Phosphorus is a crucial nutrient for cats. However, recent studies indicate that high levels of phosphorus in their diet may worsen kidney disease and even contribute to its development in healthy cats. In this article, we delve into the world of phosphorus, exploring why it is added to cat food, the latest research on its link to kidney disease, and answer some common questions about this essential mineral.
What is Phosphorus?
Phosphorus is a vital mineral found naturally in many foods. It plays a crucial role in tissue growth and repair, as well as DNA production. Additionally, phosphorus helps balance and facilitate the use of other important vitamins and minerals in the body, including vitamin D, iodine, magnesium, and zinc.
The body aims to maintain a balance between phosphorus and calcium. Ideally, the ratio of calcium to phosphorus should be close to 1:1. While calcium can be slightly higher at a ratio of 2:1, phosphorus should never exceed calcium levels in the body.
Why is Phosphorus Included in Cat Food?
Phosphorus can be divided into two main types: organic and inorganic. Organic phosphorus naturally occurs in many foods, especially grains and high-protein sources like meat. Since cats are obligate carnivores, their diets require a high protein content. Consequently, commercial cat foods, both wet and dry, often contain high levels of protein.
Canned or wet cat food with higher meat content naturally contains more organic phosphorus. Similarly, raw diets consisting primarily of meat and bones also have higher organic phosphorus levels.
On the other hand, inorganic phosphorus refers to phosphorus bound to other minerals like calcium, sodium, or magnesium. Inorganic phosphorus is often used in food as a preservative to extend shelf life and enhance texture. Examples of inorganic phosphorus include calcium monophosphate, sodium monophosphate, and sodium dihydrogen phosphate.
How Much Phosphorus Do Cats Need?
The recommended daily phosphorus intake for cats has been extensively researched and established by the National Research Council (NRC). These guidelines serve as the basis for formulating many pet diets.
The Association of American Feed Control Officials (AAFCO) also bases its nutrient guidelines on the NRC publications, although it provides more frequent updates. While pet food companies are not required to follow AAFCO guidelines, many states have incorporated them into their regulations.
Both the NRC and AAFCO express phosphorus requirements in different ways to facilitate better diet comparisons. The NRC expresses nutrients as grams per 1000 kilocalories of metabolizable energy (g/1000 kcal ME), while AAFCO uses a percentage of dry matter (%DM) in addition to the NRC’s expression.
The guidelines for feeding phosphorus to adult cats and kittens are as follows:
- Adult Cats: Recommended Daily Allowance: 0.6g/1000 kcal ME
- Adult Cats: Minimum Allowance: 0.4g/1000 kcal ME
- Kittens: Recommended Daily Allowance: 1.8g/1000 kcal ME
- Kittens: Minimum Allowance: 1.2g/1000 kcal ME
- Adult Cats: Minimum Allowance: 0.5% dry matter or 1.25g/1000 kcal ME
- Kittens: Minimum Allowance: 0.8% dry matter or 2.0g/1000 kcal ME
It’s important to note that nutritional requirements vary depending on the cat’s life stage. Kittens require higher phosphorus amounts to support their bone and tissue development. Pregnant queens have similar feeding requirements as kittens since they support the growth of their offspring. However, there are no specific guidelines for senior cats due to their varying nutritional needs based on health conditions.
Interestingly, there are no maximum phosphorus levels advised by these guidelines. This raises concerns that we will address later in this article.
Are There Upper Phosphorus Limits?
Specific upper safe limits for dietary phosphorus have not been firmly established by the NRC, AAFCO, or FEDIAF (Europe’s AAFCO equivalent). However, the 2006 NRC publication suggests an expected safe upper limit range of 2.5-3.5g/1000 kcal ME assuming an appropriate calcium-to-phosphorus ratio.
Can a High Phosphorus Diet Harm My Cat?
While phosphorus is essential, excessive intake can be detrimental. High-protein and high-phosphorus diets have been shown to worsen existing chronic kidney disease (CKD) in cats. As a result, therapeutic approaches often involve feeding cats with renal failure diets lower in phosphorus and sodium while providing high-quality protein and a neutral pH.
While the causes of CKD in cats are not fully understood, recent research has focused on the impact of dietary phosphorus on kidney health in healthy cats. A landmark study published in 2018 shed light on the subject, demonstrating that feeding healthy cats a diet extremely high in phosphorus (about 5 times the NRC recommended allowances) resulted in kidney damage markers after only 30 days.
