As kidney disease progresses, the ability of the body to keep phosphorus levels within a healthy range also diminishes. High phosphorus levels increase the risk of cardiovascular disease and can contribute to kidney disease progression so knowing if your phosphorus levels are elevated and what you can do to help lower them is incredibly important.
So that’s what I wanted to discuss today- all things phosphorus!
What is Phosphorus?
Phosphorus is a mineral, like potassium or calcium. You have more of this mineral in your body than any other except calcium. It was discovered in 1669 by Hennig Brand, an alchemist in Germany, when trying to create the philosopher’s stone, by heating residues from his own boiled down urine (as you do!). He formed a white material that glowed in the dark due to its reaction with oxygen. He named this substance phosphorus, a name derived from the Greek words phos meaning ‘light’ and phoros for ‘bearer’.
Phosphorus is present in every cell of the body. Most of the phosphorus in the body is found in the bones and teeth (85%), with the remaining 15% distributed throughout the blood and soft tissues.
What’s the difference between phosphorus and phosphate?
The terms phosphorus and phosphate are often used interchangeably but that’s not completely accurate. Phosphate is what you get when you combine the mineral phosphorus with oxygen. When you eat certain foods, phosphorus enters your intestines, when it meets up with oxygen, it becomes phosphate. When it comes to testing, phosphate is sometimes called phosphorus and vice versa but all you need to know is that they are testing the same thing.
For the purposes of this article, I’ll just stick to the term phosphorus because it’s not necessary to understand when the different terms should be used.
Role of phosphorus
Being an essential mineral, phosphorus plays many important roles within the body. It is needed for:
- Formation of bones and teeth
- Energy production
- Oxygen delivery to tissues
- Maintaining normal pH in the blood
- Helping the muscles contract
- Promoting healthy nerve conduction throughout the body
- Making DNA and RNA
How is phosphorus regulated
Dietary phosphorus is readily absorbed in the small intestine and in healthy individuals, excess phosphorus is excreted by the kidneys. A few hormones are also involved in regulating the balance of phosphorus in the body- parathyroid hormone (PTH), vitamin D and fibroblast growth factor-23 (FGF23).
Normal working kidneys can remove extra phosphorus in your blood. As kidney disease progresses, the kidneys start to lose their ability to excrete phosphorus which means levels in the body increase. As phosphorus levels rise, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) lose their ability to suppress phosphorus resorption by the kidneys, eventually resulting in elevated blood phosphorus levels.
Problems with high phosphorus
Having a high level of phosphorus in your blood is known as hyperphosphataemia.
Hyper = above normal
Phosphataemia = phosphate in blood
High phosphorus levels can cause damage to the body. Extra phosphorus causes calcium to be pulled out of the bones, making them weak and can lead to dangerous calcium deposits in blood vessels, lungs, eyes and the heart. Over time this can increase the risk of heart attack, stroke and death.
Several studies have shown an increased risk of mortality between CKD and phosphorus levels and it is also linked to faster progression of kidney disease.
High phosphorus levels can cause red eyes and itchy skin but may not cause any other obvious symptoms meaning that it can be difficult for people to know they have elevated levels. In some people, having high phosphorus levels may be associated with low calcium levels so people might experience symptoms related to low calcium including muscle cramps and spasms, numbness and tingling in the hands, feet and face, fatigue, poor appetite and difficulty swallowing.
How do you know if you have high phosphorus?
Because high phosphorus causes minimal obvious symptoms, the best way to figure out if you have high phosphorus levels is via a blood test. The normal serum phosphate concentration in adults ranges from 2.5 to 4.5 mg/dL (0.81 to 1.45 mmol/L) although these values may differ slightly between labs.
If you’re not sure what your phosphorus level is then I really recommend asking for copies of your blood tests so you can check for yourself, this will also allow you to track the level over time. If you have CKD and you haven’t had your phosphorus levels checked then ask your doctor to do this on your next round of blood tests.
