Free amino acids in human milk: a potential role in early life growth and immunity

Breastfeeding is the gold standard for infant nutrition. It promotes healthy growth, supports immune system development and provides the infant with components that could help in the protection against infections and allergies. Given the relevance of early life growth and immunity for health throughout life, there is great interest in identifying and understanding the components in human milk that could contribute to these physiological effects of breastfeeding. One group of components in human milk that could be relevant in this context, but have been only scarcely studied, are free amino acids (FAAs). In this thesis, we demonstrated that concentrations of FAAs in human milk change as the infant ages, indicating a need for an adequate FAA intake in early life. For certain FAAs, we found positive correlations between their concentrations in human milk and infant growth in the first months of life. Moreover, dietary supplementation with the FAA glutamine, which is abundant in human milk, and substitution of dietary protein for a specific mixture of FAAs was found to inhibit the development of food-allergic symptoms in mice. These findings support the hypothesis that FAAs could contribute to the unique growth- and immune-related benefits associated with breastfeeding, which calls for further research into the functions of FAAs in early life development. Understanding these functions is important considering our observation that FAA concentrations in human milk may be influenced by maternal characteristics, such as body mass index and allergy status. Moreover, this could provide opportunities for optimizing human milk substitutes, which generally contain lower concentrations of FAAs than human milk.