Diarrhea in diabetic cats is surprisingly common, and it rarely has a single cause. The condition can stem from diabetes itself damaging the gut’s nerve and muscle function, from pancreatic disease that often accompanies diabetes, from the high-protein diet your cat was switched to, or from a serious complication like ketoacidosis. Understanding the likely cause matters because some of these are manageable at home while others need urgent veterinary care.
How Diabetes Itself Disrupts the Gut
Chronic high blood sugar doesn’t just affect your cat’s energy levels. It gradually damages the nerves and tissues that control how the intestines move food along. A 2025 study in Veterinary Sciences found that poorly controlled blood sugar causes structural remodeling of the intestinal wall, particularly fibrosis (scarring) of the muscle layers. This scarring changes how the gut absorbs nutrients and how quickly contents pass through, leading to diarrhea, constipation, or both at different times.
The damage goes deeper than scarring. High glucose levels reduce the number and function of specialized nerve cells in the gut wall, along with the pacemaker-like cells that coordinate intestinal contractions. Without proper coordination, food may rush through the intestines before water and nutrients are fully absorbed. The study also found that poorly regulated diabetes promotes bacterial overgrowth in the small intestine, which compounds the problem. If your cat’s blood sugar has been difficult to control, this type of gut dysfunction is a likely contributor.
Pancreatitis: The Most Common Overlap
The pancreas produces both insulin and digestive enzymes, so it’s no coincidence that pancreatic disease and diabetes frequently coexist. Clinical studies have found signs of pancreatitis in 31 to 83 percent of diabetic cats, depending on how it’s measured. Inflammation in the pancreas can spread from the enzyme-producing tissue to the insulin-producing cells, which is sometimes how diabetes develops in the first place.
Chronic pancreatitis causes diarrhea through two routes. First, the inflammation itself disrupts digestion. Second, if enough enzyme-producing tissue is destroyed, your cat develops exocrine pancreatic insufficiency (EPI), meaning the pancreas can no longer produce adequate digestive enzymes. Without those enzymes, food passes through largely undigested. The hallmark signs of EPI are large, greasy, foul-smelling stools and weight loss despite a normal or increased appetite. Cats with diabetes who have a chronic history of loose stools should be evaluated for concurrent pancreatic disease, as the two conditions share the same organ and frequently appear together.
Dietary Changes and the Transition Period
Most cats diagnosed with diabetes get switched to a low-carbohydrate, high-protein diet. This is good for blood sugar control, but it represents a dramatic change in what the gut is processing. Research shows that 87 percent of cats in one diabetes study had been eating at least some dry food before diagnosis, and dry cat food tends to be significantly higher in carbohydrates. Moving from a 40-plus percent carbohydrate diet to one with around 12 percent carbohydrates on a caloric basis is a major shift.
High-protein diets are also higher in fat, which can be harder on the digestive system, especially for a cat whose pancreas is already compromised. If the food switch happened quickly rather than over 7 to 10 days, that alone could explain the diarrhea. Some cats also do better with moderate-carbohydrate, high-fiber formulas, since fiber slows carbohydrate absorption and helps regulate intestinal transit time. If your cat’s diarrhea started around the same time as a diet change, the food is a prime suspect.
Diabetic Ketoacidosis
This is the cause you need to rule out quickly. Diabetic ketoacidosis (DKA) happens when your cat’s body can’t use glucose for energy and starts breaking down fat at a dangerous rate, producing acidic byproducts called ketones. The resulting metabolic acidosis disrupts normal cellular function throughout the body, including the gut.
DKA typically causes vomiting more than diarrhea, but both can occur. The key warning signs that distinguish DKA from a simple stomach upset are lethargy, loss of appetite, rapid breathing, and visible dehydration. Check your cat’s gums: if they’re dry or tacky instead of slick, or if the skin between the shoulder blades stays tented when you gently pinch it, your cat is dehydrated. Sunken eyes are another telltale sign. If your cat has refused food or water for more than 12 hours alongside vomiting or diarrhea, this is an emergency.
Rare Insulin Reactions
Insulin itself almost never causes diarrhea, but true hypersensitivity reactions have been documented. One published case involved a cat that developed projectile vomiting, profuse diarrhea, facial swelling, and collapse within minutes of a glargine insulin injection. The cat continued to have severe diarrhea with blood in the stool for 24 hours after the reaction. This is an anaphylactic-type response, not a routine side effect.
If your cat’s diarrhea started immediately after beginning a new insulin type or coincides closely with injection timing, mention it to your veterinarian. Switching insulin formulations (the cat in the case report did well on porcine insulin afterward) resolves the problem. But for the vast majority of diabetic cats, insulin is not the culprit.
What Your Vet Will Test For
Because diarrhea in a diabetic cat can point to so many different problems, your vet will likely want to run a few targeted tests rather than guess. The most useful ones include a pancreatic lipase test (fPLI), which is currently the most sensitive and specific blood test for detecting pancreatitis in cats. If EPI is suspected, a different test called TLI measures how much digestive enzyme the pancreas is actually producing. Vitamin B12 levels may also be checked, since chronic malabsorption depletes this vitamin and a deficiency can worsen diarrhea on its own.
Blood sugar control will also be assessed, often through a fructosamine test that reflects average glucose levels over the previous two to three weeks. If blood sugar has been running high, the diarrhea may improve simply by achieving better glycemic regulation. Your vet may also recommend an abdominal ultrasound to visualize the pancreas and intestinal wall directly.
Managing Diarrhea at Home
While you work with your vet to identify the underlying cause, the immediate priority is preventing dehydration. Diabetic cats are already prone to fluid loss through excessive urination, and diarrhea compounds this. Make sure fresh water is always available. Some cats drink more readily from a fountain or a wide, shallow bowl.
If the diarrhea is mild and your cat is still eating and drinking, a gradual dietary adjustment may help. Switching too quickly between foods is a common trigger, so if you recently changed diets, try mixing the new food with the old over a longer period. For cats on low-carb diets that continue to have loose stools, your vet may recommend a moderate-carbohydrate, higher-fiber option, which can slow intestinal transit and firm up stools.
Probiotics are generally safe for diabetic cats and may offer a secondary benefit. A 12-week trial of one probiotic strain in diabetic cats showed an 18 percent decrease in fasting blood glucose, with no adverse effects on bloodwork or body weight. Cats on insulin even required lower doses by the end of the study. While this doesn’t mean probiotics will fix diarrhea on their own, they’re unlikely to cause harm and may support both gut and metabolic health. Choose a veterinary-formulated product rather than a human supplement, since the bacterial strains and doses differ.