Calcium is the most abundant mineral in our body; ninety-nine per cent of calcium is found in the structure of our bones and teeth. It supports normal bodily movement by keeping our tissues ridged, strong and flexible. It also serves as a calcium bank, offering a readily available source of calcium to the body if a drop in blood calcium occurs. You require calcium from your diet or supplementation as your body can not make it. If serum calcium becomes too low, there is insufficient calcium to go to nerves, muscles and wounds for healing. Symptoms of mildly low calcium can include brittle nails, muscle cramps and dry, scaly skin.
Kidney failure is known to disrupt several mechanisms that control serum calcium and normal bone metabolism. A negative or positive calcium balance in patients with kidney disease will have implications. A negative balance could increase the risk of osteoporosis and fracture, whereas a positive balance may increase the risk of vascular calcification and cardiovascular events.
Calcium is Essential for the Following:
- Formation of bone and teeth
- Muscle contraction
- Normal functioning of many enzymes
- Blood clotting
- Normal heart rhythm
- Nerve function
Only one per cent of the body’s calcium is found in extracellular and intracellular fluids, and its actions help to maintain healthy blood pressure, which we know is extremely important in those with kidney disease.
There Are Two Primary Hormones Involved in Calcium Balance.
- Parathyroid hormone (PTH): Our body is amazing at trying to keep everything in check and balanced. In the case of calcium, when the extracellular fluid contains too little calcium, our parathyroid glands will produce more parathyroid hormone. In comparison, when calcium levels increase, the parathyroid glands will produce fewer hormones.
Parathyroid hormone produces the following outcomes:
- Stimulates bones to release calcium into the blood
- Causes the kidneys to excrete less calcium in urine
- Stimulates the digestive tract to absorb more calcium
- This causes the kidneys to activate vitamin D, enabling the digestive tract to absorb more calcium
Cellin in the thyroid gland produce calcitonin, and calcitonin lowers the calcium levels in the blood by slowing the breakdown of bone.
- 1,25D (“active vitamin D”/calcitriol): Calcium-sensing receptors on the parathyroid gland pick up low serum calcium, which stimulates PTH synthesis and secretion. PTH tries to raise serum calcium by acting on the kidney proximal tubules to increase renal reabsorption of calcium. The aim is to stimulate bone osteoclast activity in order to increase bone resorption and calcium release and will also increase the renal enzyme, 1-α-hydroxylase (CYP27B1), responsible for the conversion of 25-hydroxyvitamin D (25D) to the active form of 1,25D. 1,25D helps correct low calcium by increasing calcium absorption.
Too little calcium in the blood is called hypocalcaemia, whereas high calcium in the blood is termed hypercalcaemia.
When blood levels of calcium fall too low or rise too high, three organs will respond;
- Kidneys: The kidneys are essential for calcium homeostasis. When there are normal blood calcium levels, almost all of the calcium that enters glomerular filtrate is reabsorbed from the tubular system, returning it to the blood and preserving blood calcium levels. When the tubular reabsorption of calcium decreases, calcium is lost via excretion in urine.
- Bones: Bones serve as a reservoir for calcium. Bone resorption is the resorption of bone tissue. This is the process by which osteoclasts break down the tissues in the bones and release the minerals. This results in the release of minerals and calcium into the blood.
- Intestines: The small intestine is the site where calcium from your diet is absorbed. The efficient absorption of calcium depends on the expression of a calcium-binding protein in epithelial cells.
Calcium Supplementation and Kidney Disease
Calcium levels can often be abnormal in patients with kidney disease. Raised calcium levels can present in symptoms such as headaches, nausea, sore eyes, itchy skin, aching teeth, changes in mood and confusion. Research on patients with CKD shows a link between low serum calcium and the increased time in reaching kidney failure compared to patients with normal calcium levels. Low calcium levels are also linked to low vitamin D in those with CKD. Low serum calcium may indicate that vitamin D is low due to the decline in kidney function.
It is important to note that calcium supplementation recommendations will vary in individuals. This will depend on the type and stage of an individual’s kidney disease and specific needs.
Research suggests whether or not to prescribe calcium supplements, calcium-based phosphate binders or other medications containing calcium or getting patients to add more calcium into their diets in patients with CKD should depend on the baseline calcium intake in each patient. Practitioners may avoid recommending additional calcium supplementation in patients with adequate calcium intakes of 800–1000 mg/day. Those with greater calcium intakes (>approx. 1000 mg/day) may be advised to decrease their calcium intake.
