Vitamin D plays a vital role in calcium homeostasis, but strikingly most US adults have an insufficient amount to carry out its expected function. Vitamin D helps promote intestinal absorption of calcium and phosphorus that is used to build strong and health bones, a process called mineralization. Deficiency can lead to osteoporosis, an increased risk of falls and fractures in the elderly, and has been linked to decreased immune function, bone pain and possibly colon cancer and cardiovascular disease.
Vitamin D is incorporated into our bodies from one of two pathways. The skin, when exposed to ultraviolet light, synthesizes vitamin D3 (cholecalciferol). Vitamin D2 (ergocalciferol) is ingested from fish or plant sources. The liver refines both of these precursors (D2, D3) into the main circulating vitamin called calcidiol or 25OH vitamin D. This is the form that is measured to determine whether someone is vitamin D deficient or not. The final processing step occurs in the kidneys, where calcidiol is converted to calcitriol or 1,25OH vitamin D. This is the most active form of vitamin D and is responsible for most of it regulatory functions.
Because of the complex metabolic pathways needed to make active vitamin D, a number of extrinsic and intrinsic factors can have negative impacts on its synthesis. Diet is a major limiting factor. Foods such a fatty fish, nuts and eggs are naturally rich in vitamin D, but others such a diary and cereals are usually fortified to provide sufficient levels. Malabsorptive states from celiac disease, gastric bypasss, Crohn’s and cystic fibrosis can also render an individual vitamin D deficient. People who spend little time in the sun are at greatest risk of being deficient in vitamin D. These are homebound, hospitalized or institutionalized persons, the elderly, women who wear burkas and persons living at northern latitudes. Medical conditions such as liver and kidney disease may not allow the conversion of vitamin D to its active metabolite, leaving the person functionally vitamin D deficient regardless of sun exposure or diet.
Currently, vitamin D levels above 30 ng/ml are considered necessary for overall good adult health. Individuals with levels between 20-29 ng/ml are considered insufficient and those with levels below 20 ng/ml are considered deficient. The minimum daily doses of vitamin D required to maintain normal levels is 200 IU for young adults, 400 IU for pregnant and lactating women and 800 IU for adults over the age of 50. However, most people require 800-1000 IU daily to achieve 25OH vitamin levels greater than 30ng/ml.
For vitamin D insufficiency, it is recommended that the person take 800-1000 IU of vitamin D3 daily. If the individual is vitamin D deficient, then 50,000 IU of vitamin D3 weekly for 6 weeks is given followed by 1000 IU daily there after. Levels of 25OH vitamin D are rechecked in 3 months. If the individual has kidney or liver disease, then calcitriol would be the preferred metabolite of choice, since it bypasses the need for activation to an active metabolite. As always, consult your physician to discuss any and all details of diagnosis and treatment.
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