Dogs, Vitamin D/ food sources vs synthetic, Digestion, Rickets {FB}

Vitamin D for Dogs

By CJ Puotinen

Reprinted from Whole Dog Journal, July 2016

We all need vitamin D – and so do our dogs. Without it, we suffer from bone diseases and a host of other problems.

But vitamin D is controversial and not well understood. When it comes to deciding how much is required, which sources are best, and how to supplement safely, experts disagree. Learning about vitamin D can help you make informed choices for your best friend as well as yourself.

Vitamin D research began long before it was identified and named. Between 1880 and 1930, the bone disease rickets affected children in industrialized areas where infections, crowding, and a lack of sunlight were common. Rickets causes soft, fragile bones. Cod liver oil, which contains vitamin D, was shown to prevent and cure the disease, and studies conducted on dogs and other animals proved that a nutritional deficiency of vitamin D caused rickets.

A steroid vitamin which is also classified as a hormone, vitamin D aids in the absorption of calcium and phosphate, increases bone cell activity, influences the formation and growth of long bones, and speeds the healing of fractures. But vitamin D does far more than build a strong skeleton. Adequate D levels may help prevent heart disease, joint inflammation, skin and coat problems, cancer, vision problems, depression, mental illness, infections, inflammatory bowel disease, dental problems, hyperparathyroidism, and kidney disease.

It is called the sunshine vitamin because sunlight on human skin produces vitamin D, which our bodies convert to a substance known as 25(OH)D, 25-hydroxycholecalciferol, 25-hydroxy, or vitamin D3. Sunlight is not considered a significant source of vitamin D for dogs. We can help prevent canine vitamin D deficiencies with specific foods and supplements.

Measuring D Levels

Vitamin D levels in pets and people can be measured with a blood test. Depending on the testing laboratory, results are measured in nanograms of 25-hydroxyvitamin D (calcifediol) per milliliter of blood, abbreviated ng/mL, or as nanomoles per liter, abbreviated nmol/L. To convert ng/mL to nmol/L, multiply by 2.5; to convert nmol/L to ng/mL, divide by 2.5.

Michigan State University’s College of Veterinary Medicine began offering canine vitamin D tests to veterinarians in the late 1980s. “That’s when we established a reference range based on the D levels of healthy dogs,” says Professor Kent Refsal, DVM. “The test became a diagnostic tool that helped veterinarians identify dogs with rickets, gastrointestinal disease, or other symptoms of vitamin D malabsorption or insufficiency as well as dogs with excessive vitamin D levels.”

Professor Refsal and his colleagues consult with veterinarians about their patients’ test results. The MSU laboratory’s vitamin D radioimmunoassay reference range for dogs is 60 to 215 nmol/L, or 24 to 86 ng/mL. “We consider this range to be a general indication of adequate to normal vitamin D levels for healthy dogs of all ages,” he says.

Veterinary Diagnostic Institute (VDI), which uses chemiluminescence immunoassay, reports its canine vitamin D blood test results as deficient (less than 25 ng/mL), insufficient (25 to 100 ng/mL), or sufficient (100 to 120 ng/mL). These measurements convert to less than 62.5 nmol/L (deficient), 62.5 to 249 nmol/L (insufficient), and 250 nmol/L (sufficient). However, because chemiluminescence immunoassay(CIA) and radioimmunoassay (RIA) have been shown to produce different results, a direct comparison of their reference ranges can be misleading and is not recommended.

In the May 2016 Critical Reviews in Food Science and Nutrition, veterinarians N. Weidner and A. Verbrugghe at the University of Guelph in Ontario, Canada, reviewed the current knowledge of vitamin D in dogs. After discussing the use of vitamin D tests for health screening, research on D levels and canine illnesses, and target D levels for optimum health, they concluded, “Further work is necessary before any consensus statements on blood 25(OH)D concentrations that define sufficiency in dogs can be made.”

