Updated: Mar 8, 2020
The internet is awash with recommendations, forthright opinions, and recipes for cats with chronic kidney disease (CKD). This is concerning – it's one thing for people to give bad nutritional advice for a healthy adult animal, but it's another thing when it's for an animal with a medical condition. Poor nutritional management of cats with CKD will shorten remaining lifespan and increase secondary complications - that has been well-established in cats, dogs, humans and many other species.
So in this post, I thought we would have a look at some common nutrition-related questions for cats with CKD, and also analyse an easily available free recipe and see what it looks like. This is a longer post, but if you are interested in feline nutrition, please do persist!
I'm going to get stuck into the hardest and most controversial question first, which is:
Should a cat with kidney disease be fed a protein-restricted diet?
This question has previously been a topic of contention and debate, even amongst board-certified veterinary nutritionists. Fortunately, some recent research appears to be making this a slightly more straight-forward question to try and answer. And that answer is, yes, cats with chronic kidney disease very likely benefit from a degree of protein restriction. Here's why.
Patients with chronic kidney disease accumulate many different toxins that have a wide range of detrimental systemic effects. They are called uraemic toxins, and some of these toxins are the products of fermentation of diet-derived amino acids by the gut microflora. An example is the well-researched uraemic toxin, indoxyl sulfate. Indole is produced from tryptophan by gut bacteria that express tryptophanase; this is converted to indoxyl sulfate in the liver.
So what does indoxyl sulfate do? Well, what doesn't it do! Research has shown that it adversely affects immune function, causes neuroinflammation, exhibits cardiovascular toxicity, advances the progression of kidney disease, is associated with increased mortality, and is a pathogenic factor for uraemic sarcopaenia or loss of lean muscle mass (Sato et al. 2016). This last one is interesting, as people often want to feed cats with CKD a high protein diet to try and prevent muscle loss.
It is well-established that human patients with CKD on very low protein diets have a significant decrease in uraemic toxins, which is likely due to decreased tryptophan, tyrosine and other amino acids in the diet. After one week on a very low protein diet, people with CKD had a 37% decrease in serum indoxyl sulfate (Marzocco et al. 2013). Poesen et al. (2015) also found that in healthy people, high dietary protein intake increases the concentration of indoxyl sulfate in both the serum and urine.
Two important studies on indoxyl sulfate and chronic kidney disease in cats have been done recently, and I'm sure there will be more to come. Summers et al. (2018) compared the faecal microbiome of cats with CKD, to that of healthy cats, and they also measured serum indoxyl sulfate in both groups. The authors found that cats with CKD had significantly decreased faecal bacterial diversity and richness. They also found that there were significantly higher indoxyl sulfate concentrations in IRIS stage 2 CKD cats, and also stages 3 and 4 CKD cats, in comparison to healthy control cats. Importantly, no significant difference was found in indoxyl sulfate concentrations between stage 2 CKD cats and stages 3 and 4 CKD cats, which appears to provide increased justification for earlier protein restriction (from early stage 2 onwards).
Chen et al. (2017) also found that plasma indoxyl sulfate was increased in dogs and cats with chronic kidney disease, particularly in IRIS stages 2 and 3. The authors state that "indoxyl sulfate concentrations were also correlated with the increase of blood urea nitrogen, serum creatinine, and phosphate and the decrease of haematocrit among cats." The research that we have (so far) therefore indicates that indoxyl sulfate does increase in cats with CKD, and whilst we don't yet have evidence that it has the same detrimental effects in cats as it does in other species, it seems highly likely.
An argument could be made that a high protein, raw meat-based diet is more digestible than an equivalent high protein, extruded commercial diet, so this could lead to relatively less fermentation of amino acids by gut bacteria and production of uraemic toxins. However, this still doesn't justify feeding excessively high protein raw (or commercial) diets.
Does it matter what proteins I feed?
Yes! Of course it does. Not all proteins are created equal. Aside from differences in their vitamin and mineral content, a key difference between proteins is their fat content. It is regrettable that kangaroo is often recommended for cats with CKD in Australia, because this is an extremely lean protein - it usually has only 0.5-1.5 grams of fat per 100 grams of meat. Compare this with something like standard beef mince, which has 10 grams of fat per 100 grams of meat.
Why does this matter? If you use extremely lean proteins like kangaroo or venison as the only, or predominant, proteins in your cat's homemade diet, you will significantly increase the protein and phosphorus content of the diet. You should now be able to see the problem with any product that comes with the instructions to "add to muscle meat". Chicken thigh is different to chicken breast. Beef is different to lamb. Salmon is different to tuna. Specifics matter when it comes to therapeutic diet formulation.
Should a cat with kidney disease be fed a phosphorus-restricted diet? How much phosphorus restriction is necessary? What supplements should I use?
Fortunately, the necessity for phosphorus restriction is much more widely acknowledged. Phosphate retention is the initiating factor for the development of many of the complications observed in chronic kidney disease, such as renal secondary hyperparathyroidism and bone and cardiovascular diseases. Multiple studies have demonstrated that dietary phosphorus restriction slows decline in renal function and increases mean survival time in cats with chronic kidney disease.