This study attracted attention, prompting further research into the effects of dietary phosphorus on cats’ kidneys. Other studies have highlighted the effects of inorganic phosphorus and the importance of the calcium-to-phosphorus ratio in the diet.
These emerging studies reveal concerning findings. For instance, a study published in 2019 identified that 33% of the analyzed cat diets had phosphorus content exceeding 3.6g/1000 kcal ME. Another study in the same year found that all evaluated commercial wet cat foods contained phosphorus levels above minimum requirements.
To summarize recent research findings:
- High protein diets often have higher levels of phosphorus that may harm healthy cats’ kidneys.
- Sodium salt phosphates in the diet are associated with higher phosphorus levels.
- Homemade diets may not meet recommended daily allowances.
- Highly soluble inorganic phosphates combined with low calcium-to-phosphorus ratios are linked to kidney injury.
These studies have prompted calls for reform in commercial diets to better align with recommended phosphorus allowances and appropriate calcium-to-phosphorus ratios. Transparent food labeling modifications proposed by AAFCO in 2023-2024 are expected to help pet parents make more informed choices for their cats’ health and safety.
Should Senior Cats Eat a Low Phosphorus Diet?
The effects of phosphorus on senior cats compared to young adult cats have not been specifically evaluated. However, it is crucial to recognize that older cats are more prone to developing chronic diseases. Early detection is key to better outcomes and longevity. If kidney disease is detected early, immediate efforts should be made to feed a low phosphorus cat food. Other interventions for renal support, such as increasing water intake, should also be considered.
How to Find the Phosphorus Content of Your Cat’s Food
Currently, determining the phosphorus content of a pet’s diet is challenging. Pet food labels typically include a guaranteed analysis section that outlines percentages of protein, fat, and carbohydrates, as well as an ingredients list. However, unlike human food labels, pet food labels lack a comprehensive breakdown of vitamin, mineral, and other ingredient percentages or amounts.
Change is on the horizon, though. AAFCO is proposing significant changes to pet food labels, replacing the current “Guaranteed Analysis” section with “Pet Nutrition Facts.” These new labels will provide expressed amounts of individual ingredients. These changes are expected to be implemented in 2023-2024.
Nevertheless, even with these changes, determining phosphorus levels and calcium-to-phosphorus ratios at home will remain challenging. Additionally, current pet food analysis methods cannot reliably distinguish between organic and inorganic phosphorus. Consequently, pet parents will still be unaware of the actual phosphorus levels in their pets’ diets.
Prescription veterinary diets formulated for pets with kidney disease are an exception. These renal diets must meet maximum phosphorus and minimum calcium requirements, which are listed in the guaranteed analysis section.
How Do I Choose a Diet for My Cat?
There isn’t a single diet type or brand that can be avoided entirely. Research has shown significant variation in phosphorus levels among different diet types. Inorganic phosphates and phosphate salts, such as monosodium phosphate, have been associated with kidney injury.
When selecting cat food at home, check the ingredients list for sodium salt phosphates. Although safe levels have been established, opting for calcium salts instead of sodium salts may be a consideration.
Raw diets with high phosphorus levels from meat and bones may not be the best choice. While organic sources of phosphorus have not been fully evaluated, research has shown that homemade diets often fail to meet recommended daily allowances.
Fresh-made home delivery diets from companies like Smalls, The Honest Kitchen, Raised Right, and JustFoodForCats offer fresh, nutritionally balanced ingredients with fewer preservatives. Consultation with a board-certified veterinary nutritionist is also an option to ensure a nutritionally balanced homemade diet.
While phosphorus is an essential nutrient, it can be harmful if consumed in excess. Research has shown that diets exceeding 3.6g/1000 kcal ME of phosphorus, especially when coupled with high levels of sodium salt phosphates and a low calcium-to-phosphorus ratio, can contribute to kidney disease in cats.
Recent studies have highlighted that many commercial cat diets contain phosphorus levels exceeding the recommended allowances. Although these concerns are not specific to any particular brand or diet type, pet parents can make informed choices by examining the presence of sodium salt phosphate-based preservatives. It is also essential to note that homemade diets may have their own risks, and further research is required to understand the effects of high organic phosphorus amounts in the diet.
As researchers delve deeper into the impact of phosphorus in cat food on kidney function, there is a growing need for established safe upper limits and stricter guidelines for calcium-to-phosphorus ratios in commercial diets. AAFCO’s proposed transparent food labeling changes aim to assist pet parents and veterinarians in making the best dietary choices for their cats’ health and well-being.