Hello readers –  the “Spotlight Series” is a feature I hope will help many of you sort through the veritable barrage of confidently announced nutrition misinformation that seems to increase every day.  I rarely have time these days to write up a huge blog entry, and so many topics really do require detail in order to be worth my time writing and yours reading…still, I want to provide some answers to the questions I hear pretty much daily on the Facebook groups and my Page, as well as my Inbox. So, I’m going to put the Spotlight on a few foods, nutrients and myths, as general help for my readers. (There is also a series offered at the Membership Group, which zeroes in every month on a specific nutrient and provides the same kind of detailed information.) And today, let’s start with one of the most problematic nutrients in home made diets, thanks to a lot of misinformation –  the powerful and essential Vitamin D.

Brew a cup of tea, pour a cold drink, this will take a few minutes to read, but you need to know.

To start; it’s widely known now that dogs cannot meet their Vitamin D requirements from sunlight – in case you didn’t know that one, it’s a fact. When I was starting out in this work, people were arguing about that one, all the time.  Here’s the technical write-up:

“Unlike many animals, dogs and cats have limited ability to convert 7-dehydrocholesterol in the skin to Vitamin3 and therefore must have a source (of Vitamin D3) in their diets. The level that is needed depends on the animal’s age and stage of development, as well as the concentration of calcium and phosphorus in the food. ”  Canine and Feline Nutrition:A Resource for Companion Animal Professionals, pg 109

…and of course, using the NRC Guidelines, we can add that bodyweight plays a key  role, as with all nutrients, to calculate the Recommended Allowance, and adjust it if the bigger picture requires more, or in some cases, a little less.

So, dogs can’t get Vitamin D from the sun. Where can they get it from, then, in their diets? And- what exactly is Vitamin D, and how much does a dog actually need?

Very briefly, Vitamin D is a sort of umbrella term, a “generic descriptor”  that refers to a group of compounds needed for the body to regulate calcium and phosphorus metabolism,as well as support immune system function and insulin synthesis. Vitamin D is a complex topic, and I am giving you just the surface facts today; simply put, there are two provitamin forms of D, and these are VitaminD2 (ergocalciferol) and D3 (cholecalciferol).  Vitamin D is a fat soluble vitamin, like Vitamins A, E and K. These vitamins need fats to be absorbed fully, can be stored in the body tissue and thus pose a greater risk of toxicity if over-consumed, than do the water-soluble B complex and beta-carotene. That said, you really do have to overdo it before toxicity is reached: The NRC Guidelines state that teh SUL (safe upper limit) for Preformed Vitamin A, is the dog’s Metabolic Weight  x 2099. This is possible only via supplements, a dog getting into a bottle of high dose A.  It’s not possible to ingest that much Vitamin A via food, period full stop. The same goes for D.

Vitamin D2 is plant-based; it forms when a compound called ergosterol, found in many plants, is exposed to ultraviolet light. Mushrooms in particular – WHEN allowed to sit in direct sunlight for a period of about 60minutes, convert ergosterol to VitaminD2.

VitaminD3 is  found in animal foods;  egg yolks, liver (all kinds have some) fish (content varies widely) and cod liver oil.

Studies have shown cats cannot utilize VitaminD2  efficiently, if at all; we don’t have the same amount of research specifically in dogs, but veterinary nutrition researchers affirm that since domestic dogs are closer to carnivores than omnivores, it follows that D3 is the preferable source, dietarily, for providing this important nutrient.

That has been my experience as well. In addition to a specific requirement for highly bioavailable VitaminD3,   some factors can indicate a higher need and I’ll review these in the tutorial (I only stick strictly to the RA in my recipes if there is a therapeutic indication to do so. Research indicates that the scope of Vitamin D’s health benefits go far beyond  what we know currently about it’s role in Ca:P metabolism…and that’s important enough).

Why is this important to share, today? Well the chief reason is this; it’s become popular, in some circles, to insist that dogs can derive all their dietary RA simply from eating mushrooms.

DESPITE  the fact mushrooms are not a solidly bioavailable source, this is continually promoted. . Sometimes it’s not even mentioned that they need to be exposed to sunlight (direct) before feeding.

And this: consider that  even  correctly exposed ‘shrooms, with their poorly available D2, still only actually provide very little of the vitamin. Properly prepped button mushrooms only add 7 IUs per 100 grams, and shiitakes, 15 IUs.

This probably isn’t even absorbed, or if it is, very little.

(To contextualize, a  25 kg dog requires about 200IUs daily. ..often, more is advisable, depending on the diet, age etc. We cover all of this in my courses and in the Nutrient Series on Facebook).

Those are a lot of ifs and maybes and “let’s hope for the best and not bother with the science” assumptions around the idea of relying on mushrooms for Vitamin D…assumptions I would never encourage you to rely on. Once you know the amount your dog actually requires, and the amount in the food you serve him, it’s a simple process to ensure he gets adequate intake.

