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Under industry standards, commercial cat food must contain a minimum level of phosphorus, but there is no maximum limit. Members of the Association of American Feed Control Officials will vote in August on whether to establish one.

Four weeks into the test period of a nine-month study investigating the link between cats' health and phosphorus in their food, Dr. Jonathan Stockman and his team cut the trial short.

Some of the 24 feline subjects had lost their appetites and were shedding weight and vomiting. The kidneys of 22 cats appeared concerningly bright on ultrasound images — something that happens when structural damage to the organs reflects more sound waves back than normal, causing them to look white. Stockman and colleagues knew they were observing a decline in kidney function, so before the cats got sicker, they halted the study and stopped feeding the cats the test diet.

The amount of phosphorus the cats were ingesting, 4.8 grams (g) per 1,000 kilocalories (kcal), was consistent with the high end of what cats might consume from commercial diets, a quantity that Stockman determined through an informal survey. Phosphorus is important in feline diets because it's an essential nutrient that supports key bodily functions. But too much of a good thing can be harmful.

The first study to suggest a link between high levels of dietary phosphorus and kidney disease in healthy adult cats had been published more than 20 years earlier, but now Stockman was seeing the negative effects in real time.

Stockman and colleagues at the Waltham Petcare Science Institute, which is owned by Mars, designed a second study, dropping the phosphorus to 3.6 g per 1,000 kcal. This time, the majority of cats did not demonstrate significant differences in kidney function, although the researchers noted that some cats showed a gradual increase in the waste products creatinine and urea, indicators of kidney disease.

"At that point, it was obvious that we need to probably put some sort of a limit on dietary phosphorus," Stockman said.

The research findings of both studies were published in 2018. Research published by other teams during that same year and later provided further evidence supporting a maximum.

Today, an official limit is imminent. Members of the Association of American Feed Control Officials (AAFCO) will vote in August on a recommendation to set a maximum for phosphorus in cat food. The recommendation is expected to be approved.

AAFCO is an independent organization that sets industry standards for animal feed, including pet food. Its members are employees of state and other jurisdictional agencies that enforce animal feed regulations, but AAFCO itself is not a regulatory body, nor does it have enforcement power. Still, its decisions carry significant weight in the feed and pet food industries.

Each year, AAFCO publishes the latest in its rules, ingredient definitions, nutrient profiles and more. Its 2026 Official Publication (OP) shows the phosphorus maximum for cat food in place. However, that was an editorial error, according to AAFCO Executive Director Austin Therrell. The maximum is actually not yet an approved part of the feline nutrient profile, he told the VIN News Service.

However, there's no harm in abiding by that maximum now, Therrell said. "Being confident that those recommendations are going to go forward at this point in time, could a company inadvertently see those updates in [the] 2026 OP and go ahead and voluntarily make those changes? Absolutely," he said.

The scientific inquiry

Chronic kidney disease is common in cats, especially older ones. Some 30% to 40% of cats older than 10 years old have CKD, which can affect critical bodily functions such as filtering waste products from blood and producing urine. While CKD has a variety of possible causes in cats, such as bacterial infections, the most common predisposing factor is age.

Humans with CKD have long been urged to choose diets low in phosphorus, as compromised kidneys have difficulty filtering excess phosphorus from the body. One of the first studies to look at the connection between phosphorus and cats' kidney health was in 1995. Still, there was not much published data until the last decade. A 2018 study in the Journal of Feline Medicine and Surgery documented how cats fed a diet high in phosphorus may experience impaired kidney function. That same year, Stockman's research was published.

While the evidence linking excess phosphorus in the diet to potential kidney damage is now more robust, why phosphorus has that effect is something of a mystery, said Stockman, who is now on the faculty at the University of California, Davis.

Furthermore, it's difficult to measure how much phosphorus is bioavailable — in other words, how much a cat will absorb from its food. In commercial feline diets, phosphorus typically comes from two main sources: naturally occurring phosphorus, found in meat and vegetables used in the food, and inorganic phosphoric salts, added for palatability and shelf stability, said Stockman. In general, phosphoric salts are more readily bioavailable than naturally occurring phosphorus.

