Calcium is an element that in dogs and cats must be supplied to the body with food in a bioavailable form. Together with phosphorus, it plays a structural role, constituting the basic building block of bones and teeth, and a functional role, taking part in blood clotting and the transmission of nerve impulses. The availability of calcium depends on its total amount and ratio to phosphorus, as well as on the form and source. Extrusion and cooking reduce the bioavailability of calcium [1][2]. Absorption is also influenced by the content of phytate, oxalate and fat in the [3] diet. Due to the close relationship, calcium is always considered together with phosphorus, and their ratio (Ca:P) should be approximately 1.2:1 to 1.4:1 [1]. Phosphorus is found in very large amounts in clean meat. Therefore, feeding your dog or cat only meat without adequate calcium supplementation drastically disturbs the Ca:P ratio. Its imbalance - even with the right amount of calcium in the body, leads to its deficiency [2]. More in the article calcium:phosphorus ratio.
What it contains: Meaty bones or supplements (necessary in cooked barf), such as eggshell meal, Lithothamnium calcareum marine algae, or calcium phosphate or citrate.
The phenomenon of blocking calcium by phosphorus
If there is too much phosphorus in the food, it immediately combines with calcium in the digestive system, forming hard, insoluble compounds. Such “bound” calcium becomes completely unabsorbable by the body. As a result, the animal eats a theoretically sufficient amount of calcium, but does not absorb it into the bloodstream - it will simply be excreted in the feces, and the body will experience a massive deficiency of [2]. The body then begins to use calcium from bone tissue, and the bone structure will suffer.
Meat bones and the balance down to the gram
Diets based on meaty bones do not balance perfectly in every single meal, but over several days or weeks. If on Monday the animal receives 3 grams too little of calcium, and on Tuesday it eats 5 grams too much (due to a bonier chicken neck), its body will easily compensate for this balance with [3]. Throwing meat bones and a supplement into one bowl may create unnecessary chaos for the stomach (pH changes) and the risk that the calcium quickly released from the citrate will block valuable microelements from the meat before the bone even begins to be digested. Therefore, increasing the number of, for example, necks by a dozen or so grams is more convenient for you, and more natural for the pet’s body (excess calcium will simply be excreted).
Mechanism of excretion of excess bone
(Applies only to healthy adult animals!) Calcium in raw bones is not found in free form, but in the form of an extremely hard mineral (hydroxyapatite). It is absorbed in the intestine after digestion of the bone matrix by gastric acid. If an animal eats too many bones at once, the body regulates the balance primarily at the intestinal level - it limits the absorption of calcium, and the hard, insoluble bone matrix will not be completely dissolved during the passage anyway. The undissolved surplus goes to the intestine in an unchanged, indigestible form and is simply excreted (hence the hard, chalky-white feces). Nature itself protects against overdose by limiting absorption. Calcium homeostasis is regulated primarily by fecal excretion of [3]. Note: in puppies, calcium absorption is not regulated, it occurs passively. Excess bone in a growing dog’s diet will always result in too much calcium being absorbed into the bloodstream, causing severe skeletal deformities or fractures! [2][4].
Supplements and stomach acid
Supplements such as eggshell meal are chemically calcium carbonate. It enters into an immediate chemical reaction with stomach acid, neutralizing it. If we add too much of a pure supplement “by eye” to meat, we can drastically deacidify the carnivore’s stomach. As a result, the animal will not properly digest the protein from the meat, and what’s worse - the acid will not neutralize harmful bacteria (e.g. Salmonella). Bone also engages stomach acid, but its structure makes this process slow and physiological, rather than rapid as in the case of powder. Additionally, calcium, zinc, copper and iron are minerals that fight in the intestines for the same “entrance doors” (receptors) to the bloodstream [1]. Calcium salts are quickly and immediately dissolved by stomach acid, and the intestines are filled with a huge amount of free ions. Therefore, they win the competition and block the absorption of zinc, copper or iron from a given meal. Of course, calcium carbonate is not bad, but it needs to be supplemented wisely. In turn, calcium from eaten bones is released slowly, stably and accompanied by phosphorus, so the intestines have time to calmly absorb other micronutrients.
Deficiency: Hypocalcemia is a serious electrolytic disorder, especially in seriously ill animals. Its cause may include, among others: hypoparathyroidism, kidney failure, hyperadrenocorticism or toxins. Symptoms include: anorexia, growling, nervousness, focal muscle spasms (ears and muzzle), stiff gait, nervous and muscular system disorders including tetany and seizures [5].
Excess: Calcium and phosphorus compounds are often responsible for the formation of urinary tract stones in dogs and cats, but their excess in the diet is not the only cause. Hyperparathyroidism may lead to hypercalciuria. Stones may also be formed as a result of bacterial inflammation of the bladder.
Sources
- 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
- 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
- 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/s0007114517001210
- 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
- Hall, J. A. (2024). Hypocalcemia in Small Animals. In MSD Veterinary Manual. Merck & Co., Inc. https://www.msdvetmanual.com/metabolic-disorders/disorders-of-calcium-metabolism/hypocalcemia-in-small-animals?