In May 2016, researchers at Edinburgh University’s Royal School of Veterinary Studies in Scotland announced a series of research projects on pet dogs and vitamin D. Dr. Richard Mellanby, the university’s head of small animal medicine, explained, “Our research aims to understand whether dogs’ vitamin D levels fluctuate throughout the year, which is important for making sure we’re feeding our pets the right diet. We’re also interested in how vitamin D affects recovery after surgery and whether having less vitamin D is a cause or consequence of inflammation. Untangling this complex relationship will help us to devise new approaches to improve the welfare of animals after surgery.” Dr. Mallanby’s review article “Beyond the skeleton: The role of vitamin D in companion animal health” appeared in the April 2016 Journal of Small Animal Practice.

Pennsylvania veterinarian Linda Stern, DVM, began screening feline and canine patients with the VDI test last fall. “Of the 24 dogs we have tested so far,” she says, “only 29 percent had adequate vitamin D levels.”

Dr. Stern checks her patients’ D levels, supplements as necessary and retests after 10 to 12 weeks. “Dogs with arthritis tend to have significantly low vitamin D levels,” she says, “and when their levels improve, so does their range of motion. My general observation is that dogs feel better, have more energy, and look happier and healthier when their D levels are adequate. Some show dramatic improvement right away, which happened with one of our patients with liver disease. Monitoring patients with follow-up tests ensures that they maintain safe, optimum D levels.”

Too Much Is Toxic

Because vitamin D is fat-soluble, it accumulates in body fat. Overdoses can be toxic and even fatal.

Most canine fatalities related to vitamin D stem from the accidental ingestion of prescription drugs that contain vitamin D, such as topical medications for human skin conditions like psoriasis, and from the ingestion of rodenticides, which are poisons designed to control rats, mice, and other rodents.

Cholecalciferol (synthetic vitamin D3) was registered as a rodenticide in the United States in 1984. Toxic doses lead to too much calcium in the blood, which can affect the central nervous system, muscles, gastrointestinal tract, cardiovascular system, and kidneys.

Although less common, overdoses of vitamin D from foods and supplements can occur. Excessive vitamin D causes hypercalcemia (elevated calcium levels); anorexia (loss of appetite and extreme weight loss); excessive thirst, urination, drooling, and vomiting; muscle weakness; soft tissue mineralization; and lameness. In growing dogs, excessive vitamin D supplementation can disrupt normal skeletal development as a result of increased calcium and phosphate absorption.

In 1999, DVM Nutri-Balance high protein dog food and Golden Sun Feeds Hi-Pro Hunter dog food were recalled because of excessive vitamin D3 due to a feed-mixing error. This caused the illness and death of at least 25 dogs.

Seven years later, Royal Canin Veterinary Diet recalled four products due to a misformulation in the vitamin premix. Six dogs and five cats were reported to have clinical signs consistent with vitamin D3 toxicity.

In 2010, Blue Buffalo recalled packages of its Wilderness Chicken, Basics Salmon, and Large Breed Adult Chicken dry dog foods because of a sequencing error at the dry ingredients supplier, which allowed a more potent vitamin D used in chicken feeds to contaminate the dog formulas and increase their vitamin D to unacceptable levels. Vitamin D3 toxicity from the error affected at least 36 dogs.

In March 2016, four varieties of canned Fromm Family pet food were voluntarily recalled because the company’s analysis showed that these diets may contain excessive levels of vitamin D3.

Vitamin D in Commercial Pet Foods

“It is widely assumed that properly formulated commercial pet foods contain adequate D levels for canine health,” says Susan Howell, DVM, “but that isn’t true.” Dr. Howell provides veterinary technical support for Standard Process, Inc., a nutritional supplement manufacturer. “Foods are formulated to meet minimum nutrient requirements set forth by the Association of American Feed Control Officials, or AAFCO,” she says. “They are not formulated to meet optimal requirements.”

