Is high-phosphorus cat food harming our HEALTHY cat’s kidneys? Plus: some potentially safer foods

Could high-phosphorus food be the reason that even cats on high-quality food can develop kidney disease as they age?  While the research is still unfolding, this may be an important piece of the chronic kidney disease (CKD) puzzle. With CKD affecting an estimated 30-40% of cats over 10 years and 81% of cats over 15 years, our beloved feline friends need all the help they can get!

Many of us have been told that the amount of phosphorus in cat food only matters if your cat has kidney disease. Alas, recent research is changing this story. It now appears that too much phosphorus can spell trouble for healthy cats’ kidneys, too. Plus, even some of the high-quality cat foods are high in phosphorus. Yikes.

That’s why I am shifting our healthy cats to MODERATE-phosphorus cat foods. (Bear in mind that phosphorus is an essential nutrient, so I would not choose low-phosphorus “CKD” foods for healthy cats.) However, figuring out which quality cat foods get the phosphorus right? Not easy!

Here’s what I can share to help.

What the research tells us

Studies in 2017 and 2018 found that feeding healthy cats a high-phosphorus diet caused troublesome kidney test results within just four weeks!

This sparked further investigations into the impact of phosphorus. Here are some nuggets from the research:

• Many cat foods, including many high-protein ones, have high phosphorus levels.
• It appears that inorganic phosphates, especially sodium monophosphate, stress the kidneys more than organic phosphorus (eg, from bones or milk).
• The calcium-to-phosphorus ratio is important. We want to strive for food that has more calcium than phosphorus (or at least a 1:1 ratio).

One study found that a third of 82 randomly selected cat foods had phosphorus levels shown to cause kidney dysfunction in healthy cats.

So what’s a safe phosphorus level for healthy cats?

While phosphorus precautions may be especially important for cats approaching their senior years, I suspect that the earlier we get them on safer food, the better. A Nutritional Research Council publication suggested 250–350 mg/100kcal is a safe upper limit of phosphorus in your cat’s food. 

But it isn’t just about that number—we also want to make sure there is more calcium than phosphorus. Without the proper ratio, even moderate phosphorus may cause problems. And, as we mentioned, the TYPE of phosphorus matters too.

All of this translates to these tips for feeding healthy cats (without CKD):

1. NO MORE THAN an average of 350 mg phosphorus/100 kcals in cat food for healthy cats.*
   
2. Cat food should have equal or more calcium than phosphorus–ideally somewhere between a 1:1 to 1.5:1 ratio. Daily ratios are not considered as vital as the overall balance over a week. Most cat foods seem to have more calcium than phosphorus, but I definitely found a few that don’t.
   
3. Try to avoid foods with inorganic phosphates. It appears that sodium phosphates are particularly suspect. And, while raw diets with ground bone can be high in phosphorus, their organic phosphorus may be less risky than phosphates. In this article, researcher Dr. Ellen Kienzles said there was no problem detected in cats eating a high-phosphorus diet from bone meal for 4 weeks––however, I cannot find the study on this. Also, check your raw food’s ingredients, as some do add phosphates.

*What about the DMB (dry matter basis) percentage, you ask? I get it. Most companies will give you DMB % rather than the mg. But there is no consistent DMB percentage to apply to every food. Because, depending on the amount of moisture, 350 mg phosphorus/100 kcals could be around 1.44% to 1.60% DMB. So you could avoid anything above 1.44%, but this may inadvertently rule out some reasonable foods. For example, Weruva Lamb and Mackerel is 1.59% DMB phosphorus but has an acceptable 336 mg phosphorus.

And is there a lower limit for phosphorus? Yes, the AAFCO advises not going lower than 0.5% DMB or 125 mg/100 Kcal phosphorus for healthy cats (without CKD).

Some higher-quality, moderate-phosphorus cat foods 

I’ve been looking into cat foods with safer amounts of phosphorus and calcium. And while I wanted to weed out foods with inorganic phosphates, the best I can do right now is avoid sodium phosphates (mostly!).

These are all grain-free, higher-quality foods. Bear in mind, I’m mostly listing poultry here–only because that’s what our own picky cats will eat!

UPDATE DEC 2024: Until raw cat food companies can confirm via testing that their food is free from the avian flu, I would avoid or cook raw cat food for now. A number of cats of died from avian flu recently–some from contaminated raw food and raw milk.

• Fromm Four-Star Nutritionals Chicken Duck & Salmon Pate Cat Food
• Fromm Four-Star Nutritionals Turkey Pate Cat Food
• Fussie Cat Super Premium GOLD Chicken in Gravy 
• KOHA LID Chicken Pate for Cats 
• Rawz Chicken & Chicken Liver Pate Canned [note “Rawz” brand is not actually raw]
• Rawz Shredded Chicken Canned
• Rawz Shredded Tuna & Chicken Canned 
• Rawz Turkey & Salmon Pate Canned
• Rawz Senior Turkey, Pumpkin & New Zealand Green Mussels Canned [NEW]
• Vital Essentials Raw Entrees ONLY Beef, Chicken flavors Frozen and Freeze-Dried (with water added for serving) See caution above about raw food + avian flu.
• Primal Feline Raw Frozen Nuggets – all flavors except Venison See caution above about raw food + avian flu.
• Primal Feline Raw Freeze Dried Nuggets – Chicken & Salmon, Beef & Salmon flavors (add water for serving) See caution above about raw food + avian flu.

Where to get Rawz? This is a very high-quality canned food we currently order from Hearty Pet.

TIPS: It’s ideal if you can find these foods locally so you can make sure your cat will eat them before a big investment. Either way, introduce them slowly by adding a little on the side of your cat’s plate for several days. It’s a pain (you’ll waste some of the food), but it really helps open cats to new foods.

Note: This list is short ONLY because I didn’t have the time to research all the foods, but I’ll expand it as I find more good ones. Check out the next section so you can do your own research for foods your cats like!

How do you find the phosphorus amount in cat food?

Figuring out the phosphorus levels in your cat’s food can be tricky since labels lack this crucial info! While they say the AAFCO is working on new labeling standards which hopefully will help, here are some helpful options for now.

• Ask Chewy about a specific food in the Questions area of that food’s page. So far they have been really helpful with this info!
• Primal, Rawz, and Fromm rock for posting the phosphorus, calcium, and other amounts on their websites (look for “typical analysis” or “nutritional facts” on product pages)
• Dr. Lisa Pierson’s Cat Food – Nutritional Composition chart
• FelineCRF’s Canned Food Chart (Keep in mind this is for CKD cats! But it has helpful numbers.)
• You can email the company to find out (in your abundant spare time). Any company worth their salt will get you this info promptly.

Bottom line

Phosphorus is essential for your kitty, but new research suggests too much can be harmful to healthy cats, and many cat foods go above recommended amounts!

Diets above 360 mg phosphorus/100 kcal were shown to cause kidney damage in cats, especially when they had inorganic phosphates or more phosphorus than calcium. Keeping your cat foods’ phosphorus levels below 350 mg/100 kcals, while making sure each food has more calcium than phosphorus, appears to be ideal. However, don’t go lower than 0.5% phosphorus dry matter basis or 125 mg/100 kcal phosphorus for healthy cats.

Let’s look forward to more research and label changes to help us make better-informed choices for our feline friends. In the meantime, we’ll keep doing the best we can (with limited info) to prevent kidney trouble and catch it early.

How about you, do you have any resource tips or moderate-phosphorus cat foods to recommend?