Milk is obviously a highly nutritious food for a growing infant. Mammalian milk differs in its composition slightly between species, but is generally composed of both whey and casein protein, sugar in the form of lactose, and fatty acids the composition of which are dependent on the fats in the diet of the mother. The protein component of milk is important because it provided a positive nitrogen balance and is therefore pivotal to growth. However, as well as protein, milk also contain free amino acids that have quite different functions. The free amino acid content of mammalian milk varies between species, and for example is higher in human milk than cow’s milk. The exact purpose of the free amino acid content of milk is not fully understood but the presence of amino acids, their quantities and supposition as to their function has been published in the nutritional literature1. Clues as to the importance of differing ratios of free amino acids in human milk are suggested by the significant changes to the profiles seen during the first five weeks of lactation.
Glutamic acid is the most abundant free amino acid in human milk. Glutamic acid occupies a pivotal role in amino acid metabolism and in particular glutamic acid is required as a precursor to alpha ketoglutarate. The central role of alpha ketoglutarate as an intermediate in the citric acid cycle, suggests that glutamic acid plays a role in energy metabolism in the growing infant. The importance of glutamic acid in this regard may relate to the under developed nature of the digestive system in the infant, with its incomplete compliment of digestive enzymes. In addition glutamic acid may be an important energy substrate for the growing intestinal tract along with free glutamine. A role for glutamic acid as a chelating agent to allow the efficient absorption of zinc in the growing infants has been postulated. In this role, glutamic acid may increase the absorption of zinc significantly. Other free amino acid chelating agents in human milk include histidine, methionine, tryptophan, cysteine and glycine.
The second most abundant free amino acid in human milk is taurine. Taurine is unusual amongst amino acid in that it is non-proteinogenic and as such only fulfills a role in human nutrition as a free amino acid. The most well known role for taurine is probably as a conjugating agent in bile acids. Bile acids are required in humans to allow fat absorption, and are also important in regulating plasma cholesterol levels. In infants it has been supposed that the conjugation of bile acids by taurine produces an acidic environment in the gut and this increases the growth of the beneficial bifidus bacteria. Formula feeding tends to reduce the amount of taurine incorporated into bile acids and increase bile acid conjugation to glycine. This results in less favourable fat absorption due to the poorer ability of glycine conjugated bile acids to form efficient micelles during emulsification of fats. In addition to its role in conjugating bile acids, taurine may also play an important role in the central nervous system and may modulate growth.
Taurine is the most abundant non-proteinogenic amino acids in human milk, but other free amino acids in human milk are also present. In particular, phosphoethanolamine and phosphoserine are also known to be present and have biological functions. These two phospho-amino acids make up roughly half of the phospholipids found in human milk but their exact roles are not known. Phosphoethanolamine may help modulate growth in the infant, but beyond this little is known about its functions. Urea is also present in human milk and contributes significantly to the nonprotein-nitrogen containing component of milk (along with taurine and the phospho-amino acids). As well as urea, other nonprotein-nitrogen containing component of human milk may include amino sugars present in chemicals such as oligosaccharides. These are likely used as a source of energy during bacterial fermentation and in this regard may play an important role in colonisation of the sterile infant gut to a more diverse bacterial environment.
Dr Robert Barrington’s Nutritional Advice: Studies consistently show that human milk is superior to infant formula not only with regard to infant growth but also in terms of long term disease prevention. This should be the preferred route of feeding.
RdB