Dr. Howell cites a 2015 Tufts University study funded by VDI Laboratories that examined the effects of diet on the serum vitamin D levels of golden retrievers, German shepherds, and white shepherds. Most of the study’s 320 dogs were fed commercial diets from 40 different manufacturers, and some were fed homemade diets or a combination of commercial and homemade diets. As the report concluded:

Serum 25(OH)D concentrations in dogs vary widely, which likely reflects varying dietary vitamin D content. Notable differences exist among manufacturers and brands and may reflect differences in proprietary formulations. Given the variability of measured serum 25(OH)D concentrations in dogs and the importance vitamin D appears to have on health status, dietary vitamin D content should be optimized.

The study found that dogs on home-prepared diets had some of the most deficient vitamin D levels.

“In addition,” says Dr. Howell, “I spoke to a representative from VDI who said they had recently tested three golden retrievers, all having the same body weight and all eating the same diet. Each dog had a different serum vitamin D level. This shows that every animal is unique. They are dealing with their own variances, particularly in their ability to absorb and utilize vitamin D. Vitamin D absorption depends on good digestion. In my opinion, if D levels are deficient or insufficient, it may be as much a matter of addressing digestion as an issue of providing more vitamin D.”

Increasing D Levels by Improving Digestion

Dr. Howell recommends feeding dogs a variety of meat-based diets that are free from corn, wheat, soy, rice, white potatoes, tapioca, and peas: “Those foods are alkalizing to the stomach, and dogs need an acidic stomach for food to be digested and nutrients like vitamin D to be absorbed. The other problem with these ingredients is that they cause inflammation, which decreases nutrient absorption. As animals age, their stomachs become more alkaline, which explains why older animals may have a harder time breaking down and absorbing vitamin D from their food.”

For dogs fed dry food, she suggests adding bone broth or warm water before feeding. “Adding raw organic apple cider vinegar to food helps acidify the stomach,” she adds, “and it provides prebiotics, which feed gut microbes. Add 1/8 teaspoon to each meal for small dogs; 1/4 teaspoon for dogs weighing 21 to 50 pounds; and 1/2 teaspoon for dogs over 50 pounds.”

Whole Dog Journal contributor Mary Straus, whose dogaware.com website offers nutrition and feeding tips, recommends supplementing the diet with probiotics (active beneficial bacteria), prebiotics (foods that feed beneficial bacteria), and digestive enzymes to improve digestion and the assimilation of nutrients.

Like other fat-soluble vitamins, vitamin D requires dietary fat for assimilation. In the September 2006 Journal of the American Veterinary Medical Association, John E. Bauer, DVM, compared facilitative and functional fats in the canine diet. Saturated fats are facilitative, he wrote, because they enhance palatability, provide fuel for energy, can be stored in the body for future use, do not pose a health threat unless fed in excessive amounts, and assist in the digestion and assimilation of fat-soluble vitamins.

Coconut oil and butter contain saturated fats and are often listed as good companions to fat-soluble vitamins. Consider adding 1 teaspoon per 25 pounds of body weight to your dog’s dinner to help improve his or her vitamin D levels.

D in Home-Prepared Diets

While home-prepared diets may show the greatest variation in canine vitamin D levels, Dr. Howell notes that not every home-prepared diet has to be supplemented with vitamin D. “I’ll refer you back to the Tufts study,” she says. “Animals on balanced home-prepared diets may have sufficient D levels. It’s a matter of feeding foods that contain vitamin D, fostering healthy digestion, and possibly supplementing Vitamin D in a whole-food form or in a synthetic form if necessary. What I worry about is that people may over-supplement unknowingly and cause a toxicity in their pet.”

For this reason, she recommends that pet parents ask their veterinarians for help with homemade diets or turn to Balance IT (balanceit.com), a pet diet-planning website associated with the University of California, Davis. The dogaware.com website is another source of diet-planning information.

