Vitamin D for Dogs: Can They Get It from Sunlight?

Vitamin D is essential for bone health, immune function, muscle strength, and overall vitality in dogs. Yet vitamin D deficiency is surprisingly common, especially in dogs eating heavily processed diets or living in regions with limited access to vitamin D-rich foods.

This guide covers why dogs can't synthesize vitamin D from sunlight, what vitamin D does, signs of deficiency, and the best whole-food sources that deliver bioavailable vitamin D in safe amounts.

Why Can't Dogs Get Vitamin D from Sunlight?

Humans produce vitamin D when ultraviolet B (UVB) light from the sun hits the skin and converts 7-dehydrocholesterol (a cholesterol precursor) into vitamin D3 (cholecalciferol). This is why sun exposure is often recommended for maintaining healthy vitamin D levels in humans.

Dogs, however, cannot synthesize vitamin D from sunlight for two key reasons:

1. Fur Blocks UV Penetration

Dogs have this same precursor (7-dehydrocholesterol) in their skin, but their fur blocks UVB light from reaching the skin in sufficient amounts to trigger vitamin D synthesis. Even short-haired dogs don't produce meaningful amounts of vitamin D from sun exposure.

2. Dogs Don't Reabsorb Skin Oils

Dogs do produce oils on their skin that contain vitamin D precursors. In theory, licking their fur could allow them to ingest these oils and absorb the vitamin D. However, research shows that dogs don't absorb these oils efficiently through ingestion — the amounts are negligible compared to dietary needs.

The bottom line: Dogs are entirely dependent on dietary sources of vitamin D. Sun exposure doesn't contribute to their vitamin D status the way it does in humans.

What Does Vitamin D Do for Dogs?

Vitamin D is a fat-soluble vitamin that functions more like a hormone than a traditional vitamin. It regulates numerous physiological processes:

1. Calcium and Phosphorus Absorption (Bone Health)

Vitamin D's primary role is regulating calcium and phosphorus metabolism. It:

• Increases calcium absorption in the intestines
• Regulates calcium and phosphorus levels in the blood
• Supports bone mineralization and strength

Without adequate vitamin D, dogs cannot absorb calcium efficiently — even if calcium intake is adequate. This leads to rickets in puppies (soft, deformed bones) and osteomalacia in adults (weakened bones, fractures).

2. Immune Function

Vitamin D modulates immune response by:

• Enhancing the activity of immune cells (T-cells, macrophages)
• Reducing chronic inflammation
• Supporting the body's ability to fight infections

Dogs with low vitamin D levels are more susceptible to infections, autoimmune conditions, and chronic inflammatory diseases.

3. Muscle Function

Vitamin D supports muscle contraction and strength by regulating calcium availability. Deficiency leads to muscle weakness, poor coordination, and reduced physical performance.

4. Cardiovascular Health

Vitamin D receptors are found in heart tissue and blood vessels. Vitamin D helps regulate blood pressure, supports heart muscle function, and may reduce the risk of cardiovascular disease.

5. Cancer Prevention (Emerging Research)

Vitamin D receptors are present in many tissues, and vitamin D plays a role in regulating cell growth and differentiation. Some research suggests adequate vitamin D may reduce cancer risk, though more studies are needed in dogs specifically.

Signs of Vitamin D Deficiency in Dogs

Vitamin D deficiency develops gradually and symptoms vary by severity. Common signs include:

• Bone abnormalities in puppies (rickets): Bowed legs, enlarged joints, soft bones, stunted growth
• Bone pain or fractures in adults (osteomalacia): Difficulty walking, limping, reluctance to exercise
• Muscle weakness: Poor coordination, tremors, difficulty standing or climbing stairs
• Dental issues: Weak teeth, delayed tooth eruption in puppies
• Frequent infections: Weakened immune response, slow wound healing
• Lethargy: Low energy, reduced activity levels

Severe deficiency is rare in dogs eating commercial diets (which are fortified with synthetic vitamin D), but subclinical deficiency (low-normal levels) is more common and may contribute to long-term health issues.

Best Whole-Food Sources of Vitamin D for Dogs

Vitamin D occurs naturally in two forms:

• Vitamin D3 (cholecalciferol): Found in animal-based foods; more bioavailable for dogs
• Vitamin D2 (ergocalciferol): Found in plants and fungi; less efficiently utilized by dogs

Dogs absorb and utilize D3 far more effectively than D2, so animal-based sources are preferred.

