The effects of severe protein restriction following parturition on the changes in rat milk composition during lactation were investigated using multiparous female Sprague-Dawley rats caged individually following mating and offered a high-protein diet (H; 215 g crude protein (N × 6·25; CP)/kg dry matter (DM)) ad lib. until parturition. Following parturition, half the females continued to receive diet H, whilst the remainder were offered a diet low in protein (L; 90 g CP/kg DM) ad lib. On days 2, 4, 8 and 12 of lactation groups of females from both dietary treatments were used to provide a milk sample. Milk samples were analysed for their lactose (enzymically), protein (binding to Coomassie blue), lipid (gravimetrically) and mineral (spectrophotometrically) contents. The milk lactose concentration of group H increased with stage of lactation (r2 0·85, P < 0·001). Such an increase was prevented by diet L, and from day 8 of lactation the milk lactose of group L was lower (P < 0·05) than in group H. Group H milk protein concentration did not change during lactation and averaged 90·7 mg/g. Dietary protein restriction reduced the milk protein concentration of group L so that on days 2, 4 and 12 of lactation it was lower (P < 0·05) than that of group H. On day 8 of lactation the milk protein concentration of group L had increased (P < 0·05) and was comparable with that of group H. For group H, milk lipid averaged 166·8 mg/g and was generally unchanged during lactation. Diet L increased (P < 0·01) the milk lipid concentration (205·5 mg/g) compared with diet H and this was also significant on days 4 and 8 of lactation (P < 0·05). Group L milk lipid concentration also increased between days 4 and 8 of lactation (P < 0·05). Milk Na concentration declined during lactation in both dietary groups (P < 0·01) but was unaffected by dietary treatment. Both milk Ca and P concentrations increased (P < 0·01) during lactation in both dietary groups, whilst protein restriction also increased the Ca and P concentrations (P < 0·05). Milk K and Mg concentrations were unaffected by dietary treatment or stage of lactation. This significant alteration in the milk composition of severely protein-restricted dams, while possibly favouring the disposal of greater quantities of energy-yielding nutrients, suggests that equations developed for the estimation of milk production in rats cannot be used under such conditions. Laction: Milk composition: Feed intake: Dietary protien: Rat
Adipocyte differentiation-related protein is secreted into milk as a constituent of milk lipid globule membrane