“I’m a big believer in animals getting their nutrients from real food,” says Dr. Howell. “Instead of supplementing with a synthetic form of vitamin D3, I think it’s worth getting some fresh foods into the diet that are good sources of D, such as salmon, liver, and eggs. It’s less likely that you will over-supplement if you give a food source of vitamin D rather than cholecalciferol, which is a high-dose synthetic form of vitamin D.”

“If an animal with insufficient D levels doesn’t have adequate levels after trying food sources of D,” she says, “I think it’s worth looking at digestion and then at a synthetic D supplement. A conservative amount of synthetic D can bring an animal into the sufficient range. Some popular synthetic vitamin D supplements are from Rx Vitamins and Thorne Research. These products are liquid and easy to dose and administer to your pet. Both are available by prescription and should be monitored by your veterinarian in conjunction with the diet in order to avoid over-supplementation.”

D-Insufficiency Risk Factors

Any dog can be D-deficient if his or her diet doesn’t supply the vitamin, but older animals, animals with compromised digestive health, spayed and neutered dogs, and dogs on corticosteroids, antacids, or anti-seizure medications are at added risk.

Dogs with illnesses like cancer, chronic inflammatory conditions, heart disease, renal disease, hyperparathyroidism, or inflammatory bowel disease are likely to have low vitamin D levels.

A 2014 study published in the Journal of Veterinary Internal Medicine examined the vitamin D status of 31 dogs with congestive heart failure (CHF) and 51 unaffected dogs. The dogs with CHF had significantly lower serum D levels than the unaffected dogs even though their vitamin D intake per kilogram of metabolic weight was the same. The study concluded that low concentrations of 25(OH)D may be a risk factor for CHF in dogs, that low levels were associated with poor outcomes in dogs with CHF, and that strategies to improve vitamin D status in some dogs with CHF may prove beneficial without causing toxicity.

In human heart disease, vitamin D deficiency is associated with disease progression and a poor prognosis. A 2015 cross-sectional study of dogs at different stages of chronic valvular heart disease (CVHD) found a similar correlation. As reported in the Journal of Veterinary Internal Medicine, the affected dogs’ vitamin D status declined prior to the onset of heart failure.

In the previously mentioned Tufts study, German shepherd dogs were found to have a 26-percent higher median amount of serum vitamin D than golden retrievers. “This means that intestinal absorption of vitamin D differed according to breed,” says Dr. Howell. “Spayed and neutered animals were found to have lower D levels than sexually intact dogs, and intact males had significantly higher serum D levels than intact females.”

Synthetic Vitamin D

In the wild, canines obtain vitamin D from the fat of prey animals. In the supplement aisle, vitamin D can come from natural sources but it’s more often synthetic.

The pharmaceutical drug cholecalciferol (synthetic vitamin D3) is produced by the ultraviolet irradiation of 7-dehydrocholesterol extracted from lanolin in sheep’s wool. Unwanted isomers formed during irradiation are removed in a purification process, leaving a concentrated resin that melts at room temperature.

Ergosterol, also called provitamin D2, is found in fungi such as Saccharomyces and other yeasts, mushrooms, and Claviceps purpurea, which causes the fungal disease ergot, for which ergosterol is named. Ergot affects rye, barley, wheat, and other cereal grasses. Ergosterol is converted by ultraviolet irradiation into ergocalciferol, or synthetic vitamin D2.

In 2006, the Journal of the American Academy of Dermatology reviewed vitamin D studies in order to answer the question, “How much vitamin D do you need, and how should you get it?” Although synthetic vitamin D2 is widely used as a prescription drug and is added to some processed foods, the study’s authors concluded that vitamin D3 is superior to vitamin D2 because it is less toxic at higher concentrations, is more potent, has a more stable shelf life, and is more effective than vitamin D2 at raising and maintaining vitamin D blood levels.

Food Sources of Vitamin D

If you’re interested in supplying natural vitamin D, it makes sense to look for foods that provide it, but finding them may not be easy.