1. Fatty Fish (Salmon, Mackerel, Sardines) (~400-600 IU per 3 oz)

Fatty fish are the richest natural source of vitamin D3 available. Wild-caught salmon, for example, provides:

• 400-600 IU vitamin D per 3 oz serving
• Omega-3 fatty acids (EPA/DHA) — anti-inflammatory, brain health
• High-quality protein
• Selenium — antioxidant function

Feeding 2-3 oz of salmon 2-3 times per week provides 250-400 IU vitamin D per serving — meeting or exceeding daily needs for most dogs.

2. Beef Liver (~40-50 IU per Ounce)

Beef liver provides moderate vitamin D along with a full spectrum of other nutrients:

• Vitamin A (retinol): 50x more than muscle meat
• B12, folate, iron, selenium, zinc

While not as concentrated in vitamin D as fish, liver's overall nutrient density makes it a valuable addition to any diet.

For more on liver's benefits, see Why Beef Liver is One of the Best Things You Can Feed Your Dog.

3. Egg Yolks (~40-50 IU per Yolk)

Egg yolks provide moderate vitamin D3 along with:

• Choline: Brain and liver health
• Biotin: Skin and coat health
• Selenium, lutein, zeaxanthin

Feeding 2-3 eggs per day (for a 50 lb dog) provides 80-150 IU vitamin D.

For more on egg yolks, see Choline for Dogs: The Forgotten Essential Nutrient.

4. Fish Liver Oils (Cod Liver Oil) (Very High — Use with Caution)

Cod liver oil is extremely high in vitamin D (400-1000 IU per teaspoon), making it easy to over-supplement. While it can be useful for addressing deficiency, it should be used carefully to avoid toxicity.

Fish liver oils also contain high levels of vitamin A, which can accumulate to toxic levels if over-supplemented.

5. Beef Kidney (~20-30 IU per Ounce)

Kidney provides small amounts of vitamin D along with B12, selenium, and riboflavin.

How Much Vitamin D Does My Dog Need?

Understanding vitamin D requirements requires looking at different levels:

1. AAFCO Minimum (To Prevent Deficiency)

AAFCO sets minimum vitamin D requirements at approximately 5-8 IU per pound of body weight daily. This is the baseline to prevent rickets and osteomalacia. For a 50 lb dog, that's 250-400 IU/day minimum.

2. What Most Dog Foods Contain

Most commercial dog foods meet AAFCO minimums by fortifying with synthetic vitamin D3 (cholecalciferol) after processing. A typical adult kibble provides 500-800 IU per 1000 kcal, which translates to roughly 250-400 IU per day for a 50 lb dog eating 2 cups of food.

While synthetic D3 is bioavailable, it lacks the co-factors (vitamin A, E, K, healthy fats) found in whole-food sources that enhance vitamin D utilization.

3. What Research Suggests for Optimal Health

Research suggests that optimal vitamin D intake — especially for immune health, bone density, and inflammation control — may be 10-15 IU per pound of body weight (roughly double AAFCO minimums). This supports:

• Stronger immune function and reduced infection risk
• Better bone density and calcium regulation
• Enhanced anti-inflammatory effects

For a 50 lb dog, optimal intake would be 500-750 IU/day. Blood testing shows dogs with levels of 70-100 ng/mL (vs 50-70 ng/mL on standard diets) have better health outcomes.

4. How Whole Foods Provide Vitamin D

Whole-food vitamin D comes with natural co-factors that enhance absorption and utilization. Here's how a 50 lb dog can meet optimal needs:

Daily/weekly whole-food additions:

• 3 oz wild-caught salmon 2-3x per week: ~400-600 IU per serving (averages 115-170 IU/day)
• 2 egg yolks daily: ~80-100 IU/day
• 1 oz beef liver daily: ~40-50 IU/day
• Base diet (fortified kibble/raw): ~250-400 IU/day

Total vitamin D: ~485-720 IU/day — well within optimal range (500-750 IU) without risk of toxicity.

Because whole-food vitamin D is packaged with fat-soluble vitamin co-factors (A, E, K), it's utilized more effectively than synthetic D3 alone — meaning better results with the same or lower total intake.

Vitamin D Toxicity: Can Dogs Get Too Much?

Yes. Unlike water-soluble vitamins (B vitamins, vitamin C), vitamin D is fat-soluble and can accumulate in the body to toxic levels.

What Causes Vitamin D Toxicity?