Following are some general considerations regarding calcium supplementation and kidney disease:
- Phosphorus interaction- Calcium and phosphorus levels are interconnected in the body. Kidney disease can lead to high levels of phosphorus in the body, negatively affecting bone health. Excessive calcium supplementation without addressing phosphorus levels can exacerbate this imbalance.
- Calcium binders- Some individuals with kidney disease may need to take phosphate binders. These are medications to help control phosphate levels. Calcium-based phosphate binders are particularly useful as they avoid aluminium toxicity, directly suppress parathyroid hormone and are cost-effective. Their popularity is reducing due to emerging evidence of accelerated vascular calcification.
The Institute of Medicine set new Dietary Reference Intakes (DRI) for the general population for calcium in 2010. The Tolerable Upper Limit (UL) set for calcium was 2,000 mg/day. The KDOQI guidelines suggested this as a maximum amount of calcium from dietary sources and calcium-based binders in patients with stage 3–5D CKD.
“The KDIGO guidelines do not suggest limits for dietary calcium intake or any maximum level of total intake”. However, in those with stage 3 CKD, the KDIGO outlines “recommend restricting the dose of calcium-based phosphate binders … in the presence of persistent or recurrent hypercalcemia” and “suggest restricting the dose of calcium-based phosphate binders in the presence of arterial calcification and/or adynamic bone disease and/or if serum PTH levels are persistently low”.
Present data suggests that 2,000mg/day of calcium (the UL for the general population and the maximum level set by KDOQI) is too high, even in those with moderate-stage CKD. Research suggests a more moderate target for dietary calcium intake is recommended for CKD patients. This is around 800-1,000mg/day, close to the recommended Daily Allowance (RDA) DRI of 1,000-1,200mg/day, varied by age in healthy adults.
Forms of Calcium in Supplements
You will notice calcium supplements will come in different forms. The two primary forms are calcium carbonate and calcium citrate. Calcium carbonate tends to be the best value. However, for optimal absorption, it requires stomach acid for absorption and is best to take with food. If you have gut issues and issues with absorption, this form may not be for you. Calcium carbonate is generally a tolerable form of calcium; however, some people experience mild constipation and bloating.
Calcium citrate supplements are absorbed more readily than carbonate. They can be taken on an empty stomach and are better absorbed by those who take stomach acid-reducing medication. It is the best choice of calcium in those with achlorhydria or who are taking histamine-2 blockers or protein-pump inhibitors.
Other forms of calcium include calcium lactate and gluconate. However, these are less concentrated forms and are not practical oral supplements.
Some calcium supplements are mixed with other vitamins, vitamin D or magnesium. Always check to see the form of calcium you are taking and what other vitamins are present in the supplement to make sure you are taking what you require.
As noted, there are different forms of calcium available, and each of these forms will supply you with varying amounts of calcium, as seen in the list below:
- Calcium carbonate (40% elemental calcium)
- Calcium citrate (21% elemental calcium)
- Calcium gluconate (9% elemental calcium)
- Calcium lactate (13% elemental calcium)
Calcium is best taken and better absorbed in smaller doses (500mg or less)
Do Calcium Supplements Interfere With Medications?
Calcium supplementation can interact with pharmaceutical medication. Always speak to your healthcare practitioner if you add a supplement to your regime. This medication may include synthetic thyroid hormones, bisphosphonates, antibiotics and calcium channel blockers.
Potassium-sparing and thiazide diuretics may increase calcium levels in your blood, whereas loop diuretics can cause calcium levels to go down. Calcium can also reduce the absorption of fluoroquinolone and tetracycline. Taking calcium at the same time as bisphosphonates, used to treat osteoporosis, can decrease the absorption of this medication. You may be advised to separate doses by 2 hours. Anti-seizure medications like phenytoin, carbamazepine, phenobarbital, and primidone can also lower calcium levels. Once again, dividing the dose from these medications may be advised by your healthcare provider. A particular type of cholesterol-reducing medication, called bile acid sequestrants, can increase the loss of calcium in the urine; therefore, your healthcare provider may request you add a calcium supplement.
There you go!
Everything you need to know about calcium and CKD.
I hope you found this helpful information. As always, please check with your healthcare professional before you add/wish to change your supplements.