Effect of extended lactation and diet on transferrin concentrations in rat milk.

Milk transferrin in the rat is immunochemically identical to serum transferrin. Its concentration in milk during normal lactation (10 pups for 21 d) varies biphasically, decreasing from a value of 1.5 mg/ml in colostrum to barely detectable values at d 4 and 8 of lactation, and thereafter increasing to reach values of 4 mg/ml at d 21. The effect of extended lactation on transferrin in milk was investigated in two experiments in which litters were replaced by 4-d-old litters at d 8 and 12 of lactation or at d 20 of lactation. Transferrin concentrations in milk in both experiments increased in a similar manner to reach values of 10 mg/ml at d 28 through d 36 of lactation. Serum transferrin and serum insulin and prolactin concentrations were not significantly altered in these experiments. Premature exposure of dams to older pups did not affect the pattern of milk transferrin concentrations. Milk transferrin concentrations were, however, modulated by altering the milk demand (changing litter sizes) and by restricting either the total food intake or the protein content of the diet. These restrictions led to lower transferrin concentrations.