The importance of the calcium:phosphate ratio in cat food 

Many people are aware of the need for a range of vitamins and minerals in their diet, and the same is also true for our pets. In fact, some vitamins and minerals are so crucial to bodily function, that severe disease can develop without them. But did you know that too much can also be detrimental? In fact, balanced nutrition, is not just about avoiding too much or too little: some nutrients within the body are also influenced by the concentration of other nutrients. The ratio of calcium to phosphorous is one of the most important examples of this. Understanding the importance of these minerals in your cat’s food, means understanding what they do, and what can go wrong if they are out of balance. 

Roles of Calcium 

Calcium is considered an essential mineral and is one of several nutrients that are essential for a healthy body. It is most well-known for its role in the provision of strength to bones and teeth. The need for dietary calcium changes across an animal’s life, with higher levels needed during the development of these calcium-rich structures as they grow. Pregnancy and lactation also increase the body’s demand for calcium. 

Calcium has additional roles in nerve and muscle function. Contraction of muscles to allow movement requires calcium, and impulse transmission throughout the nervous system also needs calcium. Additionally, it enters the cells of the heart and has an important role in the mechanism of the heart pumping, and therefore oxygen circulation and life. 

Moreover, blood clots require calcium to form. Clots are an essential defence mechanism of the body, both as a barrier against invading organisms, but also as prevention of excessive blood loss. These roles in the circulation of oxygen through heart beats and its part in the body’s defence against the outside world, make it a critical nutrient. 

Roles of Phosphorous 

Like calcium, phosphorous is also required for bone and teeth development. At cell level, it is involved in the maintenance and repair of cells, making it necessary for healthy tissues. Within cells, it is one of the components of ATP (adenosine triphosphate), which is the usable form of energy utilised by, and stored within, cells. At genome level, phosphorous forms part of the structure of both DNA (the body’s genetic code), and RNA (a messenger from DNA which triggers protein production). 

Balancing Calcium and Phosphorous 

In optimal conditions, about 50% of the body’s calcium will be either free or in an ionised (electrically charged) form. The rest will either be bound to proteins (40-45%) or associated with non-protein complexes (5-10%), such as phosphate. Phosphorous is usually bound to oxygen in the body to form phosphate, and this is how it is stored. Phosphate binds to calcium to make it less available when there is an excess or unbinds from it when needed. It is the concentration of calcium within the blood that drives this relationship. 

Both calcium and phosphorous are absorbed from the diet, a process that requires vitamin D. If calcium levels drop, a gland in the neck, the parathyroid gland, responds by producing a hormone called parathyroid hormone (PTH). PTH results in the separation of phosphate from calcium, and the release of both from stores into circulation. It also increases the activity of vitamin D to increase absorption from the digestive system as well as increasing the kidney’s reabsorption of calcium at the same time as decreasing phosphate reabsorption. This system works on a constant feedback loop, increasing or decreasing activity, to keep calcium levels in balance. 

A second hormone called calcitonin, produced by a different gland in the neck, the thyroid gland, also has a role in calcium balance. It is produced when there is excess calcium, increasing loss of the mineral through the kidneys as well as preventing calcium from being pulled out of stores. 

Out of Balance 

An imbalance of calcium and phosphorous in the body can lead to disease. High levels of calcium (hypercalcaemia) can result in several disease processes due to the role of this mineral. Symptoms can be subtle and slow to develop, or they may be more obvious, but overlap with many other disease symptoms. 

Vague symptoms include reduced appetite and energy, or weakness. Because of the kidney’s role in calcium excretion, urinary symptoms may develop such as changes to urination habits (increased frequency, or difficulty passing), and an increase in urination volume and thirst. This excessive loss of fluids can lead to dehydration. Vomiting may also occur, which can exacerbate dehydration also. 

Due to calcium’s role in nerve function, high levels of calcium can cause seizure activity. The mineral’s involvement in heart muscle contractions can also trigger arrhythmias (abnormal heart rhythms). An arrythmia may only be detectable when your cat goes for a check-up at the vet clinic, whereas the other symptoms will be visible at home. 

Too much phosphorous, or a low ratio of calcium to phosphorous, is toxic to the kidneys. Your cat’s sensitivity to phosphorous will likely change as they age, because kidney disease is a common condition that occurs as cats get older. One third of cats over the age of 12 have chronic kidney disease (CKD). In the early stage of CKD, it is often subclinical, which means that function is becoming insufficient but there are not yet outward signs of this. Those cats with subclinical CKD will be more susceptible to damage from phosphorous than a similarly-aged cat without CKD. 

