The kidneys are a vital organs. This means that your dog cannot survive without them. Small but mighty they play many roles in keeping your dog happy, healthy and alive. One of these roles is maintaining the complex, delicate balance between two very important ions – calcium and phosphorus.
Calcium and phosphate are both minerals that are important for your dogs health. Together, they help build strong bones and teeth, and also play a role in cell and nerve function.
It’s crucial that the kidneys regulate this balance accurately as an imbalance in their levels can lead to various disorders including seizures, arrythmias and respiratory problems.
The process by which the kidneys regulate the calcium / phosphorus level is an intricate one, involving a number of different hormones (Figure 1). It’s somewhat complicated, but let’s try to simplify how this process works in order to understand why restricting phosphorus intake is such as a vital part of a therapeutic kidney diet, if not the most important. If it all sounds a bit daunting, stay with me!
How the kidneys regulate the calcium / phosphorus balance
Situated next to your dog’s thyroid gland are four small glands called the parathyroid glands. These small but mighty glands produce a biochemical called parathyroid hormone (PTH).
When something causes your dog’s calcium levels to drop, it results in PTH being secreted from the parathyroid glands and its levels increasing. This in turn causes two things to happen – calcium levels increase again, and the kidneys convert vitamin D2 (calcidiol) into vitamin D3 (calcitriol). Active vitamin D3 also increases calcium levels, while at the same time, blocks further secretion of PTH, meaning calcium levels don’t get too high and everything is kept in check (Figure 1A).
This means that when it comes to calcium levels, PTH and calcitriol work together to increase its levels. And when calcium levels are high enough, PTH and calcitriol levels decrease, meaning there’s no further increase in calcium levels.
Figure 1: Calcium / Phosphorus regulation in healthy kidneys
A – When blood calcium (Ca2+) levels decrease, the parathyroid hormone (PTH) is secreted resulting causing an increase in calcium levels and conversion of calcidiol to calcitriol. Calcitriol further increases calcium levels, while also ensuring levels don’t get too high by shutting off secretion of PTH. B – PTH works to stimulate the kidneys to ‘dump’ phosphorus (P), meaning its levels decrease, while calcitriol encourages the kidneys to retain phosphorus, keeping its levels even. Red = decrease / inhibit; Green = increase / activation.
Contrary to this, when it comes to phosphorus, PTH and calcitriol work in opposition to one another (Figure 1 B). Calcitriol stimulates the kidneys to retain phosphorus keeping its levels even, while PTH encourages the kidneys to dump the ion, resulting in a decrease in its levels.
When your dog’s kidneys are working properly and are healthy, calcitriol and PTH work together in an almost beautiful, interlinked harmony. This harmony ensures your dog’s calcium / phosphorus levels are always balanced which in turn, helps keep them healthy.
Why failing kidneys can’t maintain the calcium / phosphorus balance
In the early stages of failure, the kidneys are unable to get rid of phosphorus properly, meaning its levels rise (Figure 2). When this happens, it causes a protein called fibroblast growth factor 23 (FGF-23) to be realised from the bones, which encourage the kidneys to ‘dump’ more phosphorus, this results in a decrease in its levels.
At the same time, FGF-23 works to inhibit calcitriol. As mentioned, calcitriol encourages the kidneys to save phosphorus, meaning that its inhibition causes phosphorus to be dumped and its levels to be decrease. Excretion of FGF-23 from the bones also inhibits PTH production by instructing the parathyroid glands to stop producing it.
Figure 2: Calcium / Phosphorus regulation in failing kidneys
When the kidneys are failing, it results in an increase in phosphorus levels. This in turn stimulates fibroblast growth factor 23 (FGF-23) to be released from the bones. Release of FGF-23 causes inhibition of PTH production from the parathyroid glands, inhibition of calcitriol production and in general, a direct decrease in phosphorus levels. Red = decrease / inhibit; Green = increase / activation
While FGF-23 works well to keep phosphorus levels low, it has a knock-on effect. By inhibiting PTH and calcitriol, FGF-23 indirectly causes a decrease in calcium levels. This drop overrides the effects of FGF-23, meaning the parathyroid glands increase PTH production to restore the body’s demand for circulating calcium.
Your dog’s body also increases calcium levels by encouraging the release of structural calcium from their bones. But this also causes phosphorus levels to increase. Combined, all of this causes your dog’s calcium / phosphorus to go completely awry. Something which can and does have consequences for your dog’s health.
What happens when calcium / phosphorus levels aren’t balanced
So why is it so bad if the calcium / phosphorus levels in your dog’s body aren’t perfectly balanced?
Well firstly, calcium is vital to helping your dog move properly. Movement of calcium ions encourages muscles to contract all over your dog’s body – from their legs to their heart and blood vessels. Meaning reduced levels of it aren’t good for your dog’s health in general and can cause problems with walking.
On top of this, when there’s an abundance of phosphorus and calcium in the blood (so-called ‘circulating phosphorus’ and ‘circulating calcium’), these two ions bind one another. When this happens, crystals form which become deposited in the soft tissues of your dog’s body, resulting in their bones becoming weaker and inflammation.
An imbalance of these ions also causes the parathyroid hormone to work in overdrive to try and keep calcium levels at the correct level. Increased activity of these glands can cause your dog to become dizzy and dazed and unresponsive.
Can the scales be balanced?
The good news is that by limiting the amount of phosphorus in your dog’s food, we can regulate phosphorus levels specifically, which then has a knock-on effect to regulate calcium levels.
If a therapeutic diet is unsuccessful in maintaining a normal level of blood phosphorus other methods include:
- Phosphate binders: These are given with food and work by binding phosphorus, meaning it can’t be absorbed by the body
- Fluid therapy: Giving fluids under the skin to boost circulation through the kidneys and in turn, phosphorus excretion.
So how much phosphorus does your dog need?
Your dog’s phosphorus needs depend on their kidney function. As all dogs are different, and will have different stages of CKD, it’s important phosphorus is reduced in line with creatinine and blood phosphorus readings.
Creatine itself is not a dangerous toxin but rather an important and easily measured indicator of the kidneys’ ability to filter the blood. As kidney function decline, creatinine clearance also goes down and the level of creatinine accumulate in the blood.
A healthy adult dog requires 100mg of phosphorus per kg of body weight to the power of 0.75 per day (BW 0.75x 100mg). For very early-stage kidney disease I start by using 80% of that number and will reduce further as creatinine levels increase.
For comparative purposes, most regular commercial diets have around 1.5g/1000 kcal of phosphorus. The phosphorus content of a typical commercial therapeutic diet for kidney disease have around 0.4g to 1.2g/1000 kcal of phosphorus.
Of course, phosphorus isn’t all bad. Far from it. An adequate amount of phosphorus is essential to keep your dog healthy. The goal is to restrict phosphorus, not remove it.