Effect of protein intake on protein and nitrogen composition of breast milk

The effect of a low and a high protein diet (approximately 8 and 20% energy from protein, respectively) on the contents of different nitrogen-containing substances in breast milk was studied on three healthy Swedish mothers in full lactation. Each experimental diet was fed during a 4-day period and milk samples were collected during the last day. The milk volume was estimated by weighing the child before and after each feeding. The 24-hr outputs as well as concentrations of total nitrogen, true protein, and nonprotein nitrogen were significantly higher when the subjects consumed the high protein rather than the low protein diet. The higher content of nonprotein nitrogen was due to increased urea levels as well as to increased levels of free amino acids. Milk urea levels were closely correlated with plasma urea levels in samples obtained after overnight fasting when the subjects had consumed the experimental diets for 4 days. The 24-hr output of lactoferrin, alpha-lactalbumin, and serum albumin were higher when the subject consumed the high protein diet as compared to the low protein diet but the differences were not significant.