Hello, I’m Dr. Hocheol Shin, or “Dr. Shin.” I study feline kidney health and aging at Greycoat Research.

I earned my Ph.D. studying cancer and immunology, and I’m now building on that experience to explore how we can help cats stay healthier for longer.

In “Reading Papers with Dr. Shin,” we’ll explore research related to feline health together. We’ll look a little deeper into how things work, what the studies actually found, and what those findings might mean for our cats. I’ll explain any unfamiliar terms as we go.

Ready to open today’s paper with Lulu?

< Three-Sentence Summary >

1. Studies feeding high-phosphorus test diets to healthy cats found changes in kidney filtration and ultrasound findings.
2. Total phosphorus, added phosphorus sources, and the calcium-to-phosphorus ratio differed together. These experiments do not provide a universal safety cutoff for every food.
3. Comparing foods requires consistent units, information about phosphorus sources, and the calcium-to-phosphorus ratio.

< Getting Started >

When choosing food for a cat with kidney disease, owners often look for the phosphorus content first. Among similar products, the lower number catches the eye.

But does “0.5% phosphorus” tell us how much phosphorus the body will absorb and handle? Does 0.5% mean the same thing in wet and dry foods? Is phosphorus from meat absorbed in the same way as added phosphate?

The number matters, but understanding it also requires knowing how the food was formulated. Today’s research examined what happened to healthy cats’ kidneys when they were actually fed high-phosphorus diets.

< Equal Amounts Do Not Always Behave Identically >

Phosphorus helps form bones and teeth and is involved in how cells use energy. Managing it in kidney disease does not make it an inherently unnecessary nutrient.

Some dietary phosphorus is absorbed in the intestine. Unabsorbed phosphorus leaves in feces. Kidney excretion is important in regulating the phosphorus that enters the body.

Phosphorus in cat food can come from ingredients such as meat or from added phosphates. Different phosphate salts vary in how readily they dissolve. Calcium also interacts with phosphorus in the intestine, making the balance between them relevant.

The units need to match too. A percentage based on the food’s total weight includes water. A dry-matter percentage excludes water. Grams per 1,000 kcal express the amount supplied with a specified energy intake. Comparing unlike units can make a difference in water content look like a difference in phosphorus.

Today’s paper uses grams of phosphorus per 1,000 kcal. A calcium-to-phosphorus ratio, or Ca:P, of 0.6 means 0.6 parts calcium for each part phosphorus.

< The First High-Phosphorus Trial >

Alexander and colleagues conducted two feeding trials in healthy adult cats, reported in 2019.

The first enrolled 48 cats. The test diet contained 4.8 g phosphorus per 1,000 kcal, with approximately 3.6 g supplied as added inorganic phosphorus. Its Ca:P was 0.6. Compared with the control diet, it had much more phosphorus and relatively little calcium.

The study stopped after four weeks, before its planned completion. GFR declined in the test-diet group, and abnormal kidney ultrasound findings appeared.

Changing the food had been followed by changes in kidney function and imaging. This was a more direct concern than a small rise in blood phosphorus alone.

< The Second Trial Changed Several Conditions >

The second trial enrolled 50 cats. Total phosphorus in the test diet was reduced to 3.6 g per 1,000 kcal, inorganic phosphorus to approximately 1.5 g, and Ca:P increased to 0.9. Follow-up lasted about 29 weeks.

This time, mean GFR did not change significantly. However, 36% of the test-diet group developed kidney ultrasound changes. Kidney stones occurred in both diet groups.

Less pronounced findings than in the first trial did not make the second formulation free of concern. The filtration measurements and the ultrasound examinations showed different aspects of the outcome.

< Why Can’t We Blame One Number? >

Total phosphorus, added inorganic phosphorus, and Ca:P all changed between the trials. The comparison cannot separate how much each change contributed.

Studies reported by Dobenecker and colleagues in 2018 also found kidney-related changes after high-phosphorus feeding. Diets using sodium phosphate and calcium phosphate produced different patterns of urinary phosphorus excretion and glucose in the urine. Actual phosphorus intake and other conditions were not identical either.

Glucose in urine is called glucosuria. The tubules normally reclaim filtered glucose, so its appearance in urine can be examined as a kidney-related finding. That observation alone is not a diagnosis of diabetes.

These results give us a reason to consider phosphorus sources. They do not establish that all inorganic phosphorus behaves identically, or that every food below one number is safe.

< Personal Thoughts >

I start with the phosphorus content, then ask how it was expressed, which ingredients or additives supplied it, and how much calcium was present relative to phosphorus. Without those details, I cannot tell how closely a food resembles the diets actually tested.

There is also a result that leads into the next article. In Alexander’s first trial, blood phosphorus actually fell in the high-phosphorus group, despite the changes in kidney filtration and ultrasound findings.

The amount eaten did not simply appear as a matching number in the blood. Absorption and excretion were also being regulated.

Hormones participate in that regulation. FGF23, the subject of the next article, signals the kidneys to excrete more phosphate. We will meet cats whose blood phosphorus remained similar while this hormone changed.

< References >

1. Alexander J, Stockman J, Atwal J, et al. Effects of the long-term feeding of diets enriched with inorganic phosphorus on the adult feline kidney and phosphorus metabolism. British Journal of Nutrition. 2019;121:249–269.
2. Dobenecker B, Webel A, Reese S, Kienzle E. Effect of a high phosphorus diet on indicators of renal health in cats. Journal of Feline Medicine and Surgery. 2018;20:339–343.
3. Dobenecker B, Hertel-Böhnke P, Webel A, Kienzle E. Renal phosphorus excretion in adult healthy cats after the intake of high phosphorus diets with either calcium monophosphate or sodium monophosphate. Journal of Animal Physiology and Animal Nutrition. 2018;102:1759–1765.

< About the Author >

Hocheol Shin, Ph.D. Greycoat Research

B.S./M.S./Ph.D. in Biological Sciences, KAIST (Korea Advanced Institute of Science and Technology)

Research background in cancer immunology, tumor microenvironment, and translational oncology. Currently engaged in companion animal health and longevity research.

[Educational Disclaimer]

This article is for educational purposes only and is not a substitute for professional veterinary advice, diagnosis, or treatment. It is not intended to diagnose, treat, cure, or prevent any disease.

Please consult your veterinarian before making changes to your cat’s diet, supplements, or care routine.