Effect of dietary protein and phosphorus levels on calcium and phosphorus metabolism of the young, fast growing rat.

Utilizing a 2 X 2 factorial design, male weanling rats were fed diets containing either 25% or 45% casein with 0.35% or 0.8% phosphorus (P). Dietary calcium (Ca) was set at 0.9% for each diet. During the 7 weeks of this experiment, food intakes and body weights were measured weekly; urine and fecal samples were collected daily. Regardless of dietary P level, increasing dietary protein resulted in significantly decreased fecal Ca output accompanied by an increase in the Ca content of urine. Increasing dietary P resulted in significantly increased fecal Ca content, decreased absolute and percent Ca absorption, and decreased urinary Ca output. Increased fecal and urinary P excretion were obtained when dietary P levels were increased. Increasing dietary protein at the 0.35% P level slightly increased the values for absolute and percent Ca and P absorption. However, when 0.8% P was fed, increasing the protein level of the diet decreased these Ca (weeks 1-5) and P absorption values. Ca and P balances were positive at all times.