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Vitamin D (Cholecalciferol) in dog and cat diets

A short guide to the role of Vitamin D (Cholecalciferol) in dog and cat nutrition.

Vitamin D (Cholecalciferol)

It belongs to the group of fat-soluble vitamins. In the body it acts as a powerful hormone, and its main task is to manage calcium-phosphorus balance — without it calcium will not be absorbed from the intestines into the blood, and bones and teeth will not be able to mineralize properly [1]. Neither dogs nor cats can produce it efficiently in the skin under the influence of sunlight (as humans do) — they must obtain it from food [2]. In both species dangerous poisoning is extremely easy (e.g. after eating a human supplement).

Where it's found: Cod liver oil, fatty sea fish (salmon, herring, sprat, sardine, mackerel), to a lesser extent egg yolks and liver. Note: muscle meat, bones, and white fish (cod, pollock) contain practically no vitamin D, and fish oil from fish flesh (e.g. salmon oil) is not cod liver oil — it provides omega-3 but does not cover vitamin D [1]. The safest tool for precisely closing the balance is a pure D3 supplement (cholecalciferol, not D2/ergocalciferol, which animals use poorly) [3].

Deficiency: Insidious — symptoms develop only after 3–6 months of feeding without sources of vitamin D, because the body keeps reserves in fat tissue and the liver [4]. It causes rickets (in growing animals), osteomalacia and pathological fractures (in adults), and "rubber jaw" syndrome. This is a frequent problem in poorly balanced home diets based exclusively on meat and bones, without fatty fish and supplementation — in cats it is additionally often compounded by calcium deficiency, which accelerates skeletal destruction.

Excess: It is stored in the body and is extremely toxic — the safety margin is very narrow. It causes hypercalcemia and dangerous calcification of soft tissues (kidneys, heart, blood vessels), increased thirst, vomiting, and apathy [5]. The most common cause in practice is not a single large dose, but chronic overlapping of sources — fish + premix with D3 + cod liver oil + a separate D3 supplement. Many premixes contain D3 doses calculated for a diet without fish.

Species difference

In dogs the cutaneous D synthesis pathway practically does not work — the level of the precursor (7-dehydrocholesterol) in the skin is too low [2]. In cats the mechanism is even more blocked: high activity of the enzyme that breaks down the precursor means synthesis does not occur at all [3]. Walks in the sun and lying on the windowsill are pleasant, but they will not provide vitamin D to either species.

A cat is especially easy to overdose — it is small, and supplements have "human" dosing, so one drop too many makes a bigger difference than in a large dog. Hypercalcemia from excess D hits the cat above all in the kidneys, which are anyway a weak point of this species, especially in older cats and breeds predisposed to chronic kidney disease [4]. In a cat with kidney disease do not supplement D on your own — this requires veterinary consultation.

Interactions

Vitamin D, calcium, and phosphorus form a system of connected vessels — without the proper Ca:P ratio even an ideal dose of D will not work, and vice versa [6]. Cod liver oil simultaneously brings in vitamin A — with liver in the diet it is easy to overdo A while trying to fix D; pure D3 is safer in this respect.

Standards

NRC 2006: dog RA = 136 IU/1000 kcal ME, SUL = 800 IU/1000 kcal; cat RA = 70 IU/1000 kcal ME, SUL = 7520 IU/1000 kcal [1]. FEDIAF gives a maximum of 7500 IU/1000 kcal [7].

Sources

  1. National Research Council (NRC) (2006). Nutrient Requirements of Dogs and Cats, chapter 5 — fat-soluble vitamins. https://nap.nationalacademies.org/catalog/10668/nutrient-requirements-of-dogs-and-cats
  2. How KL, Hazewinkel HAW, Mol JA (1994). Dietary vitamin D dependence of cat and dog due to inadequate cutaneous synthesis of vitamin D. Gen Comp Endocrinol 96:12-18. https://doi.org/10.1006/gcen.1994.1154
  3. Morris JG (1999). Ineffective vitamin D synthesis in cats is reversed by an inhibitor of 7-dehydrocholesterol-Δ7-reductase. J Nutr 129(4):903-908. https://doi.org/10.1093/jn/129.4.903
  4. Parker VJ, Rudinsky AJ, Chew DJ (2017). Vitamin D metabolism in canine and feline medicine. JAVMA 250(11):1259-1269. https://doi.org/10.2460/javma.250.11.1259
  5. Vecchiato CG, Delsante C, Galiazzo G et al. (2021). Cholecalciferol (Vitamin D3) Toxicity Observed in Five Cats. Front Vet Sci. https://doi.org/10.3389/fvets.2021.707741
  6. Stockman J, Villaverde C, Corbee RJ (2021). Calcium, Phosphorus, and Vitamin D in Dogs and Cats: Beyond the Bones. Vet Clin North Am Small Anim Pract 51(3):623-634. https://doi.org/10.1016/j.cvsm.2021.01.003
  7. FEDIAF (2021). Nutritional Guidelines for Complete and Complementary Pet Food for Cats and Dogs. https://europeanpetfood.org/self-regulation/nutritional-guidelines/