Salmon is widely described as a significant source of vitamin D, but in 2007 the Journal of Steroid Biochemistry and Molecular Biology published an evaluation of the vitamin D content in fish. It found that salmon flesh does contain vitamin D, but farmed salmon – which is far more common and less expensive than wild salmon – had only 25 percent of the vitamin D of wild salmon. The report explained:

It has been assumed that fish, especially oily fish such as salmon, mackerel, and blue fish are excellent sources of vitamin D3. However, our analysis of the vitamin D content in a variety of fish species that were thought to contain an adequate amount of vitamin D did not have an amount of vitamin D that is listed in food charts. There needs to be a re-evaluation of the vitamin D content in foods that have been traditionally recommended as good sources of naturally occurring vitamin D.

Salmon oil may provide some vitamin D along with the fatty acids for which it is famous. In the Tufts study mentioned above, dogs receiving salmon oil as a supplement had higher serum 25(OH)D (on average a 19.6 ng/mL increase) than those not receiving a supplement, but other forms of fish oil surprisingly had no effect.

Dairy products are not naturally high in vitamin D, but milk and yogurt are often fortified with synthetic vitamin D. Check labels to be sure.

Cod liver oil is the traditional food source of vitamin D. A hundred years ago, fermented cod liver oil, which can have a powerfully fishy smell, was the world’s most widely prescribed nutritional supplement. Perhaps your grandparents remember having to swallow a spoonful at a time. Cod liver oil contains vitamins D and A, both of which are essential for human and canine health. But cod liver oil’s manufacturing methods have changed, and so has its vitamin content.

Fully cleaned and deodorized (e.g., molecularly distilled) cod liver oil to which nothing has been added contains very low levels of vitamin A and little or no vitamin D. Some manufacturers add synthetic or natural vitamins A and D to their cleaned and deodorized oil.

To compare brands, read labels – especially their vitamin A and D content – and check product literature or websites for information about manufacturing methods and the source of any added vitamins A and D.

Vitamins A and D are measured in International Units, abbreviated IU. The vitamin A content of natural (unprocessed) cod liver oil is usually 2 to 10 times that of its vitamin D.

To make cod liver oil more palatable to humans, some brands are available in lemon, orange, cinnamon, mint, or other flavors. Most dogs enjoy the plain, unflavored oil.

Carlson Labs cod liver oil, which is molecularly distilled and bottled in Norway, provides 850 IU vitamin A and 400 IU vitamin D per teaspoon. According to the label, its vitamins A and D, which are added after distillation, are derived from cod liver oil (500 ml or 16.9 fluid ounces, $55.00).

Garden of Life Olde World cod liver oil, made in Iceland, is molecularly distilled and contains vitamins A (4,500 IU per teaspoon) and D (450 IU per teaspoon). According to the label, these added vitamins are naturally occurring (8 fluid ounces, $17.45).

Green Pasture’s Blue Ice fermented cod liver oil. Fermented fish livers were valued in ancient Rome as a health tonic, and the same natural process is used to make Green Pasture’s cod liver oil. Because nutrients vary in fermented foods, the manufacturer labels this product a food without listing its vitamin D content, but current values are available on request. Based on the past four years of test data, one teaspoon of fermented cod liver oil contains approximately 8500 IU vitamin A and 3400 IU vitamin D (8 fluid ounces, $44.00).

Nordic Naturals arctic cod liver oil is molecularly distilled, and no vitamins are added after distillation. Each teaspoon provides 1580 IU vitamin A and 6 IU vitamin D. While this cod liver oil contains natural rather than synthetic vitamin D, 6 IU is an extremely small amount (8 fluid ounces, $25.95).

Nordic Naturals pet cod liver oil (16 fluid ounces, $44.95) and Nordic Naturals pet cod liver oil for medium to large breed dogs (8 fluid ounces, $24.95) contain omega-3 fatty acids and vitamin A (550 IU per teaspoon) but do not contain vitamin D. This brand will not correct vitamin D deficiencies.

