With decreasing renal function the kidney becomes less able to:
- degrade parathyroid hormone
- excrete phosphorus
- convert vitamin D to its active form.
Reduction in these activities results in bone demineralisation or osteodystrophy and mineral
deposition in soft tissues, including the kidneys themselves, which further reduces
function. This mineral deposition can occur at normal plasma calcium and phosphorus
concentrations in damaged renal tissue, and it can be an early morphological abnormality
in animals with renal insufficiency.
Dietary phosphorus restriction has been reported to reduce nephrocalcinosis and osteodystrophy
even in mild renal insufficiency before increased plasma phosphorus levels (Maschio
et al. 1980). Plasma phosphorus concentrations should be kept at 5.0 mg/dl (1.6 mmol/l)
or less, and plasma calcium levels at 10 mg/dl (2.5 mmol/l) or more, with a solubility
factor for the two of less than 55 (Massry 1980).
Phosphorus restriction significantly reduced the degree of tissue mineralisation
in nephrectomised cats when compared with cats fed a normal diet (Ross et al. 1982),
and high phosphorus intake in nephrectomised dogs resulted in hyperparathyroidism and severe
bone deminerialisation as compared with dogs with reduced phosphorus intake (Rutherford et al
Studies in cats and dogs have demonstrated that phosphate binders are ineffective
at the commonly used dosages if diets with unrestricted phosphorus content are fed.