If both calcium and phosphorous are high in conjunction, mineral complexes can form. These are more likely to happen in organs with fine blood vessels. The kidneys are a prime example of this. Calcium phosphate complexes can form urinary stones, known as uroliths. Although other types of uroliths are more common in the bladder (bladder stones), calcium phosphate uroliths are more likely to form in the kidneys (kidney stones). These uroliths can be very painful but also interfere with organ function, and cause difficulty or abnormal urination, and blood in the urine. If these mineral complexes form in the intestine, they can compromise nutrient absorption. 

If there is not enough calcium or phosphorous available (or vitamin D), rickets can occur. Rickets is a disorder that results in soft, or brittle bones. This is more likely to occur during the growth period of a kitten but can occur in adults. As bones are not strong, they can bend or break, resulting in deformities, lameness, and pain. 

Calcium and Phosphorous in the Diet 

Cats need to obtain calcium and phosphorous from their food. But to actually absorb it from their diet, they need a healthy intestine and access to vitamin D. While we humans can create vitamin D from exposure to sunlight, cats cannot do this. They can only get vitamin D from their diet. Additionally, not all minerals are absorbed equally. How much of a nutrient is actually able to get through the intestine and into the blood stream, is known as its bioavailability. 

Studies have shown that the ratio of dietary calcium to phosphorous for an adult cat should be between 1-1.3 Ca:1 P. As cats get older, the risk of kidney insufficiency increases so diets for senior cats should have a ratio towards the lower end of the recommended phosphorous intake. Kitten food will usually have a higher ratio of 1.1-1.5 Ca:1 P due to the increased demand from growth. For those cats with diagnosed CKD, their phosphorous intake should be restricted. 

Phosphorous in the food can be present naturally in protein from meat or fish, bony material, or some plant products (organic sources). It is also added to food as an inorganic salt. These inorganic salts are actually better absorbed by the digestive tract and are therefore more bioavailable than the form within raw materials. Phosphorous forms can also be water-soluble or acid-soluble and these differ with how the kidneys are able to eliminate them. When looking at the ingredients list of a cat food, inorganic minerals may be collectively referred to as ‘ash’ which represents what is left when the organic material is removed. 

Feeding Your Cat the Right Food 

If you ever have any questions or doubts about how best to feed your cat, speak to your veterinary team. Many clinics have nurses dedicated to nutritional advice and support, and if your cat has medical needs, then your vet can advise how best to approach their diet. It is also important to acknowledge that your cat’s dietary needs will change across their life, even without underlying disease. It is important to feed for your pet’s life stage, as no singular diet can provide balanced nutrition for an entire life span. 

There are many major brands of cat food that will meet the recommended amount of calcium, phosphorous and vitamin D. However, there is an overwhelming choice of cat foods currently available in the UK, and not all foods are created equal. Aside from selecting a diet for the right life stage, you should look for evidence that the food meets regulatory standards such as those of AAFCO (Association of American Feed Control Officials) or FEDIAF (European Pet Food Industry Federation). Meeting these standards means consistency across batches, as well as nutrition backed by scientific research. 

For home-prepared diets, achieving appropriate levels of nutrients is critical. If you do not want to feed a commercially available diet, or your cat refuses to eat one, then it is important to speak to your vet about a referral to a veterinary nutritionist. These professionals can work with you and your cat’s individual preferences to create a recipe that works for your cat’s needs. 

As absorption of key nutrients is affected by the pH of the stomach and the health of the intestinal lining, it is also important to consider other health conditions (co-morbidities) that may prevent calcium and phosphorous from being absorbed. If the intestinal tract is unhealthy, even a well-balanced diet may not be adequate for your cat. Prescription diets exist that can aid intestinal health and improve intestinal absorption. Again, a veterinary nutritionist can help create a diet that caters to multiple health conditions if available commercial diets are not appropriate. 

Conclusion 

As essential nutrients, both calcium and phosphorous are critical to the health of the body. But equally, it is the ratio of these nutrients to each other that is key. Your cat’s life stage and overall health can affect their sensitivity to alterations in this balance, making it important to reevaluate their dietary needs across their life. For optimal health, it is important to know that the food you feed is regulated and consistent, in the case of commercially available diets, or created with the professional input of a certified veterinary nutritionist. If you ever have questions or concerns about your cat’s dietary needs, or general health, contact your veterinary team in the first instance.