When it comes to our canine companions, providing them with a well-balanced diet is essential for their overall health and vitality. One critical element in achieving the appropriate balance of vitamin D, which plays a role across the body including skeletal strength, immune function, and mood regulation.

Vitamin D is often referred to as the "sunshine vitamin” for humans because it is created in the body when our skin is exposed to UV light. Dogs do not have the ability to synthesize vitamin D endogenously through sunlight, so it must be provided in adequate amounts in the diet. Formulating with the appropriate amount of vitamin D can be a challenge for pet food suppliers as it is only safe in small amounts and is not naturally present in most foods, so typically requires carefully-measured supplementation.

What is Vitamin D?

Vitamin D refers to a group of compounds with a similar chemical structure - ergocalciferol (vitamin D2) is produced by plants and yeast while cholecalciferol (vitamin D3) is produced in the skin of some animals as well as through a complicated photosynthetic process in microalgae.  Vitamin D3 is naturally found in concentrated levels in fish - particularly fatty fish that eat plankton that synthesize D3 from sunlight - as well as the livers of mammal. Dogs’ carnivorous wolf ancestors would traditionally get their vitamin D from eating the organs of prey mammals as well as fish if available. 

Vitamin D is typically added as a supplement to dog food - both in traditional meat-inclusive food as well as plant-based formulas. Vitamin D is a relatively ‘fragile’ compound that can be destroyed by heat and exposure to oxygen, so even foods with fatty fish or liver as main ingredients can be below the minimum dietary requirement. Vitamin D3 supplements have traditionally been created from the lanolin in sheep’s wool, while a vitamin D2 supplement is made from exposing yeast to UV light. 

To serve the vegan market and avoid the use of animal-derived products, companies have developed Vitamin D3 supplements derived from lichens - a unique and complex symbiotic organism that is a partnership of a fungi and a plant (algae). The fungi and microalgae create a vitamin D2 compound that is then synthesized into vitamin D3. 

When vitamin D2 or D3 is consumed by dogs (and other mammals), it is metabolized into a different compound called 25-hydroxyvitamin D (f.k.a. 25(OH)D) that is primarily stored in the fat of an animal’s liver, which is then released into the bloodstream to be converted by the kidneys into the active, usable form of vitamin D called calcitriol. These vitamin D metabolites play a vital role in the regulation of calcium and phosphorus levels in the body. The primary function is to promote the absorption of these minerals from the intestines, ensuring proper bone development, maintenance, and overall skeletal health. Additionally, vitamin D has been shown to behave like a hormone in the body, as it binds to hormonal receptors and plays a role in many physiological functions, including immune system support, inflammation reduction, and cell growth regulation.

Vitamin D is also toxic in relatively low quantities. Rat poison is typically just a large dose of vitamin D3, which causes extremely high levels of calcium to be released into the blood that quickly overwhelms and destroys the kidneys. The relatively low tolerance for metabolized vitamin D makes the amount of dietary vitamin D - and particularly D3 - important to closely measure and monitor, as mistakes with supplementation (and poor testing programs) have resulted in pet food batches that killed many dogs. 

Benefits of Vitamin D for Dogs

Vitamin D is unique in that it functions as both a vitamin and hormone, which means that it is both critical to the normal metabolic functioning of cells but also regulates some higher-level body functions. As a result, vitamin D has a broad impact on the body, including:

• Bone health: Appropriate levels of metabolized vitamin D prevents conditions like rickets and reduces the risk of osteoporosis in dogs. It strengthens bones and teeth, making them less prone to fractures and ensuring optimal mobility, especially crucial for active dogs.

• Immune support: Vitamin D has been shown to modulate the immune system, aiding in the defense against infections and diseases. Dogs with sufficient vitamin D levels are generally more resilient and better equipped to fight off illnesses.

• Muscle function: This vitamin plays a role in muscle function and coordination, contributing to a dog's agility and overall physical performance.

• Mood regulation: Some research suggests that vitamin D3 might influence mood and behavior in dogs, potentially contributing to reduced stress and anxiety levels.

