There is an emphasis on low phosphorus cat food with some charts giving you phosphorous level in various commercial cat diets. One of the reasons why there is a need for this mineral to be low in dry, wet (canned), meats, homemade and other foods is kidney disease.
However, to properly understand the correlation between phosphorous and renal disease, it is good to briefly look at its regulation and the roles of kidneys in its regulation.
Phosphorus in cats
Phosphorus (P) is the second most abundant mineral in a cat’s body after calcium. As already seen while looking at functions and sources of phosphorus in cats, it has many vital roles.
Being reactive, it is found in the form of phosphate with about 86% is found in bones. The remainder is found in intracellular and extracellular compartments including in blood plasma.
In blood plasma, about a third of the phosphate is found as ionized (inorganic forms of HPO42- or H2PO4- ) form while the remainder is in the form of phosphate esters (organophosphates) and phospholipids.
Note that ionized phosphates level will be higher in serum than blood plasma since, during clotting, platelets release some.
Phosphate regulation
Various animals including cats must maintain a closely guarded ionized calcium and phosphate homeostasis and this involves bone, kidney and intestinal handling of these ions. A decrease in one lead to an eventual decrease in the other and vice versa.
Usually, the phosphate level is affected by its absorption, dietary intake, calcitriol, and parathyroid hormone (PTH) roles. Also, the phosphatonins such as the fibroblast growth factor 23 as well as klotho, its cofactor, may separately or together be involved.
The secretion of PTH causes calcium and phosphate resorption from bones, stimulate calcitriol production as well as reduces its renal reabsorption, i.e., promotes its dumping.
On the other hand, calcitriol increases phosphate absorption in the intestine, resorption from bones and again dumping in the kidney.
Therefore, any disease or condition that interferes with bones, kidney or intestinal phosphate handling is likely to lead to a load or deficiency (hyperphosphataemia and hyperphosphatemia in cats respectively).
Similarly, whatever affects calcitriol, PTH or fibroblast growth factor 23 or its cofactor klotho will affect phosphate levels in a cat’s body.
Note
While serum calcium level is directly detected by the parathyroid glands and a reduction in level triggers the stimulation of PTH, changes in phosphate blood level indirectly affects the secretion of PTH since being excess, it will bind with calcium, lowering the ionized calcium level. As calcium level increase, there will be an eventual decrease in inorganic phosphate.
Therefore, the PTH is the primary determinant of the calcium and phosphorus homeostasis while calcitriol plays an inhibitory role on PTH secretion.
Role of kidney and kidney disease
In the kidney, nephrons, to be specific, the glomerulus filters ionized calcium and phosphate, other nutrients, before the needed ones are reabsorption by transporters and channels in the tubular section where any excesses and toxins are excreted.
This helps in maintaining the right level of various nutrients and substances in blood plasma and the process is regulated by PTH and calcitriol or 1α,25-dihydroxyvitamin D.
Secondly, the kidney plays a role in synthesizing calcitriol or 1α,25-dihydroxyvitamin, the bioactive form of vitamin D.
On the other hand, kidney disease is a condition that alters the normal functioning of the kidney and it can be acute or chronic.
Acute kidney disease (AKD) refers to a case where the normal function is lost rapidly while in the case of chronic kidney disease (CKD), normal kidney function loss occurs gradually over time.
Therefore, in the case of kidney disease, it means that proper phosphorous regulation cannot occur normally and there is a need to give your pets diets low in this mineral coupled with other treatments.
Note that retention of phosphate often results in mineralization of soft tissues inside the kidney and in other organs.
Also, there may be an accumulation of other toxins in the bloodstream if the kidney cannot filter them out.
Why Low phosphorus foods for cats with renal disease?
Being obligate carnivores, a cat’s protein requirement is higher when compared to dogs so to supply this pet with the various amino acids which they cannot produce.
Let us look at where you can get nutritional data before looking at why phosphorus is supposed to be low.
Where to get nutritional data?
Whereas most proteins are considered to be high phosphorus, this is not always the case since various high-quality protein sources have different levels. The Cat Food – Nutritional Composition data by Lisa A. Pierson, DVM will help give this mineral’s content in various commercial foods.
Also, the United States Department of Agriculture database will allow you to search for any food and know its nutritional data.
Why low phosphate cat foods?
First, research by veterinarians from Ludwig-Maximilians-Universität München published in the Journal of Feline Medicine and Surgery has linked high phosphorous foods to kidney damage even in healthy animals if the amount exceeds daily requirements by more than five times and such levels may contribute to “the incidence of chronic kidney diseases in elderly cats.”
Secondly, impaired kidney function means that this organ cannot effectively perform its excretory role and high amounts of phosphate will burden it and worsen CKD.
However, reduced intake may improve CKD as it reduces the work kidneys must do to get rid of the excess amounts of phosphate.
In fact, according to clinical veterinarians at Tufts University, dietary adjustments can double the lifespan of a cat with CKD.
Furthermore, a study published in Journal of Veterinary Emergency and Critical Care, on “the role of phosphorus in the pathophysiology of chronic kidney disease” notes that “plasma phosphorus concentration to within the International Renal Interest Society targets for CKD patients improves survival time and reduces clinical manifestations of hyperphosphatemia” and secondary renal hyperparathyroidism.
However, whereas you may want to reduce the amount of this mineral, ensure the specific food you go for meet your feline’s minimum calorific and nutritional needs.
Finally, go for natural foods whose phosphate is in bound form as opposed to inorganic phosphates such as water-soluble potassium monophosphate normally added to help increase their shelf life of various commercial feline foods. This is because the latter is more readily available for absorption when compared to those in bound form.
Which foods to consider?
There are many low phosphorus food manufacturers including those that make wet, dry, canned or meats. However, ensure you known how to correctly interpret various contents or let your vet help you in case you cannot interpret them well.
The AAFCO (2014) recommends a minimum of 2000mg of phosphorus for lactating queens and growth while the amount should be 1250mg for adult (maintenance). Keep levels phosphorous 0.3 to 0.6% of dry mass
Try egg whites
Egg whites are a good source of high-quality lean proteins source with a lower protein to phosphorous ratio. Avoid avoiding real bones, rawhides, cereal grains, jerky treats as they have a higher phosphorus ratio.
Gradual change
When changing to low phosphate diets, do it gradually to give your feline friend time to adjust. Start with small amounts since some might not accept these new diets.
Proper rehydration and wet foods
Due to the increased risk of dehydration in felines with CKD, opt for fresh or wet foods or moisten them.
Also, provide an unlimited amount of freshwater since this pet tends to be thirsty as they are unable to conserve water by concentrating urine (their urine is dilute).
Go for renal care diets
Try commercial foods for cats with kidney disease such as
- Hill’s Prescription Diet k/d Kidney Care,
- Royal Canin Renal Support E Canned – Canned
- Purina Pro Plan Veterinary Diets NF Kidney Function Early Care Formula – Canned
- HI-TOR Veterinary Select Neo Diet – Canned
- Darwin’s Intelligent Design™ KS Veterinary Formula
According to VCA Hospitals, these diets are designed to ensure “less protein, sodium and phosphorus and increased omega-3 fatty acids.” Protein breakdown releases a lot of toxins which the kidney must remove from the body.
They are also supplemented with have B vitamins since as cats urinate, they lose a lot of these water-soluble vitamins), potassium (often low in case of CKD), and antioxidants.
In naturally occurring cases of CKD, these diets can help in modifying both parathyroid hormone and plasma phosphate concentration.[1]
These coupled with other ways of managing renal disease in cats will be helpful in ensuring your cat lives a long and happy life.