Amino acid intake during lactation and amino acids of plasma and human milk

The aim of this study was to determine the free amino acid pool in plasma and milk in marginally nourished lactating women. Twenty-eight rural women (age, 23.9 +/- 5 y; weight 50.2 +/- 4.9 kg; height, 148.2 +/- 4.8 cm) were studied under metabolic balance conditions. Subjects were divided into 6 groups (5-6 women in each), representing rural mothers postweaning and in the 1(st), 3(rd), and 6(th) months of lactation; nonpregnant, nonlactating controls were from rural and urban areas. Amino acid analyses of diet and of plasma and milk samples were performed using a Beckman 6300 amino acid analyzer. Lysine intakes were lower than the recommended intake for lactating women (RDA). Plasma amino acid profiles differed between the lactating and weaned groups: aspartate and isoleucine increased at the 6(th) month (P < 0.05), while valine declined over weaning time (P < 0.05). In milk, valine and proline decreased at the 6(th) month (P < 0.05), while serine rose at the 3(rd) month. Free amino acid pools were 1- to 15-fold higher in plasma than in milk for branched-chain amino acids and basic, aromatic, and neutral amino acids. In mammary tissue these amino acids can be channeled to tissue and milk protein synthesis or to catabolic pathways. Glutamate was 40-fold higher in milk with respect to plasma content. This was the predominant amino acid in the free amino acid pool in milk. These results suggest selective amino acid transport in mammary tissue during lactation.