Why Are Grapes Toxic to Dogs? The Tartaric Acid and OAT4 ScienceUpdated with 2024 Research
Emergency reminder: If your dog has eaten grapes or raisins, call (888) 426-4435 immediately, then read this page.
Short Answer: Tartaric Acid + Missing Kidney Transporter
Grapes and raisins contain tartaric acid and its potassium salt, potassium bitartrate (also called cream of tartar). In most species, including humans, an organic acid transporter protein called OAT4 in kidney cells efficiently clears tartaric acid from the body through urine. Dogs have very low expression of OAT4 in their kidneys. Without this transporter, tartaric acid accumulates inside the proximal renal tubule cells of the dog's kidneys, destroying those cells and causing acute kidney injury (AKI).
This is why raisins are more dangerous than fresh grapes: drying concentrates tartaric acid approximately 4-5 fold, delivering a much higher dose per gram.
This is also why individual dogs respond differently: genetic variation in OAT4 expression and baseline kidney health mean some dogs clear tartaric acid better than others. This variation is why some dogs appear unaffected by grapes while others develop AKI from a small amount.
The History: Why Older Content Still Says “Unknown Cause”
Grape and raisin toxicity in dogs was first formally documented in the 1990s through case reports to the ASPCA Animal Poison Control Center. The clinical presentation was well described: vomiting followed by progressive acute kidney injury. But despite extensive investigation, veterinarians could not identify the specific toxic agent. It was not a pesticide residue (organic grapes were equally toxic). It was not the seeds, skin, or flesh in isolation. It was not a fungal contaminant. The cause remained a genuine mystery.
The breakthrough came from an unexpected direction. In 2021, Dr Renee Schmid of the Pet Poison Helpline noted that several dogs had developed an identical clinical picture after eating foods containing cream of tartar (potassium bitartrate), which is a grape-derived product used in baking. Cream of tartar contains no fructose, no pesticides, no seeds, and no identifiable component other than tartaric acid and its potassium salt. The connection was clear.
This observation was followed by the formal case series published by Wegenast et al. in 2022 in the Journal of Veterinary Emergency and Critical Care, describing dogs developing AKI after cream of tartar ingestion, providing strong evidence that tartaric acid was the culprit in grape toxicity. Further validation came from in-vitro research (Gwaltney-Brant et al., 2023) demonstrating that tartaric acid kills Madin-Darby canine kidney (MDCK) cells in laboratory conditions. A comprehensive scoping review by Downs et al. published in the Veterinary Record in 2024 synthesised the evidence base and concluded that tartaric acid and potassium bitartrate represent the most plausible toxic agents.
This research is recent enough that most consumer-facing pet content has not caught up. Hill's, PetMD, and many other major sites still carry the “cause unknown” language. We believe owners deserve to know the current science.
The OAT4 Transporter: Plain-English Explanation
Organic anion transporters (OATs) are proteins embedded in cell membranes that act like molecular pumps, moving organic acid molecules across cell walls. They are critical to the kidney's ability to filter waste products from the blood and excrete them in urine.
OAT4 (also known as SLC22A11) is the specific transporter responsible for moving tartaric acid and structurally similar organic acids into kidney tubule cells and then out into the urine for excretion. In humans and most other mammals, OAT4 expression in the kidney is high enough to efficiently clear dietary tartaric acid, which is naturally present in many foods including grapes, tamarind, and wine.
Dogs have substantially lower OAT4 expression in their kidneys. This appears to be a species-level difference. When a dog consumes tartaric acid, it enters the proximal renal tubule cells (the cells lining the kidney's filtration tubules) but cannot be efficiently transported out for excretion. The acid accumulates inside the cells. At sufficient concentrations, this accumulation damages and kills the proximal tubule cells, disrupting kidney filtration. The clinical result is acute kidney injury.