What happens if you consistently underprovide D3 in a dog’s diet? Well it depends on other factors, including the calcium and phosphorus content of the individual diet as well as age, but the most common consequences of playing fast and loose with Vitamin D content  are

1) Rickets  (in growth diets; rickets is a serious skeletal condition)
2) Osteomalacia, a condition I have personally seen repeatedly, wherein the bones become fragile and easily broken
3) Nutritional Secondary Hyperparathyroidism, which you can read all about here: https://wagwalking.com/condition/nutritional-secondary-hyperparathyroidism

In addition, there is a strong indication that Vitamin D has cancer-protective action,  but let’s stick to what is absolutely known, for now.
Given that chronic low levels of D are a serious health issue for your dog, how to go about ensuring your home prepared diet provides enough?

Here’s what to do. 🙂

First, calculate your dog’s RA (recommended allowance) for Vitamin D, use animal/fish sources, if you can’t meet the RA in the diet, SUPPLEMENT.
To calculate the RA, take your dog’s weight in kgs to the power of 0.75,you can do this easily by setting your computer calculator to scientific and then use the button that looks like this:   x^y

and that gives you something called MW or metabolic weight.

Go on, go try it now. You can’t know how much to feed without this, or at least not with this level of accuracy.  Now you have the MW?  Multiply THAT number by 0.45, and then by 40. That’s the daily RA.  That’s it, that’s all. You may want to go higher, but you don’t want to go under this with a healthy dog. Now you tally up the dietary content – with Vitamin D it’s both easy, because the foods that provide it are so few, but harder than other nutrients too, because the USDA database doesn’t always give the content ,much of the time, so you have to do an estimate.

Here are the foods that contribute most VitaminD3, remember dairy foods are usually fortified and so are some cereals. But here are the key players:

Egg Yolk  – 40 IUs per raw large yolk

Beef liver – 42 IUs per 3 ounces cooked weight

Canned sardines, oil packed – 2 average sardines, 45 IUs

Fresh, poached salmon –  400 IUs per 3 ounces poached

Yogurt – 6 ounces whole fat (4%) plain fortified, about 80 – 120 IUs

Cod liver oil –  yes, CLO is loaded with Vitamin D3, but also very high in Vitamin A, so you may be going way over your goals with CLO. On average, and there is much variation, CLO provides 450 IUs of D3 per Teaspoon.  I rarely use it, although I have with some large breed dogs. A good quality supplement will be the answer for all cases where you need to boost significantly and the dog can’t have fish, or enough eggs/liver to cover the RA.

Five large egg yolks provide about 200 IUs of Vitamin D 3, still not enough to meet my 90 lb dog’s RA, but also add 23 grams of fat, which can push the total amount out of your dog’s digestive comfort zone.

So – the quickie version, because this is a Spotlight entry and not a  full write-up,  is this.

  1. Your dog has an absolute requirement for VitaminD, and D3, cholecalciferol, is the preferred source. D2 from mushrooms is not considered a reliable dietary source at all for cats, and while dogs *may* absorb a little, it is not considerable a viable dietary form. The bigger your dog is, the more restricted his food choices are or total energy intake, the more preposterous it is to think you can use mushrooms as a source at all.
  2. Mushrooms only provide VitaminD2, and only if properly exposed to UV light. Even after this preparation, their actual content and bioavailability is low.
  3. The foods we use to provide D3  – liver, egg yolk, fish – may still not meet the dietary requirement, depending on the size of the dog, their digestive tendencies (dogs who can’t eat a lot of fish, or egg for example, and remember that liver needs to be restricted to s smaller portion of total energy, with it’s very high copper content). These dogs need to have a good quality supplement added. Some things we’ll cover about Vitamin D at the Membership group as well as my courses – ? What conditions suggest more or less in the diet? Are the blood tests reliable? What factors may inhibit absorption of D3?  How do we figure out food content and convert from mcgs to RE or IUs or….??If we need to supplement, what do we look for in a product?I hope this has helped a little – I’ll be sharing this to my Facebook Page, so please feel free to ask questions there, or in the comments here. Always happy to help.

    Register for the Membership group at Facebook here: https://thepossiblecanine.com/product/the-possible-canine-membership-hub


  1.  National Institute of Health: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
  2.   Dietitians of Canada: https://www.dietitians.ca/getattachment/464f3006-0bb2-4f1a-a338-0b21d148bacb/FACTSHEET-Food-Sources-of-Vitamin-D.pdf.aspx
  3. Canine and Feline Nutrition, A Resource for Companion Animal Professionals, 3rd edition: Linda P. Case, MS, Leighann Daristotle, DVM, PhD, Michael G. Hayek, PhD, and Melody Foess Raasch, DVM4. National Research Council Nutrient Requirements of Dogs: https://www.nap.edu/catalog/10668/nutrient-requirements-of-dogs-and-cats
  4. Small Animal Clinical Nutrition, 4th Edition  https://s3.amazonaws.com/mmi_sacn5/2019/SACN5_6.pdf
  5. https://pubmed.ncbi.nlm.nih.gov/24001747/