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Comparison of the Essential Amino Acid, Nitrogen and Calorie Requirements of the Weanling Rat and Breast-fed Infant

Research Division, Wyeth Laboratories, Radnor, Pennsylvania 19087

Calculations from literature data indicate that if growth is measured by the increase in body weight (g/kg per day), the weanling rat grows 6.7 times faster than the breast-fed infant during the first month of life. The caloric efficiency of weight gain (g gain/kcal intake) of the weanling rat is 1.64 times greater than that of the breast-fed infant. When fed near-optimum diets, the rat retains about 53% of the essential amino acids (EAA) and 65% of the nitrogen (N) supplied in the diet. Indirect evidence from estimates of the protein concentration in infants based on body water determinations and nitrogen balance data indicates that similar retentions occur in the breast-fed infant. Assuming that the requirements of the infant are the same as the amounts supplied by breast milk, the average requirements for EAA (except for sulfur-containing amino acids) and for total N, per gram of weight gain, are similar in the two species. The differences in requirements for EAA and N per 100 kcal are proportional to the differences in the caloric efficiency of weight gain in the two species.