By DaVita® renal dietitians Helen Dorough and Sara Colman
We get vitamin D from the ultraviolet (UV) rays of the sun, but not all sunlight is equal. The amount of UV rays absorbed depends on where you live, the time of day, the season, whether it’s cloudy and if you’re using sunscreen. People who live in sunny areas at lower latitudes typically get enough vitamin D compared to people living at higher latitudes , particularly during late fall and winter.
Few foods are naturally good sources of vitamin D. The best food sources for vitamin D are fatty fish including salmon, sardines, cod, tuna and halibut. Many foods, such as some breakfast cereals and milk, are fortified with vitamin D. Milk must contain at least 100 IU of vitamin D per cup, according to federal regulations. The practice of fortifying milk with vitamin D began in the 1930s to prevent rickets, a bone disease that was common in children at the time. Other dairy products are not required to be fortified. Milk substitutes such as soy milk, rice milk and nondairy creamer may or may not have added vitamin D.
Vitamin D is responsible for:
Too much vitamin D can be toxic. The recommended maximum intake is 25 mcg (1,000 IU) for infants and 50 mcg (2,000 IU) for children and adults with normal kidney function.
It’s not unusual for people with kidney failure to have low levels of vitamin D. Your doctor may check your vitamin D level and if it’s low, order a supplement. Your doctor can tell you if a supplement is a good choice for you and if you need it. Always check with your physician before starting an over-the-counter vitamin, mineral, diet supplement or medicine. These items may be harmful to people with chronic kidney disease (CKD) if not taken properly.
Healthy kidneys are rich with vitamin D receptors and play a major role in turning vitamin D into its active form. This helps balance calcium and phosphorus in your body by controlling absorption of these minerals from the food you eat and regulates parathyroid hormone (PTH).
When kidneys fail, their ability to activate vitamin D is lost. Without the activated vitamin D to control calcium and phosphorus levels in the blood, PTH will try to overcompensate and go out of range.
PTH is secreted by the parathyroid glands that are located in the neck near the thyroid glands. In kidney failure, the parathyroid glands may incorrectly sense that there is not enough calcium in the blood and produce excess parathyroid hormone which tells the body to pull calcium out of the bones and put it in the bloodstream. This excess of PTH can cause secondary hyperparathyroidism which can result in bone pain and weak bones that fracture easily. All patients with kidney failure are at risk for secondary hyperparathyroidism. For this reason, PTH levels are routinely monitored through lab work approximately every three months or more often if needed.
There are significant problems that may result with excess calcium in the blood stream. Calcium can deposit in soft tissues leaving “little rocks” in the tissue. These deposits or calcifications will never go away. The consequences of calcification are serious. If the heart becomes calcified, blood flow may be reduced, which could cause a heart attack. Calcification in the lungs can result in difficulty breathing. And, calcification in joints can cause extreme pain.
When a person with CKD develops a high PTH, a physician may prescribe activated vitamin D to suppress PTH production. Hemodialysis patients will be given a prescription medicine intravenously during their dialysis treatment.
Those not on dialysis or on peritoneal dialysis will be prescribed an oral form of activated vitamin D or generic calcitriol. PTH levels are checked regularly to make sure the dose of the medicine is correct and that PTH is adequately suppressed but not over-suppressed. It’s a fine line that the doctor, nurse and dietitian are monitoring.
If a person has a high blood level of phosphorus or calcium, the physician will often choose not to treat the high PTH with activated vitamin D because there is an increased risk of calcium-phosphorus deposits in the soft tissues. It’s important for all kidney failure patients to keep phosphorus and calcium blood levels within a normal range. A renal dietitian regularly works with people on dialysis to ensure that they aren’t eating too many foods rich in phosphorus or calcium and to make sure phosphorus binders are taken correctly.
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