The Importance of Getting the Right Vitamin D Levels for Cats

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Now that the sun is finally starting to return, many of us are out enjoying the springtime. Though you must be careful not to burn, you may be aware of the link between sunlight and maintaining healthy vitamin D levels. Is the same true for cats? Should cats sunbathe to top up their vitamin D? What if they don’t get enough vitamin D? Can they even get too much?

What Does Vitamin D Do?

Vitamin D is a substance that is involved in a number of processes to help maintain a healthy body in many animals. The processes vary animal to animal, but vitamin D’s primary function is to manage calcium levels in the blood – this is called calcium homeostasis. Calcium is critical in animals to maintain healthy bones, help muscles contract and nerves transmit signals, help blood clot after injury, as a component of milk for lactation, and many other areas. If calcium levels drop too high or low, serious health issues can occur.

Calcium homeostasis works as follows: in the neck are two paired organs called the parathyroid glands (meaning there are 4 overall – two on each side). These measure blood calcium levels and constantly release a hormone called parathyroid hormone (PTH). If the blood calcium level drops too low (hypocalcaemia) the glands release more PTH into the bloodstream. The PTH travels to the kidneys and causes the release of enzymes that convert inactive vitamin D (calcidiol) to active vitamin D3 (calcitriol). Vitamin D3 causes more calcium to be absorbed from digested food by the intestines and reduces calcium excretion from the kidneys. PTH also causes the release of calcium from the bones. Quickly, blood calcium level return to normal, which inhibits the release of excess PTH and maintains calcium homeostasis.

Vitamin D also has other functions, including maintaining phosphorus homeostasis (another mineral that forms bone), helping bones repair and reshape following diet and exercise, maintaining dental health, regulating the immune system, reducing inflammation, aiding in cell growth, and metabolising glucose.

How Do Cats Get Vitamin D?

In many animals, vitamin D can be produced within the body with the aid of UV light – this is where the idea of getting sunlight to increase vitamin D levels comes from. Humans and other animals create a chemical from dietary cholesterol called 7-dehydrocholesterol that is stored in the skin. When UV light is absorbed by 7-dehydrocholesterol, it is converted to inactive vitamin D (cholecalciferol). This is then altered further by the liver to remain stable within the body as calcidiol. Only when needed is calcidiol converted to active calcitriol by enzymes released by the kidneys.

However, cats and dogs, unlikely many other animals, do not produce a significant amount of vitamin D from sunlight. This is because their skin contains high levels of an enzyme called 7-dehydrocholesterol reductase, which converts 7-dehydrocholesterol back into cholesterol before UV light can convert it to vitamin D. This means cats must obtain practically all their vitamin D from their diet. Prey animals store vitamin D mainly in their liver, meaning when the carnivorous cat consumes their prey, they intake vitamin D for their own use by absorbing it through the intestines. Because vitamin D can be stored easily, this also means they do not quickly become deficient if they have not eaten for a while. Cats are also more resistant to vitamin D toxicity, due to their adaptation for consuming so much in their prey.

Dietary vitamin D comes in two forms – cholecalciferol and ergocalciferol. The former is found mainly in animals, and the latter in plants. Because cats are obligate carnivores, they cannot efficiently use ergocalciferol in their bodies, unlike dogs which can utilise both forms; cats must consume cholecalciferol specifically.

What Happens if a Cat Becomes Deficient in Vitamin D?

Cats with an inappropriate diet are at risk of hypovitaminosis D, or low vitamin D levels. Those with kidney diseases are also at risk due to secondary hyperparathyroidism – this is because cats with kidney disease can have increased phosphorus levels. This causes a reduction in calcium levels (to try and inhibit phosphorus absorption). Low calcium triggers the release of PTH, but the diseased kidneys are unable to convert inactive vitamin D to vitamin D3, further increasing PTH without leading to increased calcium levels. Some cats may also become deficient due to gastrointestinal disease or certain kinds of cancer.

The main effects of hypovitaminosis D are associated with calcium. A low vitamin D level means the kidneys and intestines will excrete more and absorb less calcium, causing hypocalcaemia. Over time, these conditions progress to weakened bones. This presents as lameness, bowing of the legs, increased risk of bone breakage, curving of the spine, poor quality fur and general unwellness. Radiography will show the bones are less dense. If caused by hyperparathyroidism, calcium taken from the bone is replaced with fibrous tissue, resulting in a rubbery jaw and other bone abnormalities.