Nutra Pro virgin cod liver oil from Norway is separated from fresh cod fish livers using cold-pressing and advanced purifying technologies without the use of chemicals. One teaspoon contains 5,000 IU vitamin A and 500 IU vitamin D (8 fluid ounces, $32.99).

Rosita extra virgin cod liver oil, or EVCLO, is manufactured in Norway from wild cod livers using an ancient extraction method that does not utilize heat, chemicals, fermentation, solvents, or mechanical devices. One teaspoon contains 3,000 to 5,000 IU vitamin A and 400 to 500 IU vitamin D (150 ml or 5 fluid ounces, $49.00).

Unlike “virgin” and “extra virgin” olive oils, whose labels reflect legally defined manufacturing and grading methods, the terms “virgin” and “extra virgin” have no specific meaning when applied to cod liver oil. They imply that the product is minimally processed.

The chemistry of naturally occurring cod liver oil is complicated. According to Christopher Masterjohn, PhD, assistant professor of health and nutrition sciences at Brooklyn College in New York, “Research in the 1930s suggested that there were at least four if not six forms of vitamin D in cod liver oil, and recent research has shown that fish metabolize vitamin D into at least three other compounds and probably more.” As conventional tests measure only vitamins D2 and D3, unrefined cod liver oil may provide significant health benefits that are not reflected by its D2 and D3 content.

Vitamin A Safety

Vitamin A is essential to human and canine bone growth, reproduction, immune system health, and vision. Like vitamin D, it is fat soluble. Synthetic vitamin A (retinyl acetate, retinol acetate, vitamin A acetate, vitamin A palmitate, retinyl palmitate, retinoids, or 13-cis-retinoic acid) should be used with care to avoid accidental overdoses, which can cause bone loss, hair loss, liver damage, and confusion.

Is natural vitamin A dangerous? According to some scientists and health experts, cod liver oil’s vitamin A content makes it potentially toxic. In 2008, Dr. John Cannell of the Vitamin D Council (vitamindcouncil.org) warned against using cod liver oil because of its vitamin A.

Other scientists and health experts disagreed, noting that vitamin A by itself (such as in molecularly distilled cod liver oil or cod liver oil containing synthetic vitamins) can be dangerous but that traditional cod liver oil contains a safe and effective ratio of naturally occurring vitamins A and D. In reply to the warnings against cod liver oil, Sally Fallon Morell, founder of the Weston A. Price Foundation (westonaprice.org), reviewed cod liver oil’s history and safety:

We at the Weston A. Price Foundation have continually pointed out that vitamins A and D work together and that without vitamin D, vitamin A can be ineffective or even toxic,” she explained. “We do not recommend Nordic Naturals or any brand of cod liver oil that is low in vitamin D. But it is completely inappropriate to conclude that cod liver oil is toxic because of its vitamin A content. Similar reviews could be put together showing the benefits of vitamin A and cod liver oil in numerous studies, including studies from the 1930s. Obviously the solution is to use the type of cod liver oil that does not have most of its vitamin D removed by modern processing techniques.

A Fermented Cod Liver Oil Controversy

In 2015, fans of fermented cod liver oil were rocked by the report “Hook, Line, and Stinker” by nutritionist Kaayla Daniel, PhD, in which she claimed that Green Pasture’s fermented cod liver oil is not a cod liver oil at all but rather rancid pollock oil.

Health researcher Craig Elding at the British site Health Cloud (healthcloud.co.uk), American health writer Chris Kresser (chriskresser.com/important-update-on-cod-liver-oil), and others examined these accusations in detail. See the Weston A. Price Foundation’s review of the controversy, including Morell’s November 2015 report titled “Hook, Line, and Thinker.” Years of independent tests have never shown Green Pasture’s Fermented Cod Liver Oil to have oxidative rancidity, and its source fish, Alaskan pollock (Gadus chalcogrammus), is not a member of the pollock fish family but rather a cod (Gadidae family) fish.

