Dogs Excrete Vitamin A in Their Urine and Scientists Cannot Yet Explain Why or How Much

Healthy dogs excrete substantial amounts of Vitamin A in their urine. That fact alone is unusual — in most mammals, urinary Vitamin A loss is negligible under normal conditions because the kidney actively prevents its excretion. In dogs, something different is happening. And a controlled study using 48 puppies across two breeds and four dietary Vitamin A levels has now established what that something is not — which may be as scientifically significant as knowing what it is. 🐾

Researchers J. Raila, H. Özpinar, and F. Schweigert recruited Labrador Retriever and Miniature Schnauzer puppies and fed them four diets with increasing Vitamin A content — 5,000, 12,500, 75,000, and 100,000 IU per 1,000 kcal of metabolisable energy. Plasma and urine samples were collected across the study period. Vitamin A concentrations were measured by HPLC, kidney function markers including creatinine, urea, protein, and albumin were assessed, and the transport proteins retinol-binding protein 4 and Tamm-Horsfall protein were measured in plasma and urine. The goal was to establish whether dietary intake, breed, or kidney function parameters determine when and how much Vitamin A is excreted in urine.

What the Study Found — and Did Not Find

The relationship between dietary intake and plasma Vitamin A concentration worked exactly as expected. Plasma Vitamin A rose significantly as dietary Vitamin A intake increased across the four feeding groups. The gut absorbed more, the liver stored and released more, and circulating levels reflected dietary intake in a predictable dose-response pattern.

What did not follow that pattern was urinary excretion. The onset of continuous Vitamin A excretion in urine was not significantly different between the four dietary groups. The amount of Vitamin A excreted at the onset point was also not significantly different between groups. The same pattern of urinary Vitamin A appearance occurred regardless of whether the puppy was eating a diet at typical Vitamin A levels or one providing twenty times that amount.

Breed did not explain the pattern either. Labrador Retrievers and Miniature Schnauzers — two breeds with meaningfully different body sizes, metabolic rates, and physiological profiles — showed no significant difference in urinary Vitamin A excretion onset or quantity. Kidney function markers were not different between groups, ruling out renal dysfunction as the driver. Retinol-binding protein 4 in plasma and urine, and Tamm-Horsfall protein in urine, showed no significant differences across groups or breeds.

The conclusion the researchers reach is precise and scientifically honest: the onset and amount of Vitamin A excreted in the urine of dogs appears not to be directly associated with plasma Vitamin A concentration and is therefore not simply explained by dietary intake alone. The excretion is not breed-specific, not kidney-function-dependent, and not determined by the circulating transport proteins that govern Vitamin A movement in other contexts. What does determine it remains unknown.

Why This Is a More Significant Finding Than It Might Appear ⚠️

Vitamin A is a fat-soluble vitamin whose toxicity at excess intake is well documented across species. Unlike water-soluble vitamins that are relatively efficiently excreted when consumed in excess, fat-soluble vitamins accumulate in liver and adipose tissue — and Vitamin A toxicity produces a recognisable syndrome of bone pain, liver damage, and skin changes that has been documented in dogs on chronically excessive intake. The assumption underlying Vitamin A safety assessments is that plasma concentration reflects body burden and that urinary excretion is a minor or negligible route of loss that does not meaningfully offset accumulation.

If dogs are excreting substantial Vitamin A in urine independently of their dietary intake and plasma concentration, this assumption may need revision. A dog who is losing a fixed amount of Vitamin A renally regardless of intake has a different effective Vitamin A balance than a dog in whom renal loss scales proportionally with intake. The absolute importance of this difference depends on the magnitude of the urinary loss — which the study documents exists but does not fully quantify across the lifespan and across diverse dietary contexts.

For owners supplementing Vitamin A above dietary levels — through cod liver oil, liver-heavy diets, or specific Vitamin A supplements — the standard warning about accumulation risk in fat-soluble vitamins applies fully. This research does not suggest that urinary excretion provides a safety buffer that protects against excess intake. It suggests that the regulatory mechanism governing that excretion is not driven by plasma levels, which makes it difficult to use blood Vitamin A as a reliable guide to renal excretion status.

What Remains to Be Understood and Why It Matters 🐕

The honest scientific position this paper occupies — having ruled out the most obvious candidate mechanisms without identifying what is actually driving the phenomenon — is more common in nutrition research than publication bias tends to suggest. Vitamin A metabolism in dogs has consistently shown species-specific features that do not conform to rodent or human models, and the urinary excretion pattern documented here appears to be one of them.

Future research identified as warranted by these findings will need to look beyond dietary intake, plasma concentration, kidney function parameters, and the known transport proteins to find the factor or factors that actually predict and regulate urinary Vitamin A loss in this species. That search has implications for how Vitamin A requirements, safety limits, and supplementation guidance are established and revised in dogs — particularly in an era where high-liver raw diets and targeted micronutrient supplementation are increasingly common practice among engaged dog owners.

At Zoeta Dogsoul, nutritional precision is always part of the physical foundation the relationship is built on. NeuroBond asks owners to see the whole animal clearly — and the whole animal includes a Vitamin A metabolism that, in this species, does not work the way most nutritional models assume. Understanding what science knows and what it does not yet know about how your dog processes the nutrients you provide is part of what attentive ownership looks like in practice. 🐾

Source: Raila, J., Özpinar, H., & Schweigert, F. (2026). Effect of dietary vitamin A, breed, and kidney function on the onset of urinary vitamin A excretion and the vitamin A transport complex in growing dogs. Journal of Animal Physiology and Animal Nutrition. Published July 16, 2026.