (transitional milk) , and at 3 and 4 weeks and 2, 3, 4, 6, 9 and 12 months (mature milk) following delivery for the determination of proteins and amino acids. Results: The highest protein content was detected in colostrum among the various milk samples from mothers in both cities and it gradually decreased with increasing lactation length, during the first 3 weeks rapidly and then more slowly but a significant difference being still observed (P < 0.05), reaching a stable level at 3 months with no significant difference being found between the two cities for each lactation stage (P > 0.05). Similarly, amino acid contents significantly decreased with increasing lactation length (P < 0.05); the contents of total amino acids, essential amino acids and non-essential amino acids decreased by approximately 58%. Glutamic acid, leucine and asparaginic acid were the most abundant amino acids in breast milk, whereas methionine and tryptophan were the least abundant. Amino acid composition in breast milk differed between the cities for all lactation stages except colostrum and mature milk at 4 months. The proportion of each amino acid to the total amount remained stable with the prolongation of lactation, and essential and nonessential amino acids accounted for about 42% and 58% of the total amino acids, respectively. However, for all lactation times except colostrum, the proportion of phenylalanine to total amino acids in Beijing breast milk was higher than in Shenzhen breast milk. Conclusion: Human colostrum has the highest contents of proteins and amino acids among all lactation times, which decline with increasing lactation time. The contents of amino acids and the proportions to total amino acids but not the protein content of breast milk vary between cities.

Key words: breast milk, protein, amino acid, longitudinal study