Vitamin D in Dogs and Cats: Why Diet Matters More Than Sunlight
Vitamin D is an essential nutrient for dogs and cats, playing a key role in bone strength, immune health, and overall wellbeing. Unlike humans, however, our pets cannot rely on sunlight to make enough vitamin D. Instead, they must obtain it almost entirely from their diet.
This article explains how vitamin D works in dogs and cats, why their evolutionary history shaped this dependence on food, and how owners can provide it safely and naturally.
Why Dogs and Cats Can’t Rely on Sunlight
In people, vitamin D₃ is made in the skin when ultraviolet B (UV-B) rays from sunlight react with a compound called 7-dehydrocholesterol. Unfortunately, dogs and cats are very poor at this process. Their skin contains very little 7-dehydrocholesterol, and what is present is quickly converted into cholesterol instead. This means that even with long periods of sun exposure, they do not produce meaningful amounts of vitamin D.
Skin Oils and Grooming: A Minimal Contribution
It has been suggested that dogs (and cats) may get tiny amounts of vitamin D from the natural oils on their skin. When sunlight hits these oils, a small amount of vitamin D can form. If a dog grooms and licks its fur, some of this vitamin D may be swallowed.
However, research shows this contribution is extremely small and not enough to support health. At best, it may provide trace amounts, but it should not be relied upon as a meaningful source of vitamin D.
Evolutionary Adaptation: A Dietary Dependence
The reason for this limited skin production lies in evolution. Dogs and cats are carnivores that historically consumed whole prey animals. Prey such as rodents, birds, and fish store vitamin D₃ in their liver, fat, bone marrow, and other tissues. By eating these animals, early dogs and cats met all their vitamin D requirements through diet alone. Because their food already provided enough, they never developed the ability to make vitamin D efficiently in their skin the way humans and other mammals do.
Vitamin D2 vs D3
There are two major forms of vitamin D:
Vitamin D₂ (ergocalciferol): Found in plants, especially mushrooms exposed to UV light. For example, shiitake mushrooms can increase their vitamin D₂ levels when exposed to sunlight or UV lamps. However, in dogs and cats, this form is poorly absorbed and far less effective than vitamin D₃.
Vitamin D₃ (cholecalciferol): Found in animal tissues, and the only truly reliable source for dogs and cats.
While UV-exposed mushrooms may add a small amount of vitamin D₂ to the diet, they cannot replace the need for vitamin D₃ from animal sources.
Safe Natural Sources of Vitamin D₃
Dogs and cats get the best results from whole food sources of vitamin D₃, especially those that also contain the fats and minerals needed for proper absorption. Examples include:
Wild and pasture-raised animals: Higher levels of vitamin D in their liver, fat, and marrow.
Pork raised outdoors: Naturally richer in vitamin D thanks to sunlight exposure.
Egg yolks: Particularly from chickens, ducks, quails, or geese raised in the sun.
Fish and fish eggs: Salmon and sardines are excellent choices.
Cod liver and liver oils: Very high in vitamin D, but also high in vitamin A, so they must be used carefully.
Raw dairy products: Provide some vitamin D, depending on fat content.
The Risks of Synthetic Vitamin D
Vitamin D toxicity is a well-documented issue in processed pet foods, often due to excess synthetic supplementation. The body does not regulate isolated vitamin D the same way it does vitamin D from natural food sources.
Reaching harmful levels through food alone is very difficult—you would need to feed large amounts of vitamin D-rich foods over a long period of time. With synthetic supplements, however, the risk of overdose is much greater.
How Vitamin D Works in the Body
Once consumed, vitamin D₃ is converted by the liver into 25-hydroxyvitamin D (calcidiol), and then by the kidneys into its active form, 1,25-dihydroxyvitamin D (calcitriol). This active form regulates:
Calcium and phosphorus balance (critical for bones and muscles)
Immune function and inflammation
Cell growth and repair
Vitamin D works closely with other nutrients like vitamin A, vitamin K₂, magnesium, calcium, and phosphorus—highlighting why whole foods are the best way to provide it.
Key Takeaways
Dogs and cats cannot make enough vitamin D from sunlight due to their unique skin chemistry.
Small amounts may form in skin oils and be ingested during grooming, but this is negligible.
Evolution shaped them to rely on dietary sources—mainly from animal tissues.
Vitamin D₂ from plants (like mushrooms) is poorly absorbed, while animal-based vitamin D₃ is highly effective.
Natural food sources are safer than synthetic supplements, which can cause toxicity.
References
AVMA Journal – Vitamin D metabolism in canine and feline medicine:
https://avmajournals.avma.org/view/journals/javma/250/11/javma.250.11.1259.xmlHow, K. L., Hazewinkel, H. A., & Mol, J. A. (1994). Dietary vitamin D dependence of cat and dog due to inadequate cutaneous synthesis:
https://onlinelibrary.wiley.com/doi/epdf/10.1111/jpn.13259Tannenbaum, J., et al. (1995). Evolutionary aspects of vitamin D in carnivores:
https://www.tandfonline.com/doi/pdf/10.1080/01652176.1995.9694579Morris, J. G. (1995). Inefficient vitamin D synthesis in dogs and cats:
https://www.sciencedirect.com/science/article/abs/pii/S0016648084711543Lopez, M. J. et al. (2020). Review of vitamin D metabolism in pets:
https://www.sciencedirect.com/science/article/pii/S2666450X20300171Discussion of grooming and skin oil hypothesis:
https://antranik.org/the-offbeat-reason-why-cats-and-dogs-lay-in-the-sun-and-lick-their-fur/Dog Trainer Saratoga – Vitamin D and sun exposure in pets:
https://www.dogtrainersaratoga.com/soaking-up-the-sun-the-vital-role-of-vitamin-d-in-your-dogs-health/The Canine Dietitian – Role of vitamin D in canine health:
https://www.thecaninedietitian.co.uk/blog/do-dogs-need-vitamin-d-understanding-its-role-in-canine-health