The solubility of the phosphorus, as well as other ingredients in the diet, can influence how easily phosphorus is absorbed into the body.

"We think that also magnesium can impact absorption to some extent," Stockman said. "But, the calcium-phosphorus ratio and the form of phosphorus are the two most important parameters."

Considering these factors, the nutrient profile changes that AAFCO's members will consider in August include a phosphorus maximum (5 g per 1,000 kcal), a limitation of how much phosphorus in the diet is highly soluble (1 g per 1,000 kcal) and a calcium-to-phosphorus ratio of 1-to-1.

Though the proposed maximum is higher than the level at which Stockman and colleagues observed adverse effects, he said the additional limits on highly soluble phosphorus and the calcium-to-phosphorus ratio will make the new maximum safe. In his first study — the one that ended early — the amount of highly soluble phosphorus was approximately three times the new proposed limit. By contrast, a study published in 2021 looked at an intake of 5 g of phosphorus per 1,000 kcal, with only 1 g per 1,000 kcal coming from highly soluble phosphates, and the cats did not develop indicators of kidney disease.

Making it official

The AAFCO pet food nutrient profiles are, for the most part, a list of the minimum nutritional requirements for dogs and cats. Some listed nutrients also have maximums, but most don't. Currently, the phosphorus minimum for cats is 1.25 g per 1,000 kcal. (A phosphorus maximum, as well as a minimum, is in place for dog food. The reason for that upper limit is unrelated to the reason for the proposed maximum for cats, Therrell said.)

For AAFCO to make a change to its pet food nutrient profiles, such as instituting an upper threshold for a particular nutrient, evidence must be provided in the form of at least three peer-reviewed studies in the relevant species. According to the protocols listed in the AAFCO Official Publication, the studies must cover at least two distinct populations. The proponent must also provide a comprehensive review of the scientific literature that highlights any potentially contradictory evidence.

Prompted by the research by Stockman and others, AAFCO's pet food committee requested an expert panel to evaluate the scientific literature and make recommendations.

Such reviews sometimes result in a recommendation to make no changes, as was the case after a 2021 expert panel considered establishing a copper maximum in dog food. The panel determined that the existing literature did not sufficiently prove a link between copper in dog food and copper-associated hepatitis in canines.

The phosphorus review had a different outcome. That panel, completing its review in January 2025, recommended adopting the maximum. In the fall, AAFCO's pet food committee concurred. In April 2026, AAFCO's board of directors did, as well.

The full AAFCO membership will cast the decisive vote on the recommendation when it meets in August. Therrell anticipates the maximum will be approved. Normally, he said, he'd know by now about pushback to a proposal, and he's heard of no resistance to this one.

Typically, formulation recommendations go into effect immediately after approval and appear in the next edition of the AAFCO official publication.

Seeing those changes implemented in pet food formulations will take longer. Even relatively straightforward alterations can take significant time, according to an email from the Pet Food Institute, due to factors such as packaging redesign. More complicated changes involving reformulation require additional time to identify alternative ingredient sources, conduct safety and palatability reviews and more.

"The phosphorus changes … unfortunately, are not going to be simple or immediate to implement," the Pet Food Institute said. "Because the changes involve not only phosphorus limits themselves, but also nutrient ratios and distinctions between different phosphorus sources, some products that require reformulation could take a significant amount of time to bring into compliance."

They estimated that changes requiring a reformulation "would likely take many months to over a year."

Currently, pet food labels do not always include the exact amount of phosphorus, and the form it takes, in the diet. Some brands might have more detailed diet analysis information posted on their respective websites, although it might be limited to the minimum amount of phosphorus. In the absence of more complete information, Stockman suggests that curious pet owners contact the manufacturer of their pet's food to ask about its phosphorus content.