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1. Three factors were investigated which influence blood urea content, namely the protein content in the diet, the time after feeding and the protein quality of the diet.
2. To investigate the relationship between the protein content in the diet and the blood urea content, seventeen experiments with rats given increasing amounts of protein in the diet were carried out. The experimental results show that there is a positive correlation (r = 0.95) between the protein content in the diet and the blood urea content.
3. For investigation of the influence of time after feeding on the urea content in the blood, a pig was used having a catheter in the portal vein for blood sampling at intervals. The results from this experiment showed that the blood urea content increases for the first 3–4 h after feeding and thereafter reaches a plateau.
4. To use blood urea measurement as a technique for assessing protein quality it is necessary to work under standardized conditions, especially in regard to the two factors just discussed.
5. Forty-two feeding-stuffs of widely differing quality were used in nitrogen balance trials with rats. The results showed that there is an inverse relation between the blood urea content and the biological value of the diet which is sufficiently accurate (coefficient of variation = 53%) to provide ausefulmethod for the predictionof protein quality from measurement of urea levels.
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