Cod Liver Oil in Home-Prepared Diets

One of the pioneers of home-prepared dog diets is Wendy Volhard, whose Holistic Guide for a Healthy Dog describes years of research she conducted with Kerry Brown, DVM, as they documented the effects of raw, home-prepared diets on hundreds of dogs.

“Since 1984, when I first published my recipes,” she says, “it’s no exaggeration to say that thousands of dogs have been fed the Volhard way. My diet recommends 1 teaspoon cod liver oil daily for a 50-pound dog. This dose was established in 1973, when I started feeding my own dogs a raw, home-prepared diet, and the amount was based on guidelines from the National Science Foundation.”

Volhard’s cod liver oil dose depends on the dog’s weight (1/2 teaspoon per 25 pounds). She says, “We have found no need to adjust the diet to a dog’s age or lifestyle. Puppies grow beautifully, and old dogs thrive.” She does not recommend a specific brand but prefers a minimally processed, high-quality cod liver oil containing natural vitamins A and D.

The Importance of Vitamin K

Vitamin K, another fat-soluble vitamin, influences proper blood clotting, healthy bone growth, the conversion of glucose into glycogen for energy storage in the liver, and healthy liver function. Vitamin K is thought to promote longevity and protect against cancers that involve the inner lining of body organs.

Vitamin K exists as vitamin K1 (phylloquinone), which is abundant in many vegetables; vitamin K2 (menaquinone), which the body produces in the digestive tract and which is provided by some animal products; and vitamin K3, the synthetic form known as menadione.

Vitamin K deficiencies can cause internal or external bleeding, most commonly resulting from the ingestion of rodent poisons containing warfarin or similar chemicals, and it is used as a first-aid treatment or antidote for dogs poisoned by blood-thinning rodenticides.

Vitamin K toxicity is unusual in pets, though excessive menadione (synthetic vitamin K3) can cause fatal anemia and jaundice. Menadione, which has been banned by the FDA for use in human supplements, is an ingredient in commercial pet foods, where it is labelled Vitamin K supplement, dimethylprimidinol sulphite or bisulfate, or menadione sodium bisulfite or bisulfate,

Supporters of K3’s use argue that natural vitamin K may lose its potency during processing, intestinal disease can prevent gut bacteria from making the vitamin, and not all pet foods contain green leafy vegetables. Opponents argue that synthetic vitamin K can promote allergic reactions, weaken the immune system, cause toxic reactions in liver cells, and induce red blood cell toxicity.

The leading food sources of vitamin K1 are green tea and dark green leafy vegetables such as kale, turnip greens, spinach, broccoli, lettuce, and cabbage.

Sources of natural vitamin K2 include meat, eggs, and dairy from grass-fed or pastured animals; high-vitamin butter oil, which is extracted by centrifusion from the raw milk of grass-fed cows; and natto (a traditional Japanese food) or MK-7 supplements made from fermented organic soybeans.

Because vitamin D is said to work best in combination with vitamins A and K, some veterinarians recommend supplementing dog diets, especially home-prepared diets, with natural sources of all three vitamins combined with an appropriate fat. Look for whole foods or supplements derived from whole foods and, if using a vitamin K supplement, adjust the recommended human adult dose for your dog’s weight.

In Review

Vitamin D is an essential nutrient for canine bone, heart, joint, skin, coat, vision, dental, kidney, and immune system health. Low vitamin D risk factors include age, spaying/neutering, digestive problems, illness, and some commonly prescribed medications.

Commercial pet foods vary in their vitamin D content and sources, producing different D levels in dogs. Some home-prepared diets contain insufficient vitamin D. Although many American dogs are D-deficient, their D levels can safely be increased by improving digestion, feeding whole foods that contain D, using vitamin D supplements if needed, and monitoring vitamin D blood levels through testing.

Because vitamin D is fat soluble, it needs dietary fat for digestion and assimilation. Vitamin D combines well with saturated fats such as coconut oil and butter. Its nutritional partners are the fat-soluble vitamins A and K. Maintaining adequate vitamin D, A, and K levels is a simple but effective canine health strategy.

