Uraemic Toxins

As CKD progresses, damaged kidneys become less able to filter waste products out of the blood. These waste products, collectively known as uraemic toxins, build up and are largely responsible for the general “feeling unwell” that CKD cats experience, from poor appetite to mouth ulcers to lethargy. Over 140 individual uraemic toxins have been identified, though only a handful are well studied.

This post covers the treatments aimed specifically at reducing the overall toxin load, as opposed to correcting one specific bloodwork value like phosphorus or calcium (covered in our other treatment posts). It also covers some of the specific complications uraemia can cause and how to manage them.

Azotaemia is the technical term for the buildup of nitrogenous waste products in the blood, and uraemia refers to the collection of clinical signs that buildup causes. BUN and creatinine, the two values most commonly tracked on a CKD cat’s bloodwork, aren’t major toxins themselves, but BUN levels correlate reasonably well with the level of harder-to-measure uraemic toxins, which is part of why vets track it so closely.

One of the better studied uraemic toxins is indoxyl sulfate, a byproduct of protein digestion. Research has found that indoxyl sulfate levels in cats correlate with disease stage and may contribute directly to kidney fibrosis and scarring, and that rising levels can predict disease progression. Several of the treatments below, particularly chitosan and AST-120, are aimed specifically at reducing indoxyl sulfate.

You’ll typically start to see signs of uraemia once a cat’s creatinine is consistently over 3 mg/dl (265 µmol/L), assuming the cat is properly hydrated.

Fluid Therapy

Subcutaneous fluids, covered in more depth elsewhere on this site, help maintain hydration but aren’t given in large enough volumes to meaningfully flush toxins from the blood. Intravenous fluids, used for cats in crisis or with very high bloodwork values, can flush the kidneys more aggressively (called diuresis), but this is a hospital-based intervention, not something managed at home.

The “Gut as a Kidney” Theory

Most nitrogenous waste is excreted through the kidneys, but a smaller amount is normally excreted through the large intestine via gut bacteria. The idea behind several of the treatments below, sometimes called “enteric dialysis” or “nitrogen trapping,” is that boosting the right gut bacteria can offload some of the burden away from failing kidneys. It’s a reasonable theory, but it’s worth being upfront that the research supporting it in cats specifically is thin and mixed, more so than for phosphorus control.

Synbiotics: Azodyl and Renadyl

Azodyl (marketed for the veterinary market) and Renadyl (marketed for humans, but commonly used off-label in cats) are the best known products in this category. Both combine a prebiotic fibre (psyllium, and inulin in Renadyl’s case) with the same three patented probiotic strains. Renadyl contains three times the probiotic concentration of Azodyl per capsule.

The theory is that these specific bacterial strains have an unusually high affinity for uraemic toxins and will bind them in the gut before they’re absorbed. Some research supports modest reductions in BUN and creatinine with use, including a small feline trial and studies in other species, while other studies, including one investigating dogs on dialysis, found no improvement. A study specifically testing whether opening Azodyl capsules and sprinkling the contents on food (rather than giving the capsule whole) still worked found that it did not, at least when mixed into a full meal, though the manufacturer disputes this based on a different mixing method.

There’s also a legitimate concern worth understanding: these products may lower BUN and creatinine readings by trapping nitrogen in the gut, which could make bloodwork look better without necessarily reflecting genuine improvement in kidney function or a reduction in the toxins that matter most for how a cat feels. This doesn’t mean the products are useless, but it’s a reason not to over-interpret a falling creatinine as proof the kidneys themselves are doing better.

In practice, feedback on Azodyl and Renadyl within the CKD caregiver community is mixed. Many people feel it helps their cats feel a bit better, some see vomiting as a side effect, and some see no difference at all. If you try it, know that giving Azodyl correctly matters a lot. It needs to be given as a whole capsule (not sprinkled on a full meal) for the best chance of working, ideally mixed with a small amount of fatty food like cream cheese or butter, given on an empty stomach about an hour before a full meal.

This category of treatment works by binding directly with precursor compounds in the gut before they can be converted into uraemic toxins.

Chitosan

Chitosan is derived from crustacean shells and is thought to bind with indole (a compound produced during protein digestion) before it can be converted into indoxyl sulfate. It’s the active ingredient behind a large family of calcium carbonate-based phosphorus binders, including Ipakitine/Epakitin, so many cats on these products are getting a chitosan dose without a separate treatment being needed. Research in cats has shown reductions in phosphorus and, in at least one study using cats with naturally occurring CKD, in BUN as well.

