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Phosphorus - P in dog and cat diets

A short guide to the role of Phosphorus - P in dog and cat nutrition.

Phosphorus - P

Together with calcium it constitutes the main building component of bones and teeth — about 85% of the phosphorus in the body forms hydroxyapatite of the skeleton with calcium [1]. The remaining part plays a key role in every cell: it is an energy fuel (a component of the high-energy bonds in ATP), builds DNA and RNA, forms the phospholipids of cell membranes and takes care of the acid-base balance of the blood [1]. Phosphorus is so fundamental that in a healthy dog and cat on a meat diet there is practically never a shortage of it — by itself, however, it says little. What matters is its ratio to calcium. If the calculator signals a problem, it is highly likely to be about the Ca:P ratio, not about phosphorus itself — more in the article calcium:phosphorus ratio and Calcium.

Where it's found: Muscle meat, offal (especially liver and kidneys), fish, eggs and edible bones. Phosphorus from animal tissues is absorbed at nearly 100% — this is a fundamental difference compared to plant (phytic) phosphorus, absorbable at 30–60% [1].

Deficiency: In BARF practice it does not occur — all the basic components of a raw diet are rich in phosphorus [1]. Theoretical symptoms (muscle weakness, lack of appetite, softening of the bones) concern only diets based mainly on fat or starch, which does not happen in a correct BARF.

Excess — relative to calcium: This is the actual problem. Muscle meat has a Ca:P ratio of the order of 1:15–1:20, so without a source of calcium the body has to draw the missing calcium from the bones (secondary hyperparathyroidism) [2]. In puppies and kittens, skeletal deformations and fractures can appear within several to a dozen or so weeks; in adults, demineralisation progresses over months in hiding [3]. The optimal Ca:P ratio is about 1.2:1–1.4:1 in the dog and 1.1:1–1.3:1 in the cat; below 1:1 it is already bad [1].

NRC standard

The calculator converts the NRC standards (2006) per 1000 kcal ME. For an adult dog the recommended phosphorus intake (RA) is about 750 mg/1000 kcal (minimum 375 mg), and the upper safe limit about 4000 mg; for an adult cat the RA is about 640 mg/1000 kcal (minimum 350 mg) [1]. More important than the number itself, however, is maintaining the Ca:P ratio within the optimal range.

Species difference — kidney disease

In the cat an additional dimension comes in: chronic kidney disease (CKD) affects a large proportion of older cats, and excess phosphorus accelerates its progression [4]. This does not mean that cats are "more sensitive to phosphorus" as such — they are more exposed to a disease in which phosphorus becomes a critical parameter. With CKD (in the dog and in the cat) it is no longer only the Ca:P ratio that matters, but the absolute amount of phosphorus: less offal, considering a calcium supplement instead of bones (easier control of the balance), fattier meats. This is a topic to manage with a veterinarian, not for balancing on your own. It is worth examining the kidneys of an older animal every year (creatinine, SDMA, serum phosphorus). Normal blood calcium and phosphorus results do not rule out diet errors — the body maintains homeostasis for a long time at the expense of the bones.

Sources

  1. National Research Council (2006). Nutrient Requirements of Dogs and Cats, Chapter 7 — Minerals. National Academies Press. https://nap.nationalacademies.org/catalog/10668/nutrient-requirements-of-dogs-and-cats
  2. Stockman J, Villaverde C, Corbee RJ (2021). Calcium, Phosphorus, and Vitamin D in Dogs and Cats: Beyond the Bones. Veterinary Clinics of North America: Small Animal Practice. https://doi.org/10.1016/j.cvsm.2021.01.003
  3. Mack JK, Alexander LG, Morris PJ, Dobenecker B, Kienzle E (2019). Factorial calculation of calcium and phosphorus requirements of growing dogs. PLoS ONE. https://doi.org/10.1371/journal.pone.0220305
  4. Stockman J (2024). Dietary phosphorus and renal disease in cats: where are we?. Journal of Feline Medicine and Surgery. https://doi.org/10.1177/1098612X241283355