The Relation between Absorbed Nitrogen, Nitrogen Balance and Biological Value of Proteins in Adult Dogs

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Abstract
The relationship between nitrogen balance and absorbed nitrogen in normal adult dogs is linear in the region of negative nitrogen balance, the linearity often extending over onto the positive side, but becoming obviously curvilinear well on the positive side of nitrogen balance. The equation describing the linear portion is, NB = k (AN) - NEo where NB is nitrogen balance, AN absorbed nitrogen, NEo the excretion of nitrogen on a protein free diet and k the slope of the line. If the excretion of nitrogen of endogenous origin is constant, k is the fraction of nitrogen retained in the body of an animal. This fraction is the customary definition for the biological value of a protein. When the caloric intake was reduced below optimum, excretion of nitrogen on a protein free diet (NEo) and protein minima for nitrogen equilibrium increased, but the biological value (k) was not altered. Thus reduced caloric intake increased the utilization of body nitrogen but did not alter the utilization of dietary nitrogen. The value for k was determined for a variety of protein sources in the adult dog as follows: casein = 0.82, chicken entrails = 0.77, flounder entrails = 0.77, flounder heads = 0.52, and a protein derived from soybean = 0.39. The relationship between urine nitrogen (UN) and absorbed nitrogen (AN) was shown to be, UN = (1 - BV) AN + UNo where BV is biological value, and UNo the excretion of urine nitrogen on a protein free diet.

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