Determination of net protein utilization using whole carcass, hind leg or liver of the rat and its relationship with protein efficiency ratio determination.

Experiments were conducted to test the simplified NPU method designed by Lachance and Miller with diets of different protein quality and to determine the NPU using the liver nitrogen concentration instead of that in the hind limb. The NPU determination was done after 10- and 21-day periods instead of a 28-day period. The second part of the study was the analysis of the relationship between NPU and PER done with the same rats. The NPU calculated from whole carcass nitrogen concentration showed a high correlation with the NPU calculated from the nitrogen concentration in the leg (r = 0.990) and also with the NPU calculated from the liver nitrogen concentration. The Tukey's "t" test showed that in the 10-day experiment discrimination between the effect of the different diets upon NPU was not as good as was the case at 21 days. However, the NPU values at 10 days were in the same sequence as those at 21 days. For practical purposes it is more reliable and easier to perform the NPU determination using the liver instead of the whole carcass or leg. The statistical analysis of PER and NPU showed the same discriminating value when the test diets were isoenergetic.