The “whys and how’s” of phosphate restriction in cats with CKD

Host

Welcome to the Bova UK Podcast, where we discuss diseases from diagnosis through to management. These podcasts are aimed at registered veterinary surgeons and veterinary nurses. If you’re listening as a pet owner, we always advise that if you have any concerns about your animal, please consult your local veterinary surgeon.

Sarah

Today, it’s a real pleasure to be joined by Rosanne. Many years ago, Rosanne was a student at Bristol when I was just starting out as a lecturer. It’s been an absolute delight to watch her career develop and see all the brilliant work she and her colleagues at the RVC have done to advance our knowledge of chronic kidney disease (CKD) in cats. Thank you so much for joining me today, Rosanne.

Rosanne

Thank you, Sarah. It’s a real honour. You were a big influence in setting me on this path, and I’m thrilled to be here to discuss phosphate regulation in older cats.

Sarah

So, let’s dive in. We’ve talked about phosphate restriction at CPD events and in articles for years. Why does phosphate remain such a key aspect of CKD management in cats?

Rosanne

We need to go back to basics. As CKD progresses, kidney function—including excretion of waste products like phosphate—declines. This leads to disturbances in phosphate regulation. Clinical studies have shown that managing phosphate levels improves outcomes. Traditionally, this has been done through dietary phosphate restriction, and while the principle remains the same, we now recognise the importance of tailoring interventions depending on the stage of CKD.

Sarah

That leads nicely to early-stage disease. When should we start dietary phosphate restriction, and what degree of restriction is appropriate?

Rosanne

Typically, traditional renal diets are introduced around IRIS stage 2 or 3, when we see changes in phosphorus, creatinine, or SDMA. Earlier diagnosis—based on subtle changes in urine concentrating ability or ultrasound—has led to the development of early renal diets. These diets are less restrictive but still support kidney health. Current research is focused on identifying exactly when and how much to restrict phosphorus in early-stage CKD.

Sarah

Are there concerns about feeding more traditional renal diets to early-stage cats?

Rosanne

Yes. The balance of calcium and phosphorus is critical. Over-restricting phosphate can increase the risk of hypercalcaemia, which we want to avoid. We also want to maintain muscle condition, so protein content is another key consideration. We’re actively researching how to fine-tune these diets.

Sarah

A common challenge is managing multi-cat households. Should owners be concerned if a healthy cat eats a renal diet?

Rosanne

It depends. A young healthy cat shouldn’t be on a phosphate-restricted diet long-term, so some level of feeding separation is ideal. If it’s a household of older cats, a mixed approach using senior and renal diets may be fine. It’s about balancing what’s best for the group with practicalities of the home environment.

Sarah

What are your top tips for achieving good compliance with therapeutic renal diets?

Rosanne

Start gradually. A slow transition—over several weeks—is more likely to succeed. Today we have more diet options than ever before, with different flavours and formulations. In later stages of CKD, compliance can drop as appetite declines. At that point, partial compliance is still helpful. Combining renal with senior diets or using appetite stimulants and antiemetics can support intake.

Sarah

Yes, I tell owners that this is a long-term plan. Efforts really do pay off. How else can practices support owners?

Rosanne

Regular check-ins, ideally led by the nursing team, are invaluable. Empower owners to monitor weight and food intake. Owners feel more confident when they’re part of the care team, especially with consistent support and education.

Sarah

Agreed. And at the end stages, nutritional intake becomes the top priority—even if it means abandoning the renal diet. We must be pragmatic.

Rosanne

Exactly. The aim is always quality of life.

Sarah

Let’s move to phosphate binders. When do you typically introduce them?

Rosanne

Usually in stage 3 or 4 CKD, when phosphate remains elevated despite a renal diet. They’re not my first line for early-stage disease, because standard diets are often high in phosphate. It’s easier and more effective to transition to a renal diet. If owners refuse dietary change or have a cat with other needs (e.g., food allergies), binders may be an alternative.

Sarah

Any tips for administration?

Rosanne

Start with small amounts and build up. Wet food is easier for mixing powders or liquids. Some cats tolerate flavours, others prefer inert binders. Owners have creative approaches—like hiding binders in gel capsules or coating dry food with powders. Persistence helps.

Sarah

Do you have a preferred binder?

Rosanne

Aluminium hydroxide has historically been my go-to—it’s flavourless and inexpensive. But availability can be an issue. Calcium-based binders are more common now, but we must monitor calcium levels to avoid hypercalcaemia.

Sarah

How do you monitor binder effectiveness?

Rosanne

Recheck biochemistry after 4–6 weeks. Adjust the dose based on phosphate levels, and always consider weight changes. If you’re using calcium-based binders, monitor calcium too. It’s about finding a safe and effective balance.

Sarah

What if you’ve maxed out the binder dose and phosphate is still high?

Rosanne

Then I review the diet again—perhaps a more phosphate-restricted brand, or better diet compliance. I rarely use multiple binders, especially in late-stage CKD when there are already many treatments in play. Focus shifts to maintaining comfort and quality of life.

Sarah

Do you ever measure parathyroid hormone or FGF-23?

Rosanne

Not routinely. PTH is more of a research tool than a clinical one at this stage. FGF-23 is promising—it rises earlier than PTH and might help guide early interventions—but it’s currently only available for research.

Sarah

Any emerging areas of interest?

Rosanne

Yes. We’re studying renal mineralisation and differences in cats with nephroliths or mineral deposits. Cats developing CKD at younger ages may have distinct pathophysiology. That’s something we’re exploring further.

Sarah

Thank you so much, Rosanne. This has been incredibly insightful. I hope everyone listening has picked up some useful tips. And as always, if you’re a pet owner, please speak to your vet for individual advice.