With respect to supplementation for home-prepared diets for cats with CKD, let me be as clear as I possibly can. The only phosphorus-restricted multivitamin/mineral supplement formulated specifically for cats that I would recommend, or be comfortable to use for my own cats, is BalanceIT Feline-K. The other option would be high quality human supplements. In my opinion, either BalanceIT or human supplements should be used as part of a therapeutic diet that has been carefully formulated for your cat and their specific needs.
Remember that you are feeding a homemade diet because you want your cat to have the best possible diet. The supplements used in the diet are the most critical component to get right - a small change in a food ingredient is unlikely to be harmful, however it's vital the your cat's supplements contain (a) what the company says they will, and (b) the correct amounts and types of all essential nutrients. Don't go to all the trouble of home-preparing and then settle for poorly formulated supplements, it's really not worth it. Every batch of every BalanceIT product is fully analysed prior to sale, which means that you can be confident that the products contain what they are supposed to.
Have a look at the analysis of a diet I formulated for my cat, Whirly, using 50% raw kangaroo meat and 50% raw skinless and boneless chicken thigh, plus a popular 'meal completer' advertised as being suitable for cats with kidney disease. I followed the product instructions with respect to amounts and type of meat to use, and the amount of supplement to add, just as any other owner would:
The diet contains 71.2% of calories from protein, 22.7% calories from fat, and 6.1% of calories from carbohydrate. That's a lot of protein – 266.3% of the AAFCO minimum requirement for an adult cat. The diet also contains very little eicosopentaenoic acid (EPA) or docosahexaenoic acid (DHA); these are omega-3 fatty acids that have been shown to lower glomerular capillary
pressure, reduce proteinuria and slow decline in glomerular filtration rate. In fact, the omega-6 to omega-3 ratio is a very high 181.09!
However, the most concerning thing is the phosphorus content of this diet. Even though this product claims to produce a phosphorus-restricted diet, it clearly doesn't, with 1.83 grams per 1000 kcal ME, or 146.4% of the AAFCO minimum requirement, or 285.9% of the NRC minimum requirement. Properly formulated home-prepared renal diets typically contain less than 0.8 grams of phosphorus per 1000 kcal; Royal Canin Renal dry diet contains 0.8 grams, Hill's Prescription Diet k/d canned contains 0.85 grams.
Of course there are diets that contain more phosphorus than this one does. Protein-rich, raw meat diets are typically high in phosphorus, even without any bone, so manufacturers can argue that a diet like this contains "less phosphorus". In our opinion this is still not good enough; this product does not produce a phosphorus-restricted diet when used according to the (vague) instructions. It's true that we don't know exactly how much phosphorus restriction is ideal for cats with CKD, however there is no reason to exceed the AAFCO minimum requirement, and likely no reason to exceed the NRC minimum requirement either, particularly if the cat is hyperphosphataemic.
No problem, I hear you say, I'll just use a fattier meat to reduce the amount of phosphorus in my cat's diet, that's probably what the manufacturer would want me to do (even though it's not specified in the instructions). So here's the analysis again, for Whirly, using lamb shoulder, instead of kangaroo and chicken thigh:
As you can see, increasing the amount of fat versus lean meat in the diet does reduce the phosphorus content of the diet a lot, however it also reduces other essential nutrients as well - enough to make some of them moderately to severely deficient.
What about omega-3? Can't I just add some salmon or tuna? What oils should I add to my cat's diet?
You can probably guess the answer to this one. Like phosphorus and protein-restriction, it's important to do this properly. Canned and fresh fish do contain omega-3 fatty acids and in some cases this may be sufficient, however generally supplementary omega-3 fatty acids are required, in the form of fish oil, krill oil or calamari oil. Don't fall into the trap of thinking flaxseed or chia seed oil will do the trick; they do contain the omega-3 fatty acid, alpha-linolenic acid, but cats are poor at converting this to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), so these are a poor choice.
Unfortunately, there is a lack of research on the effect of omega-3 fatty acids on the progression of CKD in cats. However, there are two important papers by Brown et al. published in 1998 and 2000, on omega-3 fatty acids for dogs with CKD. The authors found that supplementation with omega-6 fatty acids hastened the decline of kidney function, and that omega-3 fatty were renoprotective. In an earlier study, the magnitude of proteinuria and the plasma concentrations of creatinine, cholesterol, and triglycerides were lower in the group of dogs fed a diet supplemented with fish oil. These papers provide some guidance as to why it's likely very beneficial to enrich the diet of cats with CKD with omega-3 fatty acids. They also provide some insight into what amount of supplementation is required, and what the optimal omega-6 to omega-3 ratio might be.
This ratio is the reason why just adding the same "dose" of fish oil to a cat's diet is not the best approach. A cat eating a diet of 100% chicken thigh will consume a lot more omega-6, compared with a diet of 100% lean kangaroo. Adding a small amount of fish oil to the kangaroo meat diet may be adequate to achieve a desirable omega-6 to omega-3 ratio, however it may not do much when added to the chicken thigh diet. If you want the best for your cat, it's important that the amount of omega-3 in the diet is adjusted with respect to the other ingredients in the diet.
Many recipes for cats with CKD have carbohydrate added, even online raw meat-based recipes? What's the story? Cats are ca