Protein evaluation in growing rats of breast milk and breast milk substitutes with special reference to the content of non-protein nitrogen.

The levels of individual proteins and other nitrogen containing substances differ considerably between cow's milk and human milk. Therefore, during manufacture of infant formulas, attempts are made to simulate the protein composition of human milk. However, the composition and nutritional characteristics of human milk protein are incompletely known. In this paper, the protein quality of breast milk protein with and without the non-protein-nitrogen (NPN) substances present in human milk was studied with growing rats and compared to two formulas, one "adapted" commercial infant formula and a suggested further modified, possibly improved, infant formula. Detailed examinations of protein and amino acid composition of the test diets are given. Breast milk protein with added NPN substances showed a lower protein quality than all other test proteins. Breast milk protein without NPN substances and the protein of the suggested infant formula were of similar quality while the protein of the commercial adapted formula was significantly better than all other test proteins. The use of rat growth assays in the evaluation of protein quality of infant formulas is discussed.