Most of the calcium in your body is not in your blood, but stored in your bones. Here, together with phosphate, calcium forms the basis of strong bone structure. With chronic kidney disease, calcium can easily get out of balance. In this article you will learn more about the calcium metabolism and how you can keep it in balance even with kidney disease.
Was exactly is calcium?
Minerals are essential nutrients for our body. These minerals belong to the micronutrients. Although your body needs them only in small quantities, it cannot survive without them. Since the body cannot produce them itself, we have to supply them through our food. Important minerals include: iron, magnesium and calcium. You can find these in many foods.
No mineral exists in your body in such large quantities as calcium. However, most of the calcium - 99% of it to be exact - does not float around freely in your bloodstream, but is stored in your bones and provides stability there. Only 1% of all the calcium inside your body is outside your bones, and only 0.1 - 0.2% is floating around in your blood in free form or bound to proteins. So it's no wonder that calcium performs a number of important functions in your body. These include, for example:
- Building strong bones and teeth
- Supporting your muscles in contraction and relaxation
- Transmission of nerve impulses
- Blood clotting
- Supporting the body's own enzymes
Where does the calcium in your body come from?
The mineral calcium is absorbed into your body through food. The calcium and phosphate balance in our body is controlled primarily by vitamin D and the parathyroid hormone. The parathyroid hormone is, as the name suggests, a hormone and is produced by the parathyroid glands.
Maybe you have never heard of your parathyroid glands and parathyroid hormone means nothing to you? That's probably because the parathyroid glands are four organs the size of a lentil. They are located around your thyroid gland and all together weigh less than 1 gram.
The kidneys are known to balance a variety of things in your body. This balance often gets thrown off in chronic kidney disease. This also applies to our calcium and phosphate balance, and thus of course to the metabolism in your bones. Since these are largely made up of calcium, a longstanding calcium deficiency can reduce bone strength and even lead to bone fractures. On the other hand, if you have too much calcium for too long, it can lead to ossification and calcification in your body. These calcifications can also occur in unwanted places, such as your blood vessels or in the soft tissue around your joints. For example, in the short term, significantly low calcium will cause tingling of the skin (often noticed around the mouth). Significantly high calcium can quickly lead to nausea and even vomiting, constipation, or neurological symptoms such as fatigue and even coma.
Why is calcium absent in chronic kidney disease?
People with kidney disease regularly have low calcium levels. This often has several causes, which we will briefly discuss here:
Less active vitamin D
Vitamin D is activated in the kidneys in a person with healthy kidneys. If this function is lost, such as in chronic kidney disease, calcium levels in your blood often drop as well. This is because active vitamin D increases the absorption of calcium from food in your intestines. If this function is missing, calcium levels in your blood will also drop as a result.
More phosphate in blood
In addition, kidney disease usually leads to an excess of phosphate in the blood. This is often due to the fact that phosphate can no longer be adequately filtered out of your blood in your kidneys.
Unfortunately, calcium and phosphate like each other very much in the chemical sense, which is why they bind - so they often come in a double pack! When you have a lot of phosphate in your blood, it sticks to the calcium and forms a calcium-like substance. So phosphate extracts and binds even more real, free calcium in your body.
What happens in the body when my calcium is low?
When your body's calcium levels are happily dropping, your parathyroid glands send out parathyroid hormone. Why do they do that? Parathyroid hormone is supposed to help correct this calcium deficiency in people with healthy kidneys. That's why parathyroid hormone does three jobs:
- Restricts the excretion of calcium via your kidneys
- Attempts to increase the formation of active vitamin D
- Promotes the release of calcium from the large reserve stores in your bone
With these adjustments, parathyroid hormone role is to help your kidneys keep your calcium levels stable. If the function of your kidneys decreases due to chronic kidney disease, parathyroid hormone unfortunately does not have the desired effect in your body. Your parathyroid glands get angry about this and as a result send out even more parathyroid hormone to help.