Activated Charcoal and AST-120

Charcoal-based adsorbents work on a similar principle. AST-120, marketed as Covalzin for cats (and Kremezin for humans in Japan, where it’s been used since 1991), binds indole even earlier in the conversion process than chitosan does. It’s been studied fairly extensively in human patients, with mixed results. Some trials found it helped patients feel better without necessarily lowering creatinine, while large placebo-controlled US and European trials found no benefit in slowing CKD progression, in contrast to more promising results seen in Japan. AST-120 isn’t commercially available in the US or Europe, though it is available in Japan, South Korea, Taiwan, and the Philippines.

Renaltec (Porus One)

A newer carbon-based adsorbent from Germany, Porus One’s manufacturer claims its refined form moves more smoothly through the gut than plain charcoal and binds selectively with toxins like indoxyl sulfate. A recent study using cats with experimentally induced CKD found reductions in uraemic toxin levels and some improvement in metabolic acidosis, alongside small increases in BUN and creatinine. It’s available in the US, UK, and other markets, but it’s expensive, and feedback has been mixed, with some people seeing improved bloodwork and others seeing no change.

Beyond general toxin reduction, uraemia causes some specific problems worth knowing how to manage directly.

Mouth Ulcers

Painful mouth ulcers are common as uraemia worsens, and they can make a cat reluctant to eat, which compounds the problem. Sucralfate, which forms a protective coating over ulcerated tissue, is often effective for stubborn cases and needs a vet prescription. Slippery elm bark, a gentler over-the-counter option covered in more depth in our holistic treatments post, is also commonly used and can be made into a soothing syrup. If ulcers are severe enough to stop your cat eating, softer or pureed food, including baby food in the short term, can help bridge the gap while treatment takes effect.

Gastrointestinal Bleeding

Severe uraemia can occasionally cause bleeding in the digestive tract, sometimes showing up as diarrhoea, though it can also go undetected. This is a more serious complication and warrants a vet visit rather than home management alone. Sucralfate is used here too, and in more serious cases a proton pump inhibitor like omeprazole may be recommended, extrapolating from evidence in human patients even though this hasn’t been specifically studied in cats with kidney-related GI bleeding.

Neurological Signs

In advanced uraemia, some cats develop neurological symptoms, ranging from mild disorientation to seizures. Most seizures in CKD cats are actually caused by hypertension rather than uraemia directly, so blood pressure should always be checked first if your cat has a seizure. True uraemic seizures, caused by toxin levels high enough to affect the brain, are less common and reducing toxin load becomes urgent in that scenario, though this is a situation for emergency veterinary care, not home treatment.

Given the mixed evidence, it’s worth being realistic with yourself about what these treatments can and can’t do. None of them are a substitute for phosphorus control, hydration, and a good diet, which remain the treatments with the strongest evidence behind them. Toxin-reduction products are reasonable to consider as an addition once the fundamentals are in place, particularly if your cat seems generally unwell despite good bloodwork management, but they shouldn’t be the first thing you reach for, and it’s fine to decide the cost and effort aren’t worth it for your particular cat.

Do I need to give my cat Azodyl or Renadyl? No, these are optional treatments, not essential ones. The evidence for benefit in cats is mixed, and many CKD cats do well without them. They’re worth considering as an add-on if the basics (diet, hydration, phosphorus control) are already well managed.

Can I just sprinkle Azodyl on my cat’s food instead of giving the whole capsule? Research suggests this significantly reduces effectiveness when the capsule is mixed into a full meal. If your cat won’t take capsules whole, talk to the manufacturer or your vet about the recommended alternative method of mixing the contents with a small amount of fatty food on an empty stomach, rather than sprinkling on a regular meal.

Is AST-120 (Covalzin) available where I live? It’s mainly available in Japan, South Korea, Taiwan, and the Philippines. It isn’t commercially available in the US or most of Europe, though clinical trials have taken place in both regions.

Will these treatments lower my cat’s creatinine? Some may, but a falling creatinine on one of these products doesn’t necessarily mean kidney function has improved. Some products work by trapping nitrogen in the gut, which can lower bloodwork readings without changing what’s happening in the kidneys themselves.

What’s the difference between a phosphorus binder and an oral adsorbent like chitosan or AST-120? Phosphorus binders target phosphorus specifically. Oral adsorbents in this post target precursor compounds involved in producing toxins like indoxyl sulfate. Some products, like chitosan-based binders, actually do both at once.

My cat has mouth ulcers. What should I try first? Slippery elm bark is a gentle, inexpensive option many people try first. For stubborn or severe ulcers, ask your vet about sucralfate, which needs a prescription but tends to be more reliably effective.

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