A comparison of biological methods with rats for determining the nutritive value of proteins

Abstract

The protein efficiency ratio (PER) was determined on rats, at an 8% level of protein intake, for twenty-three protein sources for which values for net protein utilization (NPU), determined from both balance data and carcass analysis, and net protein ratio (NPR) have already been reported. PER values were highly correlated with those for NPU (balance), NPU (carcass) and NPR with r=0.95, 0.87 and 0.88 respectively. The biological value (BV), true digestibility (TD), PER, NPU (carcass) and NPR of whole-egg proteins, casein and wheat gluten were determined at 4-16% (BV and TD) or 4-20% (PER, NPU (carcass) and NPR) levels of protein intake. Responses were similar for BV and NUP (carcass). Increasing the protein level from 4 to 8% had no effect for whole-egg proteins and case in but caused a sharp drop for wheat gluten; there was a continuous drop for all three proteins with further increases in protein concentration. TD was not affected by level of intake for the 3 proteins. PER increased between 4 and 8% and then fell continuously to the 20% level for egg proteins and casein; for wheat gluten there was a continuous slight rise from the 8 to 20% level of intake; no value was obtained at the 4% level. NPR dropped steadily from the 4 to 20% level for all 3 proteins. NUP, determined both from balance data and by carcass analysis, of casein, soya-bean meal, cottonseed meal and wheat gluten was reduced by a daily injection of 2.5 mg cortisone acetate. The reduction in NUP (balance) decreased with protein quality from 38% for casein to 6% for wheat gluten; the reduction in NPU (carcass) was independent of protein quality and amounted to 35-50%. N retention from a "N-free" diet and from diets containing 8% protein as casein or wheat gluten was determined from balance data or from analysis of the carcasses for N, determined directly or calculated from body-water values. NUP values for casein and wheat gluten were calculated from the 3 sets of data. Compared with analysis of the carcasses for N, the balance-sheet method overestimtaed both the N lost with the "N-free" diet and the N retained with casein and wheat gluten. The discrepancies were accentuated when N in the body was calculated from its water content. For both proteins, NPU (carcass) was independent of the method of determining body N; the values were lower than for NPU (balance.