Gram Research analysis of a 2026 study in the Journal of Veterinary Internal Medicine found that most dogs born with a congenital portosystemic shunt have abnormal vitamin B12 or folate processing, even when their blood B12 looks normal. After surgery closing the vessel, 31% of dogs with normal B12 still had high methylmalonic acid 12 weeks later.
Gram Research analysis of a study published in the Journal of Veterinary Internal Medicine looked at dogs born with a congenital portosystemic shunt, an abnormal blood vessel that lets blood skip past the liver. Researchers compared 44 affected dogs with 39 healthy dogs of similar age and weight. Most affected dogs had normal vitamin B12 levels in their blood, but many showed signs that their cells were not using vitamin B12 and folate properly. After surgery to close the vessel, many results improved, though some dogs still had abnormal levels 12 weeks later. Dogs whose vessel was fully closed were more likely to have normal results than dogs whose closure was incomplete. The findings suggest that vitamin problems are common in these dogs and may need follow-up with a veterinarian.
Key Statistics
A 2026 study of 83 dogs published in the Journal of Veterinary Internal Medicine found that 61% of dogs with normal vitamin B12 blood levels and available tests had higher-than-normal methylmalonic acid in their blood before surgery.
A 2026 study of dogs with a congenital portosystemic shunt found that 31% of dogs with normal vitamin B12 still had high blood methylmalonic acid 12 weeks after surgery to close the abnormal vessel.
In the same 2026 study, dogs whose abnormal blood vessel was completely closed were more likely to have normal methylmalonic acid levels than dogs whose closure was incomplete, a difference that was statistically significant.
The Quick Take
- What they studied: The researchers wanted to know whether dogs born with an abnormal liver blood vessel have trouble with vitamin B12 and folate, and whether closing the vessel with surgery fixes these problems.
- Who participated: The study included 83 dogs. Forty-four had a congenital portosystemic shunt, and 39 were healthy dogs matched for age and body weight. Of the affected dogs, 32 had the vessel inside the liver and 12 had it outside the liver.
- Key finding: Before surgery, most affected dogs had normal vitamin B12 blood levels, but 61% of those dogs had higher-than-normal methylmalonic acid in their blood. This marker rises when cells lack enough vitamin B12. Twelve weeks after surgery, many results improved, but 31% of dogs with normal B12 levels still had high methylmalonic acid.
- What it means for you: If your dog has been diagnosed with this vessel defect, vitamin status may be worth discussing with your veterinarian, even when standard blood tests look normal. Blood tests that measure vitamin use inside cells may give a more complete picture. Owners should not change a dog’s diet or start supplements without veterinary guidance.
The Research Details
This was a prospective study, which means the researchers followed the dogs forward in time and measured them at set points. They compared affected dogs with healthy dogs of similar age and weight to see how their vitamin levels differed.
The team measured vitamin B12, a carrier protein that moves B12 through the body, folate, and several substances that rise when vitamin B12 or folate is not being used properly inside cells. These substances include homocysteine, which is a compound the body makes during normal metabolism, and methylmalonic acid, which is measured in both blood and urine. The team also measured ammonia, a waste product that the liver normally clears.
Affected dogs were tested before surgery and again 12 weeks after the abnormal vessel was closed. The researchers also compared dogs whose closure was complete with dogs whose closure was only partial. This design lets readers see changes over time within the same dogs, not just differences between two separate groups.
Measuring both blood vitamin levels and the substances that rise when cells are short on vitamins gives a fuller picture than a single test. A dog can have normal B12 in its blood while its cells still lack enough of it. Following the same dogs before and after surgery also shows whether the problem improves when the cause is treated.
The study had a clear comparison group of healthy dogs, and the groups were matched for age and weight, which helps reduce outside influences. The researchers used many separate measurements, which makes the results more detailed. However, the sample was 83 dogs, which is modest, and the study was done in a specialized veterinary setting, so results may not apply to every dog. Because the study reports many comparisons, some results could be chance findings that need confirmation in larger studies.
What the Results Show
Before surgery, dogs with this vessel defect had lower homocysteine and higher methylmalonic acid levels than healthy dogs. Both differences were statistically significant, meaning they were unlikely to be due to chance. Folate levels were also lower in the affected dogs. These patterns suggest that their bodies were not handling vitamin B12 and folate well.
Most affected dogs had normal vitamin B12 blood levels before surgery. Only 2 of 44 dogs had low B12 in their blood, while 8 had high B12 levels. Even among dogs with normal B12, many showed abnormal markers. Of the dogs with normal B12 and available homocysteine tests, 47% had homocysteine below the normal range. Methylmalonic acid was high in 61% of these dogs in the blood and in 27% in the urine. This pattern points to vitamin B12 not being used properly inside cells, even when blood levels look fine.