The logical consequence is: You have more and more parathyroid hormone in your blood, your calcium does not really increase and your phosphate is still high.
Phew, now that was quite a complex process in the body with many moving parts. Did you understand it all? If not, just read the last chapter again. Because this is a process that some doctors (non-nephrologists, of course) can't explain right off the bat.
How high should my calcium be?
A blood test usually only determines the concentration and amount of calcium in your bloodstream. So only the calcium outside your bones is measured.
One of the international nephrology guidelines (KDIGO) recommends regular checks of calcium, phosphate and parathyroid hormone levels in your blood if you have chronic kidney disease. The same goes for your vitamin D levels. Why do they do this? Well, to keep a close eye on exactly the complicated cycle described at the beginning of this article. Decisions on therapy in case of deviations of one of these lab values should therefore always be made by looking at all of these lab values together and not on the basis of individual deviations. Do you have problems with your bones, your phosphate and calcium values? It is best to talk directly to your nephrologist, if he or she does not already have this on his or her radar.
Target values for bone metabolism in kidney disease:
- Total calcium: 2.2 - 2.6 mmol/L (By the way: since calcium binds to the protein albumin, it should always be determined together with the albumin concentration in the blood).
- Phosphate: It is recommended to aim for the target range of 1.45 mmol/L or below. You should discuss how strictly to implement this with your medical team.
- Parathyroid hormone (iPTH) in kidney disease without dialysis: The optimal iPTH level is not known.
- Parathyroid hormone (iPTH) in kidney disease with dialysis: An iPTH level between 2 and 9 times the upper normal limit may be recommended.
As always, of course, you will find out your individual target values from your treating medical team.
In simple summary, it can be said that vitamin D can no longer be activated in the kidney in the case of kidney disease. Too little vitamin D often leads to low calcium levels as well. This whole process is driven once again by high phosphate. Parathyroid hormone is secreted more in response to this problem.
How much calcium do you need if you have kidney disease?
With chronic kidney disease, you should try to support the natural function of your kidneys as much as possible, or replace them in the advanced stages. In dialysis, the machine takes over part of this function. However, through your diet and your lifestyle, you make at least as big a contribution to keeping your body in balance.
The German Nutrition Society (=DGE) recommends an intake of 1000 mg of calcium per day for adults. While a calcium intake of 800 - 1,000 mg per day is often recommended for chronic disease without dialysis, there is unfortunately no clear recommendation for people on dialysis. Individual goals apply. It's best to discuss this with your dialysis team. There is also no general goal after kidney transplantation. It is best to check with your transplant center about this.
On dialysis, it's already difficult to determine the amount of calcium you're getting daily. Not only the calcium from your diet, but also the calcium from dietary supplements, vitamin D supplements, possible phosphate binders and that which you take in through dialysis must be included. So if you have any questions or uncertainties about your calcium levels, looking at your medications and talking to your dialysis team can also help.
Calcium and your diet in kidney disease
Through the hormones and vitamins within our body, calcium, phosphate and bone metabolism, as you can see, are very closely linked. As you probably already know, if you have kidney disease, you should keep your phosphate intake low to keep your cardiovascular system fit and healthy. At the same time, low calcium levels, due to the vitamin D deficiency and the excess of phosphate are not uncommon.
Calcium is found mainly in cheese and dairy products. However, these often also contain a lot of phosphate, which is why some are not a good choice for people with kidney disease. You have to be aware of this depending on whether you also need to eat a low phosphate diet. Other foods that contain a lot of calcium are for example rhubarb, spinach, sardines or salmon. However, always pay attention to the amounts of phosphate, potassium and sodium. In the food search feature of the Mizu app, you can calculate the nutrients contained in individual foods.
As you can see, the topic of calcium in kidney disease is quite complicated. In addition to the knowledge articles in the Mizu app, you can find answers to your lab values and diet primarily from your own medical team. Nephrologists in particular understand the relationships between calcium, phosphate, vitamin D and parathyroid hormone extremely well and can help you most here!
