Abstract
An experiment was carried out to determine the effect of dietary protein quality and quantity on urinary calcium (Ca), phosphorus (P) and magnesium (Mg) excretion in 5-month-old multiparous rats. As protein sources casein and/or soy protein isolate were used, to give protein contents in the diet of 20% and 40%, respectively. Female Fischer-strain rats aged 5 months were divided into groups given a 20% casein diet (C20), a 40% casein diet (C40), a 20% soy protein isolate diet (S20) or a 40% soy protein isolate diet (S40) for 16 weeks. At 6, 11 and 16 weeks, studies of Ca, P and Mg balance were done. After the balance studies at 16 weeks, the rats were killed and the Ca, P and Mg contents of the serum, kidney and right femur were determined. Parathyroid hormone (PTH) levels in sera were also measured. The results were as follows: 1) The 40% casein diet group always exhibited higher urinary Ca and P excretion than the other groups, but did not show a negative Ca and P balance. Urinary Ca excretion in the C40 group did not decrease with aging and maintained the highest value. 2) Calcium retention in the C40 group was lower than in the S40 group, indicating a negative effect of C40 on Ca metabolism. 3) Urinary Mg excretion and Mg retention showed no significant differences among the four groups. 4) The weights of raw femur and fat-free femur, the ash, Ca, P and Mg contents of the femur, and the Ca/P ratio per gram of femur showed no significantdifferences among the four groups. 5) Kidney weight in the 40% protein diet group was significantly higher than in the 20% protein diet group. The calcium content per gram of kidney in the C40 group was significantly lower than in the other groups. 6) Serum PTH content in the 40% protein diet group was higher than in the 20% protein diet group. These results indicate that excess dietary protein may have an adverse effect on Ca metabolism.
