What are calcium and serum calcium?
Calcium is a mineral stored mainly in your bones. But it's also found throughout the body, including the muscle and blood. You need to get calcium from your diet since the body can't make it.
Serum calcium is the calcium in your blood.
You need calcium for these important jobs:
- Form bones and teeth
- Make muscles move (muscle contractions), including the heart muscle
- Clot the blood to stop you from bleeding too much if you get a cut
- Help the heart keep a steady heart beat
- Help nerves work well
Calcium moves out of bones and into the blood when needed in order to keep serum calcium levels normal. If you don't eat enough calcium, then too much comes out of the bones. When too much calcium leaves the bones, bones get weak.
If serum calcium gets too low, then enough calcium can't go to nerves, muscles, and wounds. This can cause serious problems with the heart and other muscles, and with blood clotting.
Why can serum calcium become low in people with kidney disease?
Our bodies can't make vitamin D. We can only get vitamin D from food and by exposing our skin to sunlight. Healthy kidneys can take that vitamin D we absorb and change it to an active form. That active vitamin D then helps us absorb calcium.
But in chronic kidney disease (CKD), the kidneys are less able to make active vitamin D. Without enough active vitamin D, you absorb less calcium from the food you eat, so it then becomes low in your blood. Also, extra phosphorus in the blood of people with CKD may bind to calcium in the blood. This can then lower serum calcium.
A normal serum calcium level is 8.5 - 10.2 mg/dL. A serum calcium that is either too low or too high can be dangerous and both conditions need treatment. But patients with low serum calcium, even levels at the lower end of normal, have been found to reach kidney failure faster than people with higher serum calcium levels.
Why is low serum calcium a risk factor for reaching kidney failure?
Studies on thousands of patients with CKD show a link between low serum calcium and a faster time to reaching kidney failure than patients with normal levels. Low serum calcium is also linked to low vitamin D levels caused by CKD. Therefore, low serum calcium may be a sign that vitamin D is too low because kidney function is getting worse and heading toward kidney failure.
What can I do to keep my serum calcium level normal?
Eat a well-balanced diet. Other nutrients such as magnesium and vitamin D are needed to absorb calcium. A dietitian who works with kidney patients can help you plan a diet that fits your individual needs. Dairy foods are high in calcium, but may also be high in phosphorus, which is usually limited in CKD. Your dietitian can help you find healthy alternatives to many high phosphorus foods.
You may need a vitamin D supplement, but your healthcare team will need to order the right one for you. Don't take a vitamin D supplement or a calcium supplement unless your healthcare team orders one for you. If you need a medicine called a phosphate binder to lower your phosphorus, then your healthcare team may recommend one that has calcium.
For more information:
- Speak with your healthcare team
- Visit the National Kidney Foundation at www.kidney.org