After surgery, several results improved. Dogs had lower B12 and a carrier protein called transcobalamin, along with higher homocysteine and higher folate levels than before surgery. Urine methylmalonic acid also decreased. However, some dogs still had abnormal results at 12 weeks. Of the dogs with normal B12 at that point, 31% still had high blood methylmalonic acid and 10% still had high urine levels.
Dogs whose vessel was completely closed were more likely to have normal methylmalonic acid results than dogs whose closure was incomplete. This suggests that more complete closure leads to better vitamin use in the body.
Ammonia was high in 35 of 43 affected dogs before surgery, which is common with this condition because the liver is not clearing waste well. However, ammonia levels did not match the vitamin results in any clear way, so ammonia alone does not explain the vitamin problems. Ammonia levels are expected to improve after surgery, and this high ammonia is one reason the vessel is usually treated.
Dogs with the vessel inside the liver had higher methylmalonic acid after surgery than dogs with the vessel outside the liver. This suggests that the location of the vessel may affect how well vitamin use recovers, though more research is needed to understand why.
Liver disease and bypass vessels are known to disturb nutrient handling, and vitamin B12 problems have been reported in dogs with liver disease before. This study adds detail by using several markers together in dogs with a specific congenital vessel defect, showing that normal blood B12 does not rule out a problem. It also adds information on what happens after surgery, a time period that has been less studied.
The study included 83 dogs, which is a moderate number, so rare patterns may not have been detected. Only a subset of dogs had homocysteine tests, so those results are based on fewer animals. The 12-week follow-up is useful, but it does not show whether vitamin problems persist over longer periods or whether they cause visible health effects. The study also does not prove that giving vitamin supplements would help. Finally, because the study involved specialized veterinary centers, results may differ for dogs treated elsewhere.
The Bottom Line
Research suggests that dogs with a congenital portosystemic shunt often have abnormal vitamin B12 and folate use inside their cells, even when standard blood B12 looks normal. Confidence in this finding is fairly strong because it compares affected dogs with healthy dogs and tracks changes after surgery. Research also suggests that complete vessel closure is linked to better vitamin results. Whether routine vitamin supplementation improves outcomes is not yet established, so confidence in specific supplement recommendations is low. Owners should discuss any vitamin testing or supplementation plan with their veterinarian.
Owners of dogs diagnosed with a congenital portosystemic shunt, veterinarians who treat these dogs, and veterinary nutrition specialists should find these results useful. People who own healthy dogs or who have no history of this vessel defect will not need to act on these findings. Human readers looking for nutrition advice for themselves should not apply these results directly, since the study was done in dogs.
In this study, many vitamin-related results improved within 12 weeks of surgery, but some abnormalities remained at that point. Owners should expect that recovery may take longer than the first few weeks, and that follow-up testing may be needed. Your veterinarian can advise on how often to recheck blood and urine results for your dog.
Frequently Asked Questions
Can a dog with a liver blood vessel defect have a vitamin B12 problem even if blood tests look normal?
Yes-style reassurance aside, the answer is that it can. A 2026 study found that most affected dogs had normal blood B12, yet 61% of those with available tests had high methylmalonic acid, suggesting cells were not using B12 properly. Discuss testing options with your veterinarian.
Does surgery to close a portosystemic shunt fix vitamin problems in dogs?
Surgery improved many vitamin results within 12 weeks, but not all of them. In a 2026 study of 83 dogs, 31% of dogs with normal B12 still had high blood methylmalonic acid after surgery. Complete closure produced better results than incomplete closure.
What is a congenital portosystemic shunt in dogs?
It is a blood vessel present from birth that carries blood around the liver instead of through it. Because the liver cannot filter that blood properly, toxins and waste can build up, and nutrients may not be processed well. Surgery is the usual treatment to close the vessel.
Should I give my dog vitamin B12 supplements if it has a portosystemic shunt?
Research has not yet shown that routine vitamin B12 supplements improve outcomes in these dogs. Because each dog’s needs differ, supplements should only be started after a veterinarian reviews blood and urine test results and recommends a specific plan.
Want to Apply This Research?
- Log your dog’s vet visit dates, the specific blood and urine tests ordered (such as vitamin B12, folate, methylmalonic acid, and ammonia), and the results. Record the date of surgery and each follow-up test so you can see trends over time.
- Keep a simple log of your dog’s daily food, treats, and any supplements, and bring it to each vet appointment. Do not add or change vitamin supplements without first checking with your veterinarian, since the right plan depends on your dog’s test results.
- Track your dog’s appetite, energy level, and any signs of neurological change, such as confusion or unsteady walking, in a weekly log. Review these notes with your veterinarian at each recheck. Schedule follow-up blood and urine tests as your veterinarian recommends, and store all results in one place so trends are easy to see.
This article summarizes a veterinary research study for general information only. It is not a substitute for advice from a licensed veterinarian. Do not start, stop, or change any vitamin, supplement, or diet for your dog without consulting your veterinarian, especially if your dog has a liver condition or has had surgery.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.