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Calcium - Ca in dog and cat diets

A short guide to the role of Calcium - Ca in dog and cat nutrition.

Calcium - Ca

Together with phosphorus it is the main component of bones and teeth — in the body of a dog and cat about 99% of calcium is found in the skeleton [1]. The remaining 1% circulating in the blood is, however, absolutely crucial: it governs proper muscle contraction (including the heart), nerve conduction and blood clotting. Calcium is always considered in pair with phosphorus, and their ratio (Ca:P) — optimally around 1.2:1–1.4:1 in the dog and 1.1:1–1.3:1 in the cat — is more important than the amount of calcium itself [1]. Pure meat is rich in phosphorus and poor in calcium, which is why feeding a dog or cat meat alone without a source of calcium drastically disturbs this proportion and leads to a deficiency — even when there is theoretically no shortage of calcium. Calcium deficiency is one of the most frequently identified problems of home-prepared diets [2]. More in the article calcium:phosphorus ratio.

Where it's found: Raw meaty bones (necks, wings, poultry carcasses, rabbit bones) or targeted supplements — essential in cooked BARF. Eggshells, calcium carbonate and calcium citrate raise calcium without phosphorus; bone meal and hydroxyapatite add both elements [3].

Deficiency: The body "steals" calcium from the bones to maintain its constant level in the blood (secondary hyperparathyroidism). In puppies and kittens bone deformities and pathological fractures can appear within weeks, while in adults silent osteoporosis and loosening of teeth progress [3]. The trap: a normal blood calcium result does not rule out a deficiency — the body masks the problem for a long time at the expense of the skeleton.

Excess: It produces hard, white, chalky stools, constipation (in the cat risking megacolon) and impairs the absorption of zinc, copper and iron [1]. An adult, healthy dog tolerates a moderate excess well, because it limits the absorption of calcium from the intestines [4] — but puppies of large and giant breeds cannot do this, and an excess during the growth period permanently disrupts the formation of cartilage and joints [5].

Mechanism: why an excess of bone "self-excretes"

The calcium in raw bones is not free — it occurs in the form of a hard mineral (hydroxyapatite). The stomach must first dissolve it with hydrochloric acid. If the animal eats too much bone at once, the stomach cannot keep up with acid production, and the surplus passes into the intestine in an unabsorbable form and is excreted — hence the hard, chalky-white stool. Supplements such as eggshells are chemically calcium carbonate: it reacts with stomach acid instantly, so an "eyeballed" dose can drastically de-acidify the stomach of a carnivore and impair protein digestion and the neutralization of bacteria. In addition, calcium, zinc, copper and iron compete in the intestines for the same receptors — rapidly released calcium salts flood them with free ions and block the absorption of microelements [1]. Calcium from bone is released slowly, steadily and in the company of phosphorus, so the intestines have time to absorb the rest of the components. Calcium carbonate is not bad — it just needs to be supplemented sensibly, and the diet balanced over a few-day window, not down to the gram in a single meal.

What to watch for

Calcium is not properly absorbed from the intestines without vitamin D — they are an inseparable pair [3]. In the cat this is especially important, because cats practically do not synthesize vitamin D in the skin and are almost entirely dependent on its dietary sources — liver, oily fish, yolk [6]. Cats also have a narrower margin of error than dogs, and many of them gnaw the meat off and leave the bone — the caregiver thinks the diet is "bony", while in reality calcium is lacking; ground bones, quail, day-old chicks or a calculated supplement help then. Count the actual calcium supply from a specific ingredient, not just the percentage of bone in the bowl — different bones have very different mineral content. Growth, pregnancy, lactation and kidney disease are situations in which it is worth consulting the recipe with a veterinarian.

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. Pedrinelli V, Zafalon RVA, et al. (2019). Concentrations of macronutrients, minerals and heavy metals in home-prepared diets for adult dogs and cats. Scientific Reports 9:13058. https://doi.org/10.1038/s41598-019-49087-z
  3. 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
  4. Stockman J, Watson P, Gilham M, et al. (2017). Adult dogs are capable of regulating calcium balance, with no adverse effects on health, when fed a high-calcium diet. British Journal of Nutrition 117(9):1235-1243. https://doi.org/10.1017/S0007114517001465
  5. Tryfonidou MA, Holl MS, Vastenburg M, et al. (2002). Hormonal regulation of calcium homeostasis in two breeds of dogs during growth at different calcium intakes. Journal of Nutrition 132(11 Suppl):3363S–3366S. https://doi.org/10.1093/jn/132.11.3363
  6. How KL, Hazewinkel HA, Mol JA (1994). Dietary vitamin D dependence of cat and dog due to inadequate cutaneous synthesis of vitamin D. General and Comparative Endocrinology 96(1):12–18. https://doi.org/10.1006/gcen.1994.1154