What is calcium, and what does it do in the body?
Calcium is the most abundant mineral found in the body. About 99% of the calcium in the body is in bones and teeth. The remaining one percent is found in blood and soft tissues. The body uses calcium to:
- Form strong bones and teeth
- Help muscles contract and relax for normal movement
- Transmit nerve impulses
- Make blood clot normally
- Regulate cell secretions, cell division and cell multiplication
- Assist with enzyme reactions
How does the body get calcium and how much is needed?
A balanced, healthy diet provides 1000 milligrams of calcium a day. Requirements increase during growth and with age. For young people age nine to 18, bones are still growing; 1300 milligrams of calcium each day is recommended to strengthen bones. For people age 50 or older, at least 1200 milligrams calcium a day is recommended to help slow calcium loss from bones and preserve bone density.
If calcium intake is too low, calcium will be ‘borrowed’ from the bones. Over a period of many years, low calcium intake can cause osteoporosis- a bone disease in which bones lose minerals, become very weak and break easily.
How much calcium does a person with kidney disease need?
Calcium recommendations for people with chronic kidney disease (CKD) are very different from those for the general population. As kidney function declines, patients may hold onto more calcium and developed high calcium (hypercalcemia).
Calcium and phosphorous usually keep each other in check. With progression of kidney disease, patients can have higher phosphorus which can lead to low calcium (hypocalcemia).
Also, there is a complex interplay between the calcium controlling gland (the parathyroid gland), vitamin D (made by skin, or diet) and the kidneys which are affected once kidney disease progresses (usually CKD stage IIIb or higher). To determine calcium status, your doctor will measure and evaluate calcium, phosphorus, vitamin D and parathyroid hormone (PTH) levels.
Know your calcium and phosphorus levels
Patients can work with a renal dietitian and their doctor to keep their calcium and phosphorous levels in balance. Regular blood tests will show the phosphorus and calcium levels in the blood. According to the National Kidney Foundation’s (NFK) Clinical Practice Guidelines for Bone Metabolism and Disease in Chronic Kidney Disease, CKD stages three and four patients should keep their phosphorus between 2.7 and 4.6 mg/dL. Patients on dialysis should keep their phosphorus levels in the 3.0 to 5.5 mg/dL range or as close as possible to the laboratory reference range. The NFK’s Kidney Disease Outcomes Quality Initiative (KDOQI) goal range for calcium is 8.4 to 10.2 mg/dL. Calcium levels above 10.2 are considered high and may require adjustments in diet, calcium-based binders or a decrease in vitamin D therapy.
What to eat?
Calcium is found naturally in dairy products. Unfortunately, these same foods are very high in phosphorus and are not good choices for people with kidney disease who require a low phosphorus diet. If your calcium is low, your dietitian will coach you on calcium-containing foods to include in your diet. If your doctor prescribes a supplement, ask your dietitian to discuss the best time to take it. If your calcium is high, your dietitian can help identify high calcium foods to avoid.
Many dairy foods, including milk and cheese, are naturally high in calcium and should be avoided. Many other foods contain added calcium and should be avoided as well. Nutrition Facts on food labels often do not provide information on calcium. Because it is not listed does not mean the food is calcium-free. Since food labels are not required to list the calcium content, talking to a renal dietitian and reading the label ingredients will be helpful to learn which foods contain calcium.
Foods that may have added calcium include:
- Breakfast cereals or bars
- Frozen waffles
- Granola bars
- Milk substitutes such as soy milk or rice milk
- Nutrition drinks
- Pancake or muffin mixes
- Powdered drink mixes
- Snack crackers
Labels on foods that contain added calcium may include one of the following statements:
- Calcium enriched
- Calcium fortified
- Excellent source of calcium
- Good source of calcium
High Calcium Foods*
Most of these foods are also high in phosphorous and may not be recommended for CKD patients:
- Blackstrap molasses
- Calcium fortified or calcium enriched foods (see list above)
- Canned or dry powder milk
- Cottage cheese
- Ice cream
- Milk-based coffee and chocolate drinks
- Sardines or salmon canned with bones
- Tofu prepared by calcium precipitation
- Yogurt, including frozen yogurt
*Disclaimer: The high calcium foods list is not exhaustive and does not include all foods high in calcium. Portion size also plays a role in the amount of calcium obtained from the foods you eat. Consult your dietitian and/or your physician to provide specific dietary recommendations based on your individual health and requirements.
Treating and managing calcium levels
Treatment for low or high levels of calcium may include one or more of the following:
- Testing calcium, phosphorus and PTH on a regular basis
- Reducing the amount of phosphorus you eat
- Taking a medication called a phosphorus binder
- Taking the active form of vitamin D, if prescribed
- Taking a calcium supplement, if prescribed
- Taking a calcimimetic medication, if prescribed
- Getting an operation to remove some of the parathyroid glands