Phosphorus - Nutrition

What is Phosphorus?

After calcium, phosphorus is the second most prevalent mineral in mammals, including dogs and cats. Like calcium, it is found in the highest proportion in bone (about 86%) but is also found in other tissues such as muscle (about 8.6%). Like calcium it does not exist in its free state but in the form of phosphates.

Why is it Important?

Phosphorus has many key functions, principally as a component of compounds essential to normal metabolism. These include: phospholipids and phosphoproteins that are important in membrane formation; the nucleic acids DNA and RNA; other nucleotides such as ATP that are vital components of energy metabolism; enzyme cofactors. Phosphorus also has a role in regulating acid-base balance.

Roles in the Body

In terms of its structural role, the main function of phosphorus is, when combined with calcium, in the formation of hard structures such as bones and teeth. Phosphorus is also involved in many key processes such as membrane integrity and provision of so-called energy-rich compounds vital to energy metabolism. It is also a component of enzyme cofactors. These are relatively small organic molecules, associated with larger enzyme molecules, which are necessary for the enzymes to function effectively. The cofactors often combine with and then release molecules or fragments of molecules as part of the overall enzymatic process. Dietary phosphorus absorption is controlled principally by parathyroid hormone (PTH), which regulates the renal production of 1,25-dihydroxyvitamin D. The vitamin stimulates the absorption of phosphorus from the small intestine similar to the effect on calcium.

Consequences of Phosphorus Deficiency

Dog:

There are surprisingly few direct studies of phosphorus deficiency in dogs. Normal phosphorus metabolism requires normal levels of calcium and the emphasis is usually on calcium requirement and the close relationship of dietary calcium and phosphorus – the Ca:P ratio. It is often difficult to produce clear signs of phosphorus deficiency without the almost inevitable involvement of calcium nutrition. Excess dietary calcium or a very high Ca:P ratio can adversely affect the absorption of phosphorus and this will inevitably result in impaired skeletal development. Like calcium, this is particularly serious for young puppies, where the skeleton is still developing. In adult dogs, naturally occurring phosphorus deficiency is rare though inadequate phosphorus intake (below the minimum requirement) can occur with chronic starvation, secondary to the feeding of improperly formulated home-prepared diets, or inappropriate feeding of phosphorus-restricted therapeutic diets. Clinical signs of hypophosphatemia in adult dogs include decreased cardiac function as well as platelet and red blood cell dysfunction.

Recognised Syndromes Related to Phosphorus Deficiency

1. Skeletal malformation: Jenkins and Phillips described poor appetite and skeletal malformations in German shepherd puppies fed dietary phosphorus at 0.23% on a dry matter (DM) basis. These effects were largely ameliorated when the dietary content was increased to 0.33% DM. The calcium content was 0.6% DM so the Ca:P ratios were 2.6 and 1.8 respectively, higher than the ideal ratio of around 1:1.

Cat:

As with the dog there are very few studies of dietary phosphorus deficiency in cats but the effects on skeletal development will be expected to be similar to those seen in dogs. In adult cats clinical signs of deficiency including haemolytic anaemia, permanent flexion of the hind legs, apathy and metabolic acidosis were observed in those animals receiving a diet containing 0.21% phosphorus and 0.83% calcium (DM basis). The diet therefore had a very high Ca:P ratio of about 4:1. The importance of the Ca:P ratio is shown by the observation from the same study that no clinical signs occurred when the dietary concentrations of calcium and phosphorus were both 0.21% DM.

Toxicity

Dog and Cat:

There is very little information on the effects of excessive intakes of phosphorus per se in dogs and cats. Where adverse effects have been reported they are almost invariably linked to a low Ca:P ratio or a deficiency of calcium rather than an excess of phosphorus itself. If a diet has a satisfactory Ca:P ratio it is likely that a fairly wide range of phosphorus intakes will be tolerated, especially for adult animals with normal kidney function. Animals with compromised kidney function, dietary phosphate and protein restriction may be required.

Dietary Sources

Phosphorus is more widely distributed than calcium and is found in dairy, meat, vegetable and cereal products. However in cereals and vegetables phosphorus may be combined as phytate, which is less bioavailable. Common sources in manufactured dog and cat foods include bone meal, meat and bone meal, fish meal, and inorganic phosphate salts – usually those of calcium, sodium, potassium or ammonium. Home-made diets that are mostly composed of meats typically have a very abnormal Ca:P ratio.