Other diseases that have been shown to be associated in dogs with low vitamin D include certain kinds of cancer, gastrointestinal diseases, and congestive heart failure – it is unclear if these may be linked to vitamin D in cats too. There is also some evidence that cats can have increased resistance to certain infectious diseases such as Mycobacterium (tuberculosis) when supplemented with vitamin D, so may be more vulnerable if their levels are too low.

Cats also can get a condition called hypoparathyroidism, where the parathyroid gland does not produce enough PTH. This can occur due to cancer, inflammation, infection or surgical removal of the glands. While it does not directly cause vitamin D deficiency, it can lead to vitamin D not being activated, resulting eventually in hypocalcaemia and other conditions associated with low vitamin D levels.

Thankfully, many cases of hypovitaminosis D and its associated symptoms can be reversed if caught early enough and corrected. It can be diagnosed by testing the blood for vitamin D, as well as associated chemicals like calcium, phosphorus, urea and creatinine.

Can Cats Get Too Much Vitamin D?

On the other end of the scale cats can overdose on vitamin D, a disease termed hypervitaminosis D, though as stated above it is less likely than in dogs. In both species it is thankfully rare.

Sudden, or acute, hypervitaminosis D may result in rapid hypercalcaemia due to the action vitamin D has on the kidneys and intestines. This can lead to excessive drinking and urination, anorexia, vomiting, constipation, muscle tremors, depression and seizuring, and soft tissue calcinosis where the tissues are replaced with calcium. It can be corrected via intravenous administration of fluids, diuretics, steroids and bisphonates. Longer-term chronic hypervitaminosis D is more likely to cause bone disorders due to the leeching of calcium from the bones, as well as organ calcinosis leading to diseases like kidney disease.

Hypervitaminosis D has rarely been caused by dietary problems, such as rare cases of problems with the manufacturing process in pet food factories, or cats receiving excessive inappropriate food. It can also be caused by ingestion of certain rodenticide rat baits that cause the uncontrolled production of calcitriol, leading to hypercalcaemia. Many rat baits also cause clotting disorders too. Any animal known to have ingested rat poison should immediately see a vet. It has also been known for certain human medications for psoriasis when ingested by animals to cause hypervitaminosis D, due to their high vitamin D content.

How to Ensure Your Cat Receives Enough Vitamin D

Although we know that animals need to receive a certain amount of vitamin D to stay healthy, there isn’t an exact answer for how much this should be, because the dietary form of vitamin D must be converted before use and it can be difficult to measure the active form in blood. Therefore, estimates of the amount an animal must consume have been based on studies in growing puppies and any bone abnormalities they developed from their dietary intake. The studies did not measure serum concentrations of vitamin D after feeding certain diets, meaning the amount of vitamin D consumed may not necessarily reflect the final vitamin D level in the body. This will be relevant to any animal with disease that reduces their absorption of vitamin D from their diet – they may require higher vitamin D levels in their food to compensate. These studies were also limited as they did not look into the effects of vitamin D on other parts of the body.

Despite the shortcomings, by their calculations an adult cat requires approximately 60-80IU of vitamin D per 1000kcal of metabolisable energy, ME (that being the energy left from food after waste is excreted). As per UK Pet Food, an average 4kg neutered active cat requires around 190kcal of ME per day, meaning around 11-15IU of vitamin D per day is required to remain healthy. We checked a selection of popular cat foods appropriate for this cat, and found that all contained more than the recommended vitamin D per portion. The safe upper limit for vitamin D has been reported to be ~7500IU/kg of ME in cats (compared to only 800 in dogs), making overdose practically impossible. Proper scientific studies of these diets would be needed to determine if their reported vitamin D levels are accurate, and how much of this reaches a cat’s body to be utilised. We should also note that some diets stated the vitamin D ‘additive’ while others the ‘total’ vitamin D, meaning the actual levels may vary than what is stated. Furthermore, some diets, mainly fish-based, did not record vitamin D additives, perhaps indicating the natural vitamin D in the fish was already sufficient for health.

What all these complicated calculations means is that for the average cat, most commercial cat foods should contain sufficient vitamin D to maintain healthy bones, teeth, muscles, nerves and other critical components. This may not necessarily be the case for cats on non-commercial diets, particularly home-cooked diets. We suspect that plant-based diets are unlikely to contain sufficient useful cholecalciferol vitamin D, and may only contain ergocalciferol vitamin D that a cat cannot use; these diets are not recommended for the average cat.