Maternal dietary protein and amino acid intake is not associated with the amino acid composition of human milk in an affluent environment

Amino acids (AA) are essential nutrients in human milk (HM) and critical for infant growth and development. Several maternal lifestyle factors have been suggested to influence HM AA composition, with possible consequences for the breastfed infant. Whether maternal dietary protein and AA intake is associated with AA concentrations in HM is still largely unknown. Therefore, the aim of this study was to investigate the association between maternal dietary AA intake and AA concentrations in HM over the first month postpartum. Data from the observational longitudinal Amsterdam Mother's Milk study were used, consisting of 123 lactating women in their first postpartum month. HM samples were collected three times, on day 10, 17 and 24 postpartum. Maternal dietary protein and AA intake on these collection days was assessed using three 24-h recalls. HM protein-bound and free AA (BAA and FAA, respectively) were analysed by liquid chromatography. Associations between maternal AA intake and AA concentrations in HM were assessed using linear mixed models. Maternal intake was negatively associated with milk concentrations of free arginine (-0.0003; P = 0.01) and free lysine (-0.0004; P = 0.03) and was positively associated with free glutamine (0.002; P = 0.03) and free threonine (0.0008; P = 0.03). However, these associations were attenuated after correction for multiple testing. Both the quality and quantity of dietary protein intake in lactating women do not seem to influence the amino composition of their breast milk when living in an affluent environment.