[The effect of long-term increased protein administration on mineral metabolism and kidney function in the rat. I. Renal and enteral excretion of calcium, magnesium, phosphorus, sulfate and acid]

Abstract

The influence of continuous imbalanced high protein intake on the metabolism of minerals (calcium, magnesium, phosphorus) and renal function was the subject of a long-term experiment with rats. In the first part of the study particular attention was directed to the contribution of protein-induced endogenous acid production and renal excretion of hydrogen ions and sulphate to the development of hypercalciuria. For 61 weeks 200 male Wistar rats in eight groups were fed isocaloric diets, whose protein contents were increased from 13 to 26 and 40 J% at the expense of carbohydrate intake. The fat content of the diets was 40 J%. In two groups with 13 and 26 J% protein the effect of different kinds of animal protein was also studied, replacing casein by beef. Mineral contents were kept constant in these diets. To examine the excretion mechanisms of calcium and phosphorus especially under conditions of excessive protein intake, the ratio of calcium to phosphorus was varied in three diets with 40 J% protein by increasing both minerals alternatively or together from 0.6 to 1.2%. An increase in dietary protein content from 13 to 26 or 40 J% produced a sustained hypercalciuria and also hypermagnesiuria over a period of more than 400 days (after 58 weeks: 3.3, 5.9, and 6.8 mg calcium/day; 2.2, 3.3, and 3.4 mg magnesium/day; p less than or equal to 0.05). No adaptation to high protein intake occurred. Hypermagnesiuria, which equally hasn't been described before as a result of high protein intake, was accompanied by a reduced fecal excretion of magnesium. With increased protein intake (casein and beef) hypercalciuria and also hypermagnesiuria were positively correlated with an increased formation and renal excretion of hydrogen ions and sulphate, which resulted from protein catabolism. The dietary protein source influenced the extent of hypercalciuria, irrespective of a constant phosphorus intake. Although leading to equal increases in renal total acid and sulphate excretion, beef as the main protein source caused a lower calciuria than casein. High phosphorus intake caused the highest total acid excretion of all groups, but resulted in a reduced hypercalciuria and hypermagnesiuria and counteracted the influence of an increased protein intake.