Vitamin D, a fat-soluble vitamin, has come under a great deal of scrutiny in the last few years, as studies have questioned daily requirements and deficiency. Recent changes in guidelines to vitamin D supplementation have set the levels for daily requirements and the safe dose higher than previously. Do not take vitamin D supplements without discussing the dosage with your doctor first.
Function
Vitamin D assists with calcium absorption. If you have normal calcium levels but are deficient in vitamin D, you won't be able to absorb calcium from the intestine. Vitamin D contributes to strong bones as well as assisting in muscle and nerve function and in boosting the immune system. Vitamin D isn't active when first taken into the body; activation occurs in the liver. For this reason, liver disease can affect vitamin D and calcium levels.
Sources
Very little vitamin D comes from food; fortified foods provide the most intake of vitamin D in the United States. Vitamin D is also synthesized in the skin after sunlight exposure, but in the winter months in northern states, ultraviolet light is inadequate for vitamin D synthesis, so people in these populations benefit from supplements. People who most often require vitamin D supplementation include people with dark skin, the elderly, who don't absorb vitamin D as well, people who don't go outside often and those who live in colder climates. People who always use sunscreen may also develop deficiencies; SPF 8 sunscreen reduces vitamin D production by 95 percent.
Recommended Doses
The current recommended dose in 2010 is 600 IU for those older than 14 and 800 mg daily for adults older than 71. Children older than 1-year-old also need 600 IU. The safe upper limit of vitamin D is 4,000 IU for those older than 9, 3,000 IU for children ages 3 to 8 and 2,500 IU for children ages 1 to 3. Doses up to 10,000 IU rarely cause side effects, the Linus Pauling Institute states.
Risk Factors
Very high doses of vitamin D rarely occur from food, since few foods contain enough vitamin D to cause an overdose. Sunlight also can't cause a vitamin D overdose, although most people synthesize at least 10,000 IU in half an hour of full body sun exposure, according to John Jacob Cantrell, M.D. of the Vitamin D Council. Supplement overdose can cause toxicity.
Symptoms
Symptoms of hypervitaminosis D include hypercalcemia, or high calcium levels. Hypercalcemia can cause bone loss, kidney stones and calcifications in major organs like the heart and kidneys over time. Symptoms of hypercalcemia can occur with intakes of 50,000 IU, according to the Linus Pauling Institute. Lack of appetite, nausea, vomiting, constipation and weight loss can also occur. Vitamin D overdose can also lead to irregular heartbeat, confusion, disorientation and weakness.
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