Adequate vitamin D intake is necessary to prevent many health problems, such as osteoporosis, bone fractures, prostate, breast and colon cancer, hypertension, depression, and autoimmune diseases. But too much vitamin D, especially when taken as a supplement, can also cause problems, such as kidney stones.
Kidney Stones
According to the National Institute of Diabetes and Digestive and Kidney Diseases, a kidney stone is a "solid piece of material that forms in a kidney out of substances in the urine." There are four main types of kidney stones. The most common type is made of calcium, usually mixed with other waste products, and the most frequent combination of these is called calcium oxalate. Another type of stone is made from struvite, which contains magnesium and ammonia. Struvite stones may form after an infection in the urinary system. Another type of stone is a uric acid stone. A rare type of stone is the cystine stone.
Vitamin D
As stated by the National Institutes of Health, one of the side effects of taking too much vitamin D is hypercalcemia, or too high a level of calcium in the blood. Hypercalcemia can also cause kidney stones.
Evidence
The Women's Health Initiative, or WHI, has conducted the largest trial to date on the subject, which showed a link between vitamin D supplementation and kidney stone formation. Sponsored by the National Institutes of Health, the WHI trial studied over 36,000 postmenopausal women ages 50 to 79, and randomized half of them to receive extra calcium plus vitamin D supplementation, and the other half to receive a placebo. The group that received the calcium plus vitamin D had a 17 percent higher risk of kidney stones than the placebo group. The study was published in The New England Journal of Medicine in 2006.
Complicating Factors
John Jacob Cannell, M.D., Executive Director of the Vitamin D Council, agrees that excess vitamin D may increase the risk of kidney stones, but notes that this is true only in certain circumstances. He notes that diets low in magnesium, low in potassium, high in sodas, and high in acid residue are necessary for vitamin D to cause stones. All of these factors are present in the modern American diet. To prevent kidney stones, especially if you are taking vitamin D supplements, he suggests drinking plenty of fluids and taking in adequate magnesium and potassium.
Safe Doses
According to the National Institutes of Health, the safe upper limit for vitamin D intake varies by age. For infants, 1,000 to 1,500 international units, or IU, per day is safe. For children 1-8 years old, 2,500 to 3,000 IU per day is safe. For everyone 9 years old or older, 4,000 IU per day is considered safe. If you are taking vitamin D supplements, be sure to ask your doctor if the amount you are taking is safe for you.
References
- Vitamin D Council: Vitamin D Science: Vitamin D Research
- National Kidney and Urologic Diseases Information Clearinghouse: What I need to know about kidney stones
- NIH: Office of Dietary Supplements: Dietary Supplement Fact Sheet for Consumers: Vitamin D
- National Cancer Institute: Hypercalcemia
- "The New England Journal of Medicine;" Calcium plus Vitamin D Supplementation and the Risk of Fractures; Rebecca Jackson et al for the Women's health Initiative Investigators; February, 2006
- Vitamin D Council: The Vitamin D Newsletter, August 2006: Vitamin D and Kidney Stones
