The composition of human milk is the biologic norm for infant nutrition. Human milk also
contains many hundreds to thousands of distinct bioactive molecules that protect against infection
and inflammation and contribute to immune maturation, organ development, and healthy
microbial colonization. Some of these molecules, e.g., lactoferrin, are being investigated as novel
therapeutic agents. A dynamic, bioactive fluid, human milk changes in composition from
colostrum to late lactation, and varies within feeds, diurnally, and between mothers. Feeding
infants with expressed human milk is increasing. Pasteurized donor milk is now commonly
provided to high risk infants and most mothers in the U.S. express and freeze their milk at some
point in lactation for future infant feedings. Many milk proteins are degraded by heat treatment
and freeze-thaw cycles may not have the same bioactivity after undergoing these treatments. This
article provides an overview of the composition of human milk, sources of its variation, and its
clinical relevance.