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Objective—To evaluate changes in protein and nutrient composition of milk throughout lactation in dogs.

Sample Population—Milk samples collected from 10 lactating Beagles.

Procedure—Milk samples were collected on days 1, 3, 7, 14, 21, 28, 35, and 42 after parturition and analyzed to determine concentrations of nitrogen, nonprotein nitrogen, casein, whey proteins, amino acids, lipids, lactose, citrate, minerals, and trace elements. Optimum conditions for separating casein from whey proteins and distribution of milk proteins throughout lactation were assessed by use of polyacrylamide gel electrophoresis.

Results—Protein concentration was high in samples collected on day 1 (143 g/L), decreased through day 21 (68.4 g/L), and increased thereafter. Concentration of nonprotein nitrogen did not change throughout lactation (5.7 to 9.9% of total nitrogen content). Casein-towhey ratio was approximately 70:30 and remained constant throughout lactation. Lactose concentration increased from 16.6 g/L on day 1 to 34.0 to 40.2 g/L on days 7 to 42. Lipid concentration ranged from 112.5 to 137.2 g/L. Citrate concentration increased from day 1 (4.8 mM) to day 7 (6.6 mM), then gradually decreased until day 42 (3.9 mM). Iron, zinc, copper, and magnesium concentrations decreased during lactation, whereas calcium and phosphorus concentrations increased. Calcium-to-phosphorus ratio remained constant throughout lactation (approx 1.6:1). Energy content of milk ranged from 1,444 to 1,831 kcal/L.

Conclusions and Clinical Relevance—Protein and nutrient composition of milk changes throughout lactation in dogs. These data can provide valuable information for use in establishing nutrient requirements of puppies during the suckling period. (Am J Vet Res 2001;62:1266–1272)

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in American Journal of Veterinary Research



To evaluate changes in the nutrient and protein composition of cat milk during lactation.


12 lactating domestic shorthair cats.


Milk samples collected on days 1, 3, 7, 14, 28, and 42 after parturition were analyzed for concentrations of nitrogen, nonprotein nitrogen, casein, whey proteins, amino acids, total lipids, lactose, citrate, minerals, and trace elements. Individual milk proteins (caseins and whey proteins) were analyzed by use of polyacrylamide gradient gel electrophoresis.


True protein concentration ranged from 6.3 to 8.6% and was as high in mature milk as in colostrum. Nonprotein nitrogen as a portion of total N was constant (approx 8%), as was the whey-to-casein ratio (approx 50:50). Total lipid concentration was high (9.3%) in colostrum, rapidly decreased, then increased to 9% in mature milk. Lactose concentration was constant at 4%. Milk calcium, iron, and copper concentrations increased markedly during lactation, and magnesium and zinc values remained constant. Colostrum and early milk had a low Ca-to-P ratio of 0.4:0.9. Although calcium concentration increased with time, phosphate concentration also increased so that the Ca-to-P ratio remained constant in mature milk at 1.0: 1.2. The major whey proteins had molecular weights of approximately 14,000, 19,000, 40,000 and 80,000. The 80,000 protein (possibly lactoferrin) decreased in concentration during lactation. Two major casein subunits of approximately 28,000 and 33,000 were found, and both increased during early lactation.


Nutrient composition of cat milk and, thus, provision of nutrients to nursing kittens changes over time. (Am J Vet Res 1997;58:370-375)

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in American Journal of Veterinary Research