Note on the determination of net protein utilization by carcass analysis.

In theory the measurement of net protein utilization (N.P.u.) refers to the particular protein under test. In practice there is a small amount of other nitrogen (including that from the vitamins) in the basal diet so that the actual measurement evaluates a mixture of test protein and this other nitrogen. In the diets that we have been using with rats (Miller & Bender, 1955; Bender, 1956) the test diet was made up to contain 1.69% N ; the non-protein diet contained 0-05-0.1 yo N. In the subsequent discussion the following symbols are used: I,, total dietary N, comprising Ip, N from test protein and IK , other N ; F, faecal N ; M , metabolic N ; U, urinary N ; E, endogenous urinary N ; B, B, and BK, initial carcass N, carcass N of protein-fed animals and of non-protein group respectively. Our original calculation (Bender & Miller, 1953) evaluated the total nitrogen of the diet, from the equation (1) B,BK iI ,