Vitamins are essential components of the diet, required for cell function and growth; there are 13 essential vitamins. Vitamin D is a fat-soluble vitamin important to the maintenance of normal blood levels of calcium and phosphorus. Vitamin D aids in calcium absorption and bone growth. Vitamin D deficiency can result in softening and weakening of the bone, or osteomalacia, a condition called rickets in children. Recent studies suggest that Vitamin D may help protect against osteoporosis, hypertension, cancer, and autoimmune disease.
Recommended Vitamin D levels
Adequate Intake (AI) levels have been established by the Food and Nutrition Board at the Institute of Medicine of the National Academies; these AI levels represent the amount of Vitamin D consumed daily that will maintain bone health and normal calcium metabolism in healthy people. The AI levels established in 1997 are 200 International Units (IU) for all individuals under 50 years of age, 400 IU for people 50-70 years old, and 600 IU for those over 70. The recommended upper intake level (UL) limit has been set at 2000 IU because very high levels of Vitamin D can result in toxicity.
Dietary sources of Vitamin D
Vitamin D is found in fish, and is present in small amounts in cheese, egg yolk, and beef liver. It is also added to fortified foods, and these are the main source of Vitamin D in the American diet. Vitamin D was added to milk in the U.S. starting in the 1930s to combat rickets. It is now also added to some breakfast cereals, some dairy products, and some calcium-fortified juices.
There are two forms of Vitamin D: Vitamin D2 (ergocalciferol), which can be produced by plants and by mushrooms if they are exposed to UV light, and Vitamin D3 (cholecalciferol), which is synthesized in the skin of animals upon exposure to UV light.
Vitamin D from Sun Exposure
Upon exposure to UV radiation, 7-dehydrocholesterol in the skin is converted to previtamin D3, which then is changed into Vitamin D3. Precise guidelines for how much sun exposure is needed to produce sufficient Vitamin D are not available, but it has been estimated that 5-30 minutes of sun exposure at least twice a week is enough. It is important not to get excessive sun exposure, since UV light is also known to increase the risk of skin cancer.
Vitamin D toxicity can cause nausea and vomiting, and can raise the blood level of calcium which can result in confusion and heart rhythm abnormalities. Excessive levels can also increase the risk of kidney stones. Toxicity is most likely to result from taking dietary supplements, since it is difficult to obtain toxic levels of Vitamin D through the diet, unless large amounts of cod liver oil are consumed. Also, excessive sun exposure does not result in toxic levels of Vitamin D because Vitamin D is broken down as well as formed by sunlight.
Vitamin D and Disease Prevention
There is increasing evidence that Vitamin D may be protective against several other diseases besides those related to bone health. Some studies indicate a relationship between lower serum Vitamin D levels and increased colon cancer risk, increased cardiovascular disease, and increased risk of stroke. Increased intake of Vitamin D was associated with a decreased risk of autoimmune disease such as rheumatoid arthritis. All of these results are preliminary, but intriguing. Further research is required to establish the relationships between serum Vitamin D levels and measures of biological function and disease.