Nearly all of the calcium in your body – about 99 percent of it, in fact – is tied up in your bones and teeth. While the tiny fraction that resides in your soft tissues and bloodstream may seem insignificant, your body works very hard to maintain constant calcium concentrations in the fluids bathing your cells and tissues. Vitamin D, in concert with several of your organs and endocrine glands, plays a pivotal role in helping to preserve a state of calcium “homeostasis.” If you take too much vitamin D, your calcium levels can increase to dangerous levels. Your physician can help you determine the best vitamin D dosage for you.
Vitamin D Synthesis
Whenever you are exposed to sunshine, a cholesterol-like molecule in your skin is converted to cholecalciferol, or vitamin D3. Cholecalciferol enters your circulation and travels to your liver, where it is transformed to 25-hydroxyvitamin D3, or calcidiol. Calcidiol is then transported to your kidneys where, under the influence of parathyroid hormone from your parathyroid glands, it is converted to 1,25-dihydroxyvitamin D3, or calcitriol. Calcitriol, the active form of vitamin D, exerts profound effects on nearly every tissue in your body, but one of its principal functions is to help regulate calcium levels.
Calcium Functions
In addition to lending strength to your bones and acting as a cofactor for a variety of enzymes and other proteins in your cells, calcium plays a vital role in generating the electrical currents that allow your heart, muscles and nerves to perform properly. Even small changes in calcium concentrations can interfere with these tissues’ functions, and wide fluctuations in calcium levels can be life-threatening. According to “The Merck Manual of Diagnosis and Therapy,” vitamin D toxicity is one of the principal causes of hypercalcemia, or high calcium levels.
Toxicity
Vitamin D toxicity can occur in infants who receive more than 40,000 IU daily for one to four months. In adults, toxicity can occur with doses of 2,000 to 3,000 IU daily for several years, or with higher doses – 100,000 IU daily – for a few months. Symptoms of vitamin D toxicity, which result from hypercalcemia, include nausea, vomiting, abdominal pain, loss of appetite, constipation, excessive thirst, frequent urination, weakness, confusion, psychosis and coma. Kidney stones, soft tissue calcifications and kidney failure can also occur as a result of vitamin D toxicity.
Considerations and Recommendations
Vitamin D was in the nutritional spotlight in the early and mid-20th century when it was discovered that vitamin D-containing foods could prevent rickets. Since the dawn of the 21st century, vitamin D has once again garnered attention for its immune-regulating properties. However, the optimal dosage of vitamin D remains a point of contention. The Food and Nutrition Board recommends a daily vitamin D intake of 400 to 800 IU, depending on your age, and has established a tolerable upper intake limit of 2,000 IU daily. Dr. Reinhold Vieth, a nutritionist at Mount Sinai Hospital in Toronto, Canada, contends that healthy adults should get 2,000 IU of vitamin D daily and that the upper limit should be reset at 10,000 IU daily. Ask your doctor about the best vitamin D dosage for you.
References
- Linus Pauling Institute at Oregon State University: Calcium
- Office of Dietary Supplements: Vitamin D
- “The Merck Manual of Diagnosis and Therapy, 18th Edition: Hypercalcemia”; Mark H. Beers, M.D., Editor-in-Chief; 2006
- “The Journal of Nutrition”; Critique of the Considerations for Establishing the Tolerable Upper Intake Level for Vitamin D: Critical Need for Revision Upwards; R. Vieth; April 2006