Analogy: A Drain with Reduced Flow
Think of the OAT4 transporter as a drain in a sink. In humans, the drain flows well and tartaric acid is quickly removed. In dogs, the drain is partially blocked. Tartaric acid slowly accumulates in the sink (kidney cells). If enough builds up, the sink overflows and the cells are damaged. The amount needed to overflow the drain depends on how blocked the drain is - which varies between individual dogs, explaining why some dogs are affected by small amounts while others seem more resistant.
Why Individual Dogs Respond So Differently
The most puzzling aspect of grape toxicity for both owners and veterinarians has always been the extreme variability between individual dogs. Some dogs have eaten handfuls of raisins with apparently no ill effect. Others have developed acute kidney injury from a small number of grapes. This variability is not random - it reflects genuine biological differences between individuals.
OAT4 expression variability
Genetic variation between individual dogs in the SLC22A11 gene (which codes for OAT4) may result in some dogs having higher OAT4 expression than others. A dog with relatively higher OAT4 expression may be able to clear more tartaric acid before tubule cell damage occurs.
Baseline kidney function
Dogs with pre-existing kidney disease or reduced kidney reserve have less capacity to tolerate any additional insult. An older dog with early chronic kidney disease may develop AKI from a dose that a younger dog with full kidney function could partially tolerate.
Individual gut absorption
How much tartaric acid is absorbed from the gut into the bloodstream may vary between dogs based on gut microbiome composition, stomach acidity, and other factors.
Hydration status
A well-hydrated dog may dilute and excrete more tartaric acid than a dehydrated dog where kidney concentrating ability is already stressed.
Critically, there is currently no test available to determine how sensitive your individual dog is to tartaric acid. You cannot know in advance whether your dog is in the “high tolerance” or “low tolerance” category. This is why all grape or raisin ingestions must be treated as potentially serious.
Key Research Papers
Schmid R, Brutlag A. (2021)
Initial tartaric acid hypothesis presentation
Pet Poison Helpline / ASPCA APCC conference
Dr Renee Schmid first presented the tartaric acid hypothesis publicly, noting the clinical similarity between grape/raisin toxicity cases and cream of tartar ingestion cases in dogs.
Wegenast C et al. (2022)
Acute kidney injury following ingestion of cream of tartar in dogs
Journal of Veterinary Emergency and Critical Care
Formal case series documenting dogs developing AKI after cream of tartar ingestion. This provided strong evidence linking tartaric acid specifically to the clinical syndrome seen after grape ingestion.
Gwaltney-Brant S et al. (2023)
In-vitro tartaric acid toxicity in Madin-Darby canine kidney cells
Published abstract / proceedings
Laboratory research confirming that tartaric acid kills dog kidney cells (MDCK cells) in controlled in-vitro conditions. This provided mechanistic evidence for the clinical observations.
Downs M et al. (2024)
Grape, raisin and currant toxicity in dogs: a scoping review
Veterinary Record
Comprehensive scoping review of the evidence base for grape, raisin, and currant toxicity in dogs. Concluded that tartaric acid and potassium bitartrate represent the most plausible toxic agents and synthesised what is currently known about dose, mechanism, and prognosis.
What the Research Does NOT Yet Settle
The tartaric acid / OAT4 hypothesis is the current leading explanation but is not yet definitively proven. Several questions remain open in the veterinary literature:
- ?The exact LD50 (lethal dose 50%) in dogs has not been determined - the current minimum documented AKI doses are from case reports, not controlled studies.
- ?The specific genes responsible for OAT4 variation between dog breeds and individuals have not been fully characterised.
- ?Whether some grape varieties have higher or lower tartaric acid content than others, and whether this explains why some grapes appear more toxic in case reports.
- ?The role of potassium bitartrate (cream of tartar) versus free tartaric acid - whether the salt or the acid is the more directly toxic form.
- ?Whether any supportive treatments (such as alkalinisation of urine) might improve clearance of tartaric acid in affected dogs.