It is important to note that increasing vitamin D beyond the recommended amount doesn’t increase bone strength or immune function. The critical factor is whether the dog or human is over the threshold for regular metabolic function - i.e. is not deficient in vitamin D. Studies have found that supplementing additional vitamin D in humans that were not below the recommended threshold did not impact bone strength. However, over 40% of (human) Americans are likely vitamin D deficient for at least part of the year, as sunlight exposure is often inconsistent and human diets are not as carefully planned and supplemented as dog food. 

Vitamin D2 vs Vitamin D3

Vitamin D supplementation is particularly important in meat-free diets, as it is naturally found in very low concentrations in most plant-based ingredients. There are multiple choices for supplemental vitamin D as well as debate over which form is the best for dogs and humans.

There is evidence to suggest that vitamin D3 is more bioavailable - i.e. able to be absorbed and converted into a form that is usable by the body - than vitamin D2. Studies in humans have shown that a vitamin D3 supplement increases the amount of metabolized vitamin D (i.e. the amount available in the blood) more than equivalent amounts of a vitamin D2 supplement. Vitamin D2 supplements are still used as they are effective at raising metabolized vitamin D, but require higher consumption to do so.

The difference between D2 and D3 supplementation in dogs has not been conclusively studied, although feeding different variants of vitamin D have been shown to have different impact on the level of the active metabolite in dogs’ blood. Both vitamin D2 and vitamin D3 supplements are approved for use in dog food by AAFCO, have a long history of use in U.S. dog food, and the leading veterinary nutritionists consider the supplements as functionally equivalent in dogs.

A study in cats, which are considered to be obligate carnivores as they cannot endogenously synthesize many nutrients found only in meat and animal-dervied ingredients, found that vitamin D2 was metabolized ~30% less efficiently than vitamin D3. A meta-analysis of studies concerning vitamin D in dogs and cats explained:
“[Cats] do not use ergocalciferol as efficiently as they do cholecalciferol (Morris, 2002b), unlike dogs, which can efficiently use both forms (Parker, Rudinsky, & Chew, 2017). This may perhaps be explained by the eating habits of these animals in the wild. Cats are considered strict carnivores, so their historical dietary intake of ergocalciferol is close to null. In contrast, as dogs are considered omnivores, their intake of ergocalciferol should be higher than cats, and, consequently, they developed the ability to convert ergocalciferol into 25(OH)D in the liver.

Vitamin D3 is the only approved supplemental version of vitamin D for pet food in Europe, as the definition of vitamin D was revised to only include vitamin D3 five years ago. Vitamin D2 was used in European pet food prior to 2018, but the regulatory agency (FEDIAF) is now seeking more data to demonstrate that vitamin D2 is bioavailable in cats. In the meantime, vitamin D2 has been removed as an approved supplement for all non-human animal species (despite evidence of nutritional comparability in every species except cats). 

Importance of Vitamin D3 in Meat-Free Formulas

Calcium absorption: Plant-based diets often include natural compounds that can inhibit calcium absorption. Including vitamin D3 in meat-free formulas helps overcome this challenge by enhancing the uptake of calcium and phosphorus from plant-based sources.

Bone health in dogs with dietary restrictions: Some dogs have allergies, sensitivities, or medical conditions that limit their consumption of animal-based products. Vitamin D3 in meat-free diets ensures these dogs receive the necessary nutrients for optimal bone health.

Higher bioavailability in food: Vitamin D is fat-soluble, which means that is does not dissolve in water and are absorbed and transported similarly to fats. This means that vitamin D is more easily digested and absorbed into the bloodstream when it is paired with fats through the digestive system. As a result, most nutritionists recommend consuming vitamin D as part of meals (rather than supplemental pills) where it is paired with fat in the diet. 

In conclusion, vitamin D is as a crucial nutrient in a dog's diet, contributing to bone health, immune function, and more. For those choosing to provide their dogs with meat-free diets, using a bioavailable form of vitamin D - and in this case an algae-derived vitamin D3 - becomes even more important to compensate for the absence of natural animal sources. Always consult a veterinarian before making significant dietary changes for your furry friend, ensuring that their unique nutritional needs are met for a happy, healthy life.

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