Picture this: you and your dog are basking together in those precious golden rays of late summer sunshine. While you’re soaking up vitamin D through your skin, is your furry friend doing the same?
As summer begins its gradual transition into autumn, now is the time for those of us living in the Northern hemisphere to soak in the remaining hours of precious summer sunshine. If you’re anything like my dog Boone and me, we absolutely love basking under those golden rays (at least before it gets too hot). And here in northern Germany, you learn quickly never to take any of the sun’s rare appearances for granted!
While it might be obvious to most dog owners that sunlight exposure provides sensory enjoyment to dogs, the direct health benefits of sun exposure might be less clear. The answer to whether dogs synthesize vitamin D from sunlight might surprise you — and it reveals a fascinating story about evolution, diet and the trade-offs that canids have made over millions of years.
So, do dogs generate vitamin D from sun exposure?
The science on this question appears to be settled: domestic dogs and cats, unlike humans, are unable to synthesize sufficient vitamin D to meet their biological needs from exposure of their skin to sunlight. And before you attribute it to their fur coats, the evolutionary reason for this inability has nothing to do with hair coverage and everything to do with diet.
The foundational research comes from a 1994 study published in General and Comparative Endocrinology by How et al., which directly compared domestic dogs and cats to rats (the control group that CAN synthesize vitamin D). The researchers found that dogs and cats had about 10 times less of the precursor molecule (7-dehydrocholesterol) needed for vitamin D synthesis in their skin. Even when exposed to UV light in laboratory conditions, dog and cat skin showed no increase in vitamin D production, while rat skin showed a 40-fold increase.
The mechanism behind this deficiency is that carnivorous species have high activity of an enzyme called 7-dehydrocholesterol reductase, which rapidly converts the vitamin D precursor into cholesterol instead, leaving insufficient raw material for vitamin D synthesis.
If you’re wondering if I'm feeling sufficiently brave to utter the dreaded “C word” in canine nutrition – by which I mean “carnivore” ;) – yes, I am. And with good reason. This inability to produce vitamin D is a characteristic shared by carnivores generally. A 2015 study examined skin from 22 different carnivorous species (including African wild dogs and gray wolves) and confirmed the same pattern: carnivores consistently showed reduced vitamin D synthesis compared to omnivorous controls.
What about all that fur?
The fact that many animals WITH fur coats can produce vitamin D demonstrates that hair coverage isn’t the limiting factor. After all, sheep and goats can synthesize vitamin D effectively despite their thick woolly coats and dairy cows produce vitamin D across their entire body surface, even in areas covered by dense hair.
So, how do fur-covered animals manage this? It turns out that UVB rays can in fact penetrate through most animal fur to reach the skin beneath, especially when the animal moves, which causes strands of fur to shift. Moreover, animals have areas with thinner or no fur coverage, such as around the nose, ears and belly, where vitamin D synthesis can occur efficiently. In dairy cows, the udder provides another prime location for synthesis. Ultimately, the key is having adequate levels of the precursor molecule in the skin, which carnivores lack regardless of their fur situation.
The evolutionary trade-off
Here’s where it gets fascinating from an evolutionary perspective. Animals that consumed dietary sources of vitamin D in sufficient quantities – specifically carnivores feeding on liver, fat and blood of prey animals – didn’t need to maintain the metabolically expensive process of vitamin D synthesis. Through evolution, these species “reinvested” that biological energy elsewhere, losing the synthesis capacity in what’s called an “evolutionary trade-off.”
The pattern is remarkably clear: Herbivores (rabbits, guinea pigs, ruminants) and omnivores (humans, rats, pigs) CAN synthesize vitamin D. Conversely, carnivores, whether obligate or facultative (cats, dogs and other carnivorous species) CANNOT effectively synthesize vitamin D.
This evolutionary perspective adds compelling evidence to the ongoing debate about whether dogs are carnivores or omnivores. While I’ve personally placed myself on Team Omnivore, the vitamin D synthesis data has me seriously reconsidering. Team Carnivore is looking pretty compelling! For now I’ll compromise with this: dogs are preferential carnivores with the capacity to derive much of their nutritional requirements from plant-sources. (However, just because dogs can derive nutrition from plants doesn’t mean that plant-based foods are better than animal-based foods, because in most cases they’re not. But that’s a discussion for a future post!)
Vitamin C synthesis for the win
Just in case you’re feeling sorry for dogs not being able to generate their own vitamin D, don’t sweat it! Dogs (and cats) are able to synthesize vitamin C in their own bodies to meet their biological needs – and humans can’t! This is another example of an evolutionary trade-off, but this time it's specific to humans and our primate relatives.
Most mammals, including dogs, cats and cattle, can synthesize vitamin C from glucose in their livers. However, humans, other primates, guinea pigs, as well as some fruit bats have all lost this ability due to mutations in a gene called GULO (gulonolactone oxidase).
The evolutionary story here mirrors the vitamin D situation perfectly, just in reverse. As our primate ancestors developed fruit-heavy diets rich in vitamin C, there was no longer strong selective pressure to maintain the metabolically costly vitamin C synthesis pathway. Mutations that knocked out this function weren’t harmful when dietary vitamin C was abundant, so they accumulated over evolutionary time. The energy once spent making vitamin C could be “reinvested” elsewhere.
Meanwhile, dogs and other species without reliable dietary sources of vitamin C retained the capacity to generate it endogenously based on their body’s needs. So while we humans are scrambling for vitamin C-infused juices and supplements as soon as we feel a tickle in our throats, dogs just have to exist! Their bodies automatically adjust vitamin C production up or down depending on stress, illness, or other factors that increase their needs.
The more you know
As you can see from this discussion, canine nutrition is complex and full of nuance. From evolutionary trade-offs to debates over the carnivore vs. omnivore classification, navigating optimal nutrition for your dog can feel overwhelming. If you’re feeling uncertain about finding the best foods for your canine companion, I’m here to help. Reach out today for a free 15-minute discovery call to see if my personalized nutrition consulting services might benefit you and your dog.