• Over-supplementation with synthetic vitamin D — Especially high-dose supplements or cod liver oil
• Accidental ingestion of rodenticides — Many contain cholecalciferol (vitamin D3) as the active ingredient
• Contaminated commercial dog food — Rare but has occurred due to manufacturing errors

Toxicity from whole-food sources (fish, eggs, liver) is extremely rare because the amounts in food are naturally balanced.

Signs of Vitamin D Toxicity (Hypervitaminosis D)

• Hypercalcemia (elevated blood calcium): The primary mechanism of toxicity
• Vomiting and loss of appetite
• Excessive thirst and urination (polydipsia/polyuria)
• Weakness and lethargy
• Kidney damage — Calcium deposits in kidneys (nephrocalcinosis)
• Calcification of soft tissues — Heart, lungs, stomach

Severe toxicity can be fatal. If you suspect vitamin D toxicity, seek veterinary care immediately.

Safe Upper Limit

The safe upper limit for vitamin D in dogs is approximately 100-150 IU per pound of body weight per day. For a 50 lb dog, that's 5000-7500 IU/day — far higher than typical dietary intake from whole foods.

Toxicity is a concern primarily with high-dose synthetic supplements, not whole-food sources.

Whole-Food Vitamin D vs Synthetic Vitamin D

Most commercial dog foods are fortified with synthetic vitamin D3 (cholecalciferol) to meet AAFCO minimums. While synthetic D3 is bioavailable, whole-food sources offer advantages:

Food-Based Vitamin D3 (Fatty Fish, Liver, Eggs)

• Form: Naturally occurring cholecalciferol, already integrated into the food matrix
• Absorption: Enhanced by co-factors (healthy fats, fat-soluble vitamins A, E, K)
• Safety: Self-regulating — difficult to over-consume from whole foods
• Co-benefits: Comes packaged with omega-3s, protein, vitamins, minerals

Synthetic Vitamin D3 (Cholecalciferol)

• Form: Isolated compound added to food or supplements
• Absorption: Good, but lacks co-factors that enhance utilization
• Safety: Risk of over-supplementation, especially with high-dose supplements
• Co-benefits: None — provides only vitamin D

For more on whole-food nutrients vs synthetics, see Whole Food vs. Synthetic: Why Bioavailability Matters in Your Dog's Nutrition.

Testing Vitamin D Levels in Dogs

If you're concerned about your dog's vitamin D status, your veterinarian can test blood levels of 25-hydroxyvitamin D [25(OH)D] — the most accurate marker of vitamin D status.

Optimal Levels

• Deficient: <30 ng/mL
• Insufficient: 30-50 ng/mL
• Optimal: 50-100 ng/mL
• High (potential toxicity risk): >150 ng/mL

Dogs eating commercial diets typically fall in the 50-80 ng/mL range. Dogs eating fresh, whole-food diets with fatty fish and organ meats often have levels in the 70-100 ng/mL range — optimal for long-term health.

Why Watts Uses Vitamin D-Rich Whole Foods

At Watts, we prioritize beef liver and other organ meats because they provide vitamin D along with a full spectrum of co-factors that support absorption and utilization:

• ~40-50 IU vitamin D per ounce from beef liver
• Co-factors like vitamin A, selenium, and healthy fats that enhance vitamin D function
• No risk of over-supplementation — whole-food vitamin D is naturally balanced

We don't rely solely on synthetic vitamin D because:

• Lacks co-factors: Isolated vitamin D doesn't provide the supporting nutrients found in whole foods
• Higher risk of imbalance: Easier to over-supplement with synthetic forms
• Less comprehensive: Whole foods deliver vitamin D plus omega-3s, protein, vitamins, and minerals

By using organ meats and encouraging fatty fish in the diet, Watts delivers vitamin D the way dogs have evolved to absorb it — naturally, safely, and effectively.

Final Thoughts

Vitamin D is an essential nutrient that dogs cannot produce from sunlight. They rely entirely on dietary sources — and many don't get enough, especially those eating heavily processed diets with minimal animal products.

The best sources of vitamin D for dogs are fatty fish (salmon, sardines), egg yolks, and organ meats (liver, kidney). These whole foods provide bioavailable vitamin D3 along with co-factors that enhance absorption and utilization — without the risk of toxicity that comes with high-dose synthetic supplements.

If you want to ensure your dog gets adequate vitamin D naturally, add fatty fish 2-3 times per week, include egg yolks daily, and incorporate organ meats like beef liver into their regular diet.

And if you want a simple, pre-portioned way to give your dog whole-food vitamin D (plus all the other nutrients that come with it), that's exactly what Watts is designed to do.