Amino acid composition of human and animal milk, bovine colostrum, and infant formula: a standardized comparative analysis

Milk composition varies markedly among mammalian species, reflecting evolutionary adaptations to the nutritional requirements of their offspring. While total protein content largely determines quantitative amino acid (AA) supply, qualitative AA profiles are shaped by the balance between caseins and whey proteins and by their individual fractions. To provide a systematic comparison, we analyzed the chemical composition and detailed AA profiles of 105 samples, including human, donkey, cattle, buffalo, goat, sheep, and dromedary camel milk, as well as bovine colostrum and infant formulas (0-6 and 6-12 mo). Amino acids were quantified by RP-HPLC after acid or alkaline hydrolysis and expressed both as mg/100 mL and as percentages of total true protein (TP). Results revealed substantial interspecific and product-related variability. Monogastric milks (human, donkey) and infant formulas had lower protein and AA contents but higher lactose than ruminant milks; their qualitative profiles were enriched in cysteine and tryptophan. Infant formulas matched human milk in quantitative AA supply but diverged in relative composition, with higher lysine and methionine and lower cysteine and tryptophan. Donkey milk resembled human milk in protein content but differed in several essential and conditionally essential AAs, particularly methionine and arginine. Camel milk was broadly similar to ruminant milks, with modest differences in methionine, isoleucine, and arginine. Within Bovidæ, buffalo and sheep milk were more concentrated than cow and goat milk, respectively, with consistently higher AA contents; qualitative differences were most pronounced for methionine and tyrosine. Bovine colostrum was distinguished by exceptionally high protein, whey protein, and AA levels, and by a qualitative profile richer in most essential and conditionally essential AAs. Overall, these results provide a comprehensive reference for interspecies comparisons of milk AA composition and highlight the nutritional implications of using different milks, colostrum, and formulas in human diets.