Vitamin D is found in a limited number of foods, such as cheese, fortified milk, butter, margarine, fish, oysters, fortified yogurts and fortified cereals. Vitamin D is also called the sunshine vitamin because it is synthesized after exposure to sunshine. However, UV rays are not strong enough for people living north of the line between Boston and the northern border of California from November through February, and cutaneous vitamin D synthesis cannot occur at this time. For this reason, many Americans rely on vitamin D supplements.
Roles
The role of vitamin D with bone health has been known for a long time. Vitamin D is responsible for promoting calcium absorption in the intestines in addition to regulating blood calcium levels. Vitamin D deficiency can lead to rickets in children and osteomalacia in adults. More recently, new roles have been discovered for vitamin D, including regulation of neuromuscular function, modulation of the immune system and reduction of inflammation.
Recommendations
The Institute of Medicine set the adequate intake for vitamin D at 5 mcg, or 200 IU, daily for healthy adults up to the age of 50 years old. The recommendation is doubled for people between the age of 51 and 70 to10 mcg or 400 IU and tripled for those over the age of 70 to15 mcg or 600 IU.
Tolerable Upper Intake Level
To avoid hypervitaminosis D, the Institute of Medicine also set a tolerable upper intake level, or UL. The UL for vitamin D is 50 mcg, or 2000 IU, a day for healthy adults. A vitamin D intake exceeding the UL over a period of time can lead to serious side effects.
Controversy with Vitamin D UL
The Office of Dietary Supplements of the National Institutes of Health reports that the tolerable upper intake level has been challenged by many researchers since it was published in 1997. According to more recent data, the UL could actually be as high as 250 mcg, or 10,000 IU, a day. Clinical trials have involved vitamin D supplements that exceeded the current UL without any harmful effects, but more evidence is needed before recommended intake levels will be changed.
Side Effects
Toxic amounts of vitamin D can lead to non-specific symptoms such as nausea, vomiting, lack of appetite, constipation, weight less and weakness. Too much vitamin D can also dangerously elevate blood calcium levels and induce heart rhythm abnormalities, confusion and kidney stones.
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