Milk Composition

Human milk is rich in carbohydrates but relatively poor in amino acids and fat compared to non-primate milk. The amino acid and lipid composition of human milk is similar to that of the great apes. A recent quantitative analysis of milk oligosaccharides and their ratio to lactose, the most common glycan in milk, has revealed that human milk is even richer in oligosaccharides than other great ape milks, with the possible exception of orangutans for which only colostrum has been analyzed. Human milk differs from all other mammalian milks by the predominance of type I oligosaccharides (containing galactose beta1-3 N-Acetylglucosamine) rather than type II oligosaccharides  (galactose beta 1-4 N-Acetylglucosamine). All hominid milks are rich in sialic acids, bound to proteins (such as kappa casein, the principal glycoprotein of milk and gangliosides found in the milk membrane vesicles) but human milk completely lacks the non-human sialic acid N-glycolylneuraminic acid (Neu5Gc) found in the milk of all the apes. The content of sialylated oligosaccharides varies over time and between individuals. Human milk contains more than 130 different oligosaccharides, consisting of a lactose  core with various elongations by glucose, galactose, N-Acetlygalactosamine, fucose, and sialic acid. The free milk oligosaccharides of humans appear to be longer than in gorilla or chimpanzee. Many of these milk oligosaccharides may act as prebiotics, i.e. they are not metabolized in the small intestine and reach the distal digestive tract where become substrate supporting the infant gut flora. They may also act as efficient "pathogen blocks" by blocking pathogenic microbes from attaching and invading the infant gut. Human infants fed baby formula (based on bovine milk) have lower sialic acid content in their brain and different gut flora. It has been suggested that human milk may also represent "brain food" providing critical molecules for the rapidly growing infant brain. With regards to amino acid composition of human milk, there is evidence for species-specific relative abundance from comparisons between mammals including several primates. In depth comparative analyses of milk composition between humans and great apes are in their infancy.