Changes in nutrient and protein composition of cat milk during lactation

Objective - To evaluate changes in the nutrient and protein composition of cat milk during lactation. Animals - 12 lactating domestic shorthair cats. Procedure - 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. Results - 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. Conclusions - Nutrient composition of cat milk and, thus, provision of nutrients to nursing kittens changes over time.