Calcium, phosphorus, vitamin D, and parathyroid hormone (the hormone that regulates them) are all crucial for healthy bone development in puppies, and for maintaining healthy bones in adult dogs. The exact amount of each nutrient needed in the diet varies with growth, size, pregnancy or lactation, and overall health. Their amounts relative to each other are crucial too. Let’s take a look at why they are so important.

Why are calcium and phosphorus important?

Calcium and phosphorus are both essential nutrients in dogs, meaning they have to be provided in the diet in the correct amounts, and be ‘bioavailable’ (i.e. can be absorbed from the diet into the bloodstream). Calcium is the most abundant mineral found in the body, followed by phosphorus. 

Both calcium and phosphorus are essential for healthy bones and teeth. On top of these structural roles, they both have important functional roles in the body too. For example, calcium has an important role to play in sending nerve signals, and in communication between cells. This means it has an important role to play in heart health, the health of the nervous system, and in keeping muscles healthy. Phosphorus forms part of the cell membrane (the barrier, or ‘wall’, around a cell), it forms a part of the structure of DNA and it is involved in providing energy to cells. 

Why is the calcium-phosphorus ratio important in dogs?

Having too much or too little of these nutrients in the diet can cause health issues, as can having the wrong ratio to each other. Imbalance of these nutrients during growth can lead to deformities and an increased risk of bone fractures during normal activity (known as pathological fractures). 

In puppies, the correct balance of calcium to phosphorus is essential for healthy bone growth, and especially so in large breed puppies which are fast-growing. The exact requirements depend on the age of the puppy and their expected adult size. Since puppies grow more quickly than human babies, ensuring their nutrients are exactly right is even more important. Remember, puppies will be fully grown in only one or two years – 10 times faster than humans!

Calcium and phosphorus are the two main components of bone. They need to be considered together, because their levels are linked and dependent on each other. If a growing puppy eats too much or too little of either, they will be at greater risk of developmental orthopaedic disease (such as joint dysplasia or bone deformities). 

Calcium deficiency

Too little calcium (or too much phosphorus) in a puppy’s diet can cause ‘nutritional hyperparathyroidism’. Low calcium levels cause a hormone called parathyroid hormone to be secreted into the blood, leading to calcium being released from the bones (known as demineralisation). This causes bones to be underdeveloped and weak, making them much more prone to fracture, or to being deformed. Symptoms can include bone pain, limping, stiffness and neurological signs.

If a puppy is either fed too much food, or is fed a poor quality food that cannot be easily digested, their stools will be bulkier than they should be (known as increased faecal dry matter). This can lead to more calcium and phosphorus being passed out (lost) in the stool than usual. These puppies will therefore have a higher requirement for calcium and phosphorus in their diet, to compensate for the extra lost in the stool. This highlights the importance of a good quality, easily digestible, complete puppy food.

Calcium excess

Too much calcium can also cause issues. Too much calcium can lead to calcium phosphate deposits in soft tissues, known as mineralisation. These soft tissue mineralisations cause inflammation and damage to the soft tissues, and are especially dangerous in organs, such as the kidneys. 

Leaving diet aside for a moment, high blood calcium can be caused by various illnesses, such as kidney disease and some forms of cancer. Signs of high blood calcium levels (hypercalcaemia) include poor appetite, weakness and lethargy (sleepiness).

Too much calcium can also cause a relative deficiency of other important minerals, such as phosphorus. In puppies, an imbalance in calcium to phosphorus ratio puts them at risk of developmental orthopaedic diseases. Too much Vitamin D in the diet can also cause high calcium levels, further highlighting the importance of a carefully formulated, nutritionally complete diet!

Urinary Health

Urine stones can form when too many minerals are excreted in the wee, making the urine ‘saturated’ with mineral or organic compounds, which precipitate into crystals. There are other factors affecting this process too, such as the pH of the urine or urine infections. These crystals can then form bladder stones (known as uroliths). Many of the uroliths found in dogs are made up of calcium oxalate monohydrate or magnesium-ammonium-phosphate (known as struvite).

Hypercalcaemia (too much calcium in the blood) can lead to too much calcium in the urine, as this is one of the ways the body tries to remove the excess calcium. Fortunately, dogs don’t usually pass much calcium out in the urine, even when there are high levels of calcium in the diet. However, some individuals, and some breeds, are at greater risk. 

Kidney disease

Hypercalcaemia can cause soft tissue mineralisation, and so possibly damage the kidneys. How high the calcium levels are in the blood, and how long they are high for, affect how badly they are damaged. Calcium regulation within the body is usually very effective though, so there is often an underlying cause for the hypercalcaemia, and it is rarely caused by diet alone in dogs. 

Calcium and phosphorus metabolism are both affected in kidney disease. As kidney disease advances, the kidneys are less able to excrete phosphorus in the urine, meaning it builds up in the bloodstream. This can cause soft tissue mineralisation, which can cause more damage to the kidneys, therefore worsening kidney disease.

Specific prescription kidney diets are carefully formulated with controlled levels of phosphate, and some are even tailored to the stage of kidney disease. 

How much calcium and phosphorus should be in dog food? 

Usually, a calcium:phosphorus ratio of 1.2:1 is ideal. A ratio of 1.1:1 to 1.4:1 is generally considered safe. The National Research Council sets out a safe upper limit for calcium of 4.5 g of calcium per 1000 kcal of metabolizable energy for puppy food. 

The exact amount of each mineral required varies with growth, pregnancy, lactation and illness. So, it’s really important to feed a diet which is appropriate for your dog’s life stage, breed and overall health. If you aren’t sure, ask your vet or vet nurse for advice. 

Take home message

Incorrect calcium and / or phosphorus levels in the diet can have a number of detrimental effects on a dog’s health, especially in growing puppies.

Good quality, complete puppy or dog foods will contain the correct amount of calcium and phosphorus. Some pet food manufacturers even make ‘large breed’ puppy food, taking into account the different requirements of fast growing puppies. 

When feeding a complete dog food, calcium supplementation is not necessary, and can even be harmful. However, getting the ratio right in homemade dog food is very tricky. If you are considering a home cooked diet for your dog, it’s safest to do so under the guidance of a veterinary nutritionist. 

Further reading

Nutritional secondary hyperparathyroidism in Dogs (Canis) – Vetlexicon

Calcium and phosphorus – getting the balance right – Royal Canin

Which is more important in dog food – ingredients or nutrients? – Vet Help DirectIs more meat always better in a dog food? – Vet Help Direct