Natural, unprocessed cod liver oil is a food source of vitamins D and A. Supplements containing synthetic vitamin D or vitamin A are more concentrated and require more careful monitoring.

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References

Bauer JE. Facilitative and functional fats in diets of dogs and cats. Journal of the American Veterinary Medical Association September 1, 2006; 229 (5).

Kraus MS, et al. Relation of vitamin D status to congestive heart failure and cardiovascular events in dogs. Journal of Veterinary Internal Medicine, 2014 Jan-Feb;28(1):109-15.

Lu Z, et al. An evaluation of the vitamin D3 content in fish: Is the vitamin D content adequate to satisfy the dietary requirement for vitamin D? Journal of Steroid Biochemistry and Molecular Biology. 2007 Mar; 103(3-5), 642-644.

Mellanby RJ. Beyond the skeleton: The role of vitamin D in companion animal health. Journal of Small Animal Practice. April 2016; 57 (4):175–180.

Osuga T, et al. Vitamin D Status in Different Stages of Disease Severity in Dogs with Chronic Valvular Heart Disease Journal of Veterinary Internal Medicine, 2015 Nov-Dec;29(6):1518-23.

Sharp CR, et al. The effect of diet on serum 25-hydroxyvitamin D concentrations in dogs. BMC Research Notes. 2015;8;442.

Volhard W. Holistic Guide for the Healthy Dog. 2^nd edition. Howell Reference Books, 2000. volharddognutrition.com.

Weidner N, Verbrugghe A. Current knowledge of vitamin D in dogs. Critical Reviews in Food Science and Nutrition. May 2016. DOI:10.1080/ 10408398.2016.1171202.

Wolpowitz D, Gilchrest B. The vitamin D questions: How much do you need and how should you get it? Journal of the American Academy of Dermatology. 2006 Feb;54(2):301-17.

Resources

Linda Stern, DVM, Healing Creatures Animal Hospital, Camp Hill, PA. (717) 730-3755; healingcreatures.com.

Susan Howell, DVM. Standard Process, Inc., standardprocess.com. Technical support for veterinarians.

Carlson Labs Cod Liver Oil, carlsonlabs.com. Sold in natural food stores and online.

Garden of Life Olde World Cod Liver Oil, gardenoflife.com. Sold in natural food stores and online.

Grassroots Health, grassrootshealth.net. Information and affordable at-home vitamin D blood tests for humans. (Not a canine information site.)

Green Pasture’s Blue Ice Fermented Cod Liver Oil and X-Factor Gold High-Vitamin Butter Oil, greenpasture.org. Sold online.

Michigan State University College of Veterinary Medicine Diagnostic Center for Population and Animal Health, animalhealth.msu.edu. Vitamin D blood tests for dogs.

Nutra Pro International Virgin Cod Liver Oil and Grass-Fed High-Vitamin Butter Oil, nutraprointl.com. Sold online.

Rosita Extra Virgin Cod Liver Oil, rositarealfoods.com or evclo.com. Sold online.

Rx Vitamins, rxvitamins.com. Liqui-D3 supplement provides 2,000 IU synthetic vitamin D per drop. Sold to veterinarians.

Thorne Research, thorne.com. Liquid synthetic vitamin D3, or D3 combined with vitamin K-2, 500 IU vitamin D per drop.

Veterinary Diagnostics Institute, vdilab.com. Vitamin D blood tests for dogs.

Weston A. Price Foundation, westonaprice.org. Information about vitamin D and cod liver oil.

Author’s Note

This article, which was published in the July 2016 Whole Dog Journal (whole-dog-journal.com), won the Dog Writers Association of America’s Maxwell Award for the outstanding canine health-related magazine article of 2016.

CJ Puotinen, author of The Encyclopedia of Natural Pet Care (McGraw-Hill, 2000) and other books, writes for The Whole Dog Journal and other publications. She lives in Montana.