By Chip Coombs, D.V.M.
Presented with the permission of the author

Phosphorus is one of the body main macrominerals and common dietary sources are meat, fish and poultry. It is difficult to discuss the nutritional role of phosphorus without involving calcium, as it is difficult to discuss the nutritional role of phosphorus without involving calcium, as both minerals are intimately related.

Phosphorus and calcium also have an important co-relationship in the diet. Feeding pets a diet with an improper Ca:P ratio, either through an inappropriate diet or an inappropriate dietary supplement, will lead to imbalances resulting in a variety of medical problems, especially skeletal disease.

In dogs and cats affected with chronic kidney disease, diet plays an important role in managing the effects of functional loss of kidney tissue. In addition to managing the quality and quantity of protein, sodium, potassium and water soluble vitamins, phosphorus levels also play an important role in treating kidney disease.

In human subjects, diets high in phosphorus have been shown to increase kidney damage through the depositing of calcium and phosphorus within the kidney tissue. Although the effects of high phosphorus diets on the kidney function of healthy dogs or cats are unknown, there is evidence that excess dietary phosphorus is a factor in the progression of kidney disease once kidney failure has begun. As kidney failure progresses, there is a decrease in the ability of the kidneys to filter impurities from the bloodstream.

The resulting phosphorus retention and high blood phosphorus levels cause higher calcium blood levels which leads to further depositing of calcium and phosphorus in the kidney tissues; and the degenerating cycle continues.

In early stages of kidney disease, dietary manipulations of phosphorus levels may be sufficient to normalize blood phosphorus levels and to prevent further kidney damage. Since protein is a major source of phosphorus, lowered protein levels in therapeutic kidney diets help to achieve this effect.

In more advanced kidney disease, however, dietary restriction of protein alone is usually not adequate to impact significantly on blood phosphorus levels. Hence, veterinarians may also prescribe drugs that help bind phosphorus in the intestinal tract to prevent its absorption into the bloodstream.

Whereas clinical features of kidney failure do not manifest themselves until 75% of kidney function has been lost, calcium deposition within the kidneys occurs when only 50% of kidney function has been lost. In other words, kidney damage is occurring well before any damage can readily be detected by blood testing or clinical signs.

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