Determination of net protein utilization, digestibility and biological value
The method protein efficiency and net protein ratios was based on the weight gain and feed consumption. For more precise quality of a protein, we need to know the extent of the utilization of protein in the animal system. The amount of protein absorbed and that retained in the body should be determined. Since protein after digestion and absorption gets incorporated into nitrogenous compounds – both proteinaceous and non-proteinaceous of the body – a measure of nitrogen intake, absorption and retention would be an ideal practical approach.
Based on nitrogen balanced studies, Net Protein Utilization (NPU), Digestibility (D) and Biological Value (BV) are calculated.
The nitrogen content of the test diet is determined. The feed consumption over a five-days-period is measured, leading to a calculation of a total N-intake. The faecal and urinary nitrogens on these five days are also determined. From these values calculation can be made, to know the ‘N retained’ and ‘N absorbed’. NPU, D and BV are then calculated as follows:
» Wistar male rats (weighing 65 to 68g)
» Metabolic cages
» Balances for weighing rats
» Plastic container with tight lids
» N-free mixture
Soyabean oil 5.2%
N-free starch 80.6%
Preparation of Diets
Five hundred g diet (dry weight basis) is prepared for each treatment. Initially, determine the protein and dry matter content in the test samples, then calculate the amount of sample required to give 7.5g N on dry weight basis. To this add 20g mineral mixture and 5g vitamin mixture in order to make up 500g diet (dry wt. basis) and rest of the amount from N-free diet after determining the moisture in N-free diet.
Procedure to Determine Metabolic and Endogenous Nitrogen
Feed a separate group with four per cent freeze-dried, ether-extracted egg protein as the protein source. Since egg protein at this level is completely utilized by rats, nitrogen in the urine and faeces must be of endogenous origin.
Collect the determined nitrogen in the urine and faeces as described, and incorporate in the calculation.
where I is the intake nitrogen (nitrogen in the diet), F = faecal nitrogen, Fk = endogenous faecal nitrogen, U = urinary nitrogen and Uk = endogenous urinary nitrogen.
1. The salt mixture and vitamin mixture compositions are given in the preceding experiments.
2. Casein fortified with one per cent DL-methionine is used as internal standard. This diet has an NPU of 88.
1. Pellet, P L and Young, V R (1980) In: Nutritional Evaluation of Protein Foods United Nations Univ Food Nutr Bull Supp 4.
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