Phosphorus and kidney disease
The kidneys are the primary organ responsible for regulating phosphorus balance. In people with CKD, the kidneys can start to lose the ability to excrete phosphorus. Initially, various compensatory mechanisms in the body work to keep phosphorus levels in balance but as kidney function declines the body is less able to compensate. Once the GFR falls below 20 or 30, hyperphophataemia is common.
Tips for reducing phosphorus intake
Reducing phosphorus intake through the diet is important for people with elevated phosphorus levels. Phosphorus is present in many foods so it’s impossible (and wouldn’t be beneficial) to avoid it completely but you can take steps to reduce your intake.
One thing to be aware of is that the bioavailability of phosphorus in the foods you eat differs. The amount of phosphorus absorbed from animal protein is higher than what our body can absorb from plant proteins. This is because the phosphorus in plants is bound by phytate. Humans lack the enzyme (phytase) needed to release phosphorus from phytate, so we don’t absorb as much phosphorus from plant foods.
Phosphorus is also added to many foods in the form of an additive or preservative. It’s found in foods such as fast foods, ready to eat foods, canned and bottled drinks, processed meats and most processed foods. This type of phosphorus is almost completely absorbed by the gut so is highly bioavailable. Avoiding these types of food is really important for your kidney health in general and to reduce the amount of phosphorus you are getting from your diet.
Here are some strategies to help lower phosphorus levels:
- Replace cow’s milk with soy, rice, or almond milk (double check they have no added phosphate)
- Avoid dairy
- Avoid sodas/soft drinks
- Avoid chocolate, nuts and nut butters
- Substitute whole wheat, bran and oat cereals with those made from rice or corn
- Avoid meat and other animal products
- Choose vegetarian protein over animal protein
- Avoid fast foods & ready to eat meals
- Avoid foods with phosphorus containing additives- check the ingredient list for anything with PHOS
- Maintain calcium levels within range
Boiling causes demineralisation of food which reduces phosphorus levels in both vegetable and animal derived products. The amount of phosphorus lost in this process is proportional to the amount of boiling water that is used, the size of the pieces of food and the cooking time.
Boiling can reduce phosphorus content of vegetables by about 51%, legumes by about 48% and meat by about 38%.
Dietary phosphorus restriction alone may be enough initially to control high phosphorus levels, but it is often inadequate for people with advanced kidney disease or those on dialysis. If phosphorus levels remain elevated, despite a low phosphorus diet, phosphate binders may be required.
Phosphate binding agents work by binding to phosphorus in the gastrointestinal tract which means the body isn’t able to absorb it from the food you eat.
There are four different types of phosphate binders commonly used. They are:
Calcium-based phosphate binders: these are commonly used and may also serve as calcium supplements. Because some people need to take several phosphate binders with each meal, they aren’t appropriate for people with already elevated calcium levels. Additionally, there is some concern about the calcium from these binders depositing in small blood vessels, causing organ damage. Common adverse effects: constipation, hypercalcaemia, hypoparathyroidism.
Aluminium-based phosphate binders: have been shown to have toxic side effects that cause bone disease and damage the nervous system if used longer term therefore they are rarely prescribed as a long-term phosphorus binder today.
Aluminium free, calcium free phosphate binders (eg. Sevelamer or Lanthanum): don’t contain aluminium or calcium so don’t cause problems with excess aluminium or calcium load. Sevelamer binds to bile acid so also reduces cholesterol levels. Contraindicated in people at risk for bowel obstruction. Common adverse effects: nausea, vomiting, diarrhoea.
Iron-based phosphate binders (eg. Sucoferric oxyhydroxide & Ferric citrate): have a dual role in increasing iron levels. Common adverse effects: diarrhoea, discoloured faeces and nausea.
So there we are. All you need to know about phosphorus and CKD. The first step is knowing what your phosphorus levels are, and if they are elevated, the second step is doing something about it. Hopefully the tips I’ve outlined here have given you a good place to start!
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