Porus One vs Kremezin: Choosing the Right Uremic Toxin Adsorbent
Introduction: what can a uremic-toxin adsorbent actually do?
In feline CKD we tend to focus on BUN, creatinine and SDMA. But those numbers are mostly a dashboard of how well the kidneys filter; the compounds that do direct harm are different.
The main ones are protein-bound uremic toxins such as indoxyl sulfate (IS) and p-cresyl sulfate (pCS). In cats and dogs, higher blood IS has been linked to faster CKD progression (Chen 2018) — and the hope that lowering it might slow progression is where oral adsorbents come from.
1. Porus One vs Kremezin/Renamezin: what's the difference?
They act at the same step, on the same target: indole (the precursor of IS) made by gut bacteria is adsorbed before it can be absorbed, and leaves in the stool. What differs is the form, the regulatory status and how much feline evidence exists.
| Porus One | Kremezin / Renamezin | |
|---|---|---|
| Ingredient | Renaltec — spherical porous carbon | AST-120 — spherical carbon adsorbent |
| Mechanism | Both adsorb uremic-toxin precursors such as indole in the gut and pass them in stool; both use pore size to take up only small molecules | |
| Surface area | About 1,500 m² per g Maker: one 500 mg sachet = 3 tennis courts (about 750 m²) | Over 1,600 m² per g |
| What it is | A feline product (Porus GmbH, Germany; distributed by Dechra) | A human prescription drug (over 30 years in Japan); Renamezin is a Korean product with the same ingredient |
| How it's given | Powder mixed into food (one 500 mg sachet a day) | Capsules or granules, given separately or in a little food |
| Other drugs | Both should be given at least 2 hours apart from other oral medication — adsorbents can bind drugs too (Porus One's own guidance says the same) | |
| What has been studied | 3 feline studies (1 randomised) — lower toxin levels | Lower toxin levels shown in humans; in cats only one 5-cat study, too small to judge |
| Difference in effect | Being the same kind of material, expect a similar effect. The gap in studies reflects who paid for feline research, not a difference in effect (section 3) | |
2. Where uremic toxins come from
- Protein intake
Dietary protein is broken down and absorbed as amino acids in the small intestine. - Undigested protein reaches the colon
Whatever is not absorbed moves on to the large intestine. - Bacterial fermentation
Tryptophan becomes indole; tyrosine and phenylalanine become p-cresol. - Where adsorbents work
Both Porus One and Kremezin bind indole and similar compounds at this step and carry them out in stool. - The liver makes the toxins
Absorbed indole and p-cresol are converted to IS and pCS in the liver. - Cleared by the kidneys
Being protein-bound, IS and pCS are mainly secreted by the renal tubules. As kidneys fail they accumulate, and accumulation is associated with tubular inflammation and fibrosis.
So there are three levers: less protein reaching the colon (digestibility), different fermentation (fibre), and binding in the gut (adsorbents).
3. The feline evidence, in numbers
| Study | Subjects · design | Result | Funding |
|---|---|---|---|
| Porus One 2019 congress abstract | 12 healthy senior cats vs 6 controls, 8 weeks | IS down more than 60% | Maker-linked |
| Porus One Paschall 2024, Vet Sci | 13 cats with experimental CKD, non-randomised, 56 days | IS −8.5% (once daily) / −30% (twice daily); pCS −16.5% / −47% | Dechra donation; one author a Dechra consultant |
| Porus One Elzenbeck 2026, JFMS | Randomised pilot, IRIS 2–3, 10 vs 9 controls, 6 months | At the end, lower IS (P=0.045), UPC (P=0.001) and faecal dysbiosis index (P=0.009). SDMA no different; 2 progressed in each group | Maker supplied product and lab costs (stated no role in analysis or writing) |
| Kremezin National Taiwan University 2016 | 5 cats with CKD, 400 mg/day, 3 months | No clear change in IS or clinical values — "larger studies needed" | Thesis |
| Kremezin Human EPPIC trials, 2015 | About 2,000 people with CKD, randomised, placebo | Did not delay dialysis, transplant or doubling of creatinine | Manufacturer |
How to read it:
- Porus One now has evidence that lab markers fall. It is currently the only adsorbent with a randomised feline trial
- But all three studies are linked to the maker or distributor, the largest had 19 cats, mostly stage 2, and there is no stage 4 data
- The 2026 trial had no placebo, so everyone knew who was treated, and the Porus One group also received Add One treats, so the microbiome change cannot be pinned on either
- Neither product has evidence for slower progression or longer life. That the same mechanism (Kremezin) failed in large human trials is another reason for modest expectations
The two were tested on different questions. The Porus One studies asked "do toxin levels fall?" — yes. For Kremezin that answer is already yes in humans. Where Kremezin failed was the next question: "does that delay dialysis or a doubling of creatinine?" Porus One has not been tested on that yet, and in the 2026 trial SDMA and progression (2 vs 2) did not differ — the same pattern.
