Protein metabolism during lactation.

Lactation increases considerably the demand for amino acids over that required for basal needs. The demand imposed varies with the quantity of milk produced and, between species, with the protein content of milk. Using data on milk composition from Blaxter (1989) and assuming milk production of 60, 1000 and 50 000 ml/d for a rat, human subject and dairy cow respectively, the ideal protein required can be calculated as 5.7, 32 and 1465 g/d. (This calculation assumes the efficiency of use of the first limiting amino acid to be 0.85; Oldham, 1987). If this is expressed relative to the minimal requirement for amino acids, it gives a measure of the relative cost of lactation to the animal. Maintenance needs for amino acids can be considered to arise from obligatory losses occurring within the body (such as catabolism of amino acids, skin and hair loss) and those associated with protein loss to the gut. The latter varies with intake and, hence, dietary characteristics and has been estimated to account for 60 % of maintenance needs (Moughan, 1989). Thus, minimal requirements can be estimated as 40 96 of maintenance needs, calculated from Emmans & Oldham (1988). The relative amino acid cost of lactation for the rat, human subject and dairy cow, when producing the daily quantities of milk given previously are thus 11.0, 1-4 and 12.1 respectively. The cost for the rat and high-producing dairy cow are somewhat similar and substantially higher than that for human subjects.