Composition and biology of human milk.

Human milk is a complex tissue that is uniquely transferred from one individual to another and changes over time. Beyond energy, macronutrients, minerals, and vitamins, human milk contains numerous cell populations and key compounds that communicate to the offspring and protect them from a potentially harmful environment. In recognition of this evolving complexity, this review will describe the compositional changes of human milk across lactation stages, beginning with colostrum, identify nutritional gaps that may exist for some term and most preterm infants provided an exclusive human milk diet, and highlight key differences between term, preterm, and donor human milk that are relevant for infant caregivers.