The difference in the number of studies is about money, not effect: Kremezin is an off-patent human drug with no company to fund cat trials, while Porus One must sell in the veterinary market and funds research. Daily doses are similar too, about 100 mg per kg for both.
"One worked and the other didn't" is usually down to something else that changed at the same time — fluids, diet, recovery from dehydration, a different lab machine. The IS that adsorbents lower is not on a routine blood panel, and creatinine moves for reasons unrelated to adsorbents. To check a difference in one cat you would have to switch with everything else unchanged and compare at least twice at the same lab — and even then creatinine is a poor judge.
4. Digestibility matters more than which meat
An earlier version of this article said turkey is lower in toxin precursors such as tryptophan. Rechecked against USDA source data, lean meats turned out almost identical.
| Per 100 g | Protein | Tryptophan | Per g |
|---|---|---|---|
| 🍗 Chicken | 31.0g | 362mg | 11.7mg |
| 🦃 Turkey | 29.5g | 364mg | 12.3mg |
| 🐇 Rabbit | 29.1g | 384mg | 13.2mg |
| 🐟 Tuna | 29.2g | 313mg | 10.7mg |
Per g = tryptophan per gram of protein. Source: USDA FoodData Central (SR Legacy), roasted lean meat (chicken and turkey breast); tuna is yellowfin. Tyrosine and phenylalanine (p-cresol precursors) are similar too, at 32–41 mg per g of protein.
The spread is 10–13 mg, so switching meats gains almost nothing. The earlier turkey figures did not match the source data and have been replaced with the table above.
5. Cutting tryptophan backfires — amino-acid balance
A cat uses protein efficiently only when every essential amino acid is present in proportion. If lysine, leucine and methionine are at 100% but tryptophan is at 60%, overall protein use can be capped near 60%, and the surplus is broken down into BUN.
So trimming tryptophan to cut toxin precursors is counterproductive. The goal is not minimisation but balanced protein that is well digested.
6. Gut bacteria: the same meal, different toxin loads
- Proteolytic bacteria — make indole and p-cresol
- Saccharolytic bacteria — ferment carbohydrate and fibre and make fewer precursors
Prebiotic fibres such as FOS, GOS, inulin and resistant starch are used to tilt fermentation toward the saccharolytic side. Feline evidence that "added fibre lowered IS" is still small-scale, and too much fibre can soften stools or reduce appetite, so start low.
(1) Eating enough — a cat that doesn't eat breaks down muscle and makes more uremic waste
(2) Proven care: phosphorus control, hydration, blood pressure
(3) Balanced, highly digestible protein
(4) Then, if you like, an adjunct such as an adsorbent (Porus One) — optional either way, and the first thing to drop if it puts the cat off food
Digestive enzymes were removed from this list: there is no evidence for them in CKD cats.
Related reading
- Does Kremezin actually lower creatinine? — reading the numbers after starting an adsorbent
- Care compendium: Porus One — evidence grade and real-world use