Vitamin D promotes calcium absorption and regulates calcium levels in the blood. Sunlight is a major source of vitamin D because your body makes vitamin D when your skin is exposed to ultraviolet rays. Low levels of vitamin D can cause weak and softened bones, but having levels that are too high can be dangerous too.
Hypervitaminosis D
Vitamin D becomes toxic when you take excessive amounts of supplements. High blood levels of vitamin D cause high levels of calcium that can damage bones, muscles and kidneys. According to MedlinePlus.com, symptoms include constipation, decreased appetite, dehydration, fatigue, irritability, muscle weakness and vomiting. The Office of Dietary Supplements says the toxic dosage threshold for vitamin D is 10,000 to 40,000 IU per day, but the Institute of Medicine has set the tolerable upper limit at 4,000 IU for adults.
Testing
Your doctor will order blood tests to determine if you have hypervitaminosis D. The Office of Dietary Supplements says levels of serum 25-hydoxyvitamin D, the stored form of vitamin D, should be no higher than 150 nmol/L. Your doctor might also test for 1,25-hydroxyitamin D, the active form of vitamin D; serum calcium; and phosphorus levels, and might order X-rays to assess your bone health.
Treatment and Prevention
Treatment of hypervitaminosis D is to stop taking vitamin D, but other measures might be necessary in severe cases. Talk to your doctor if you think you might have vitamin D levels that are too high. You can prevent hypervitaminosis D by avoiding doses of vitamin D greater than 4,000 IU per day. The Office of Dietary Supplements says sun exposure will not cause hypervitaminosis D and that it is very unlikely to be caused by food sources.
Recommended Amounts
The Institute of Medicine recommends that adults get 400 IU of vitamin D per day from foods or supplements. Vitamin D is found in oily fish, such as salmon and tuna, and fortified foods, such as cereal and milk. Small amounts are found in liver, cheese and egg yolks. Don't take large doses of vitamin D, or any other vitamin, without speaking to your doctor first.
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