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# Vitamin D Deficiency

vi·ta·min D de·fi·cien·cy - VY-tah-min dee deh-FISH-en-see

## Definition

Vitamin D deficiency occurs when blood levels of 25-hydroxyvitamin D - the primary circulating form of vitamin D - fall below 20 ng/mL (50 nmol/L), though many experts consider levels below 30 ng/mL suboptimal. Vitamin D is unique among vitamins because the body can produce it when skin is exposed to UVB sunlight, yet deficiency is remarkably common - affecting an estimated 42% of American adults and over 1 billion people worldwide.

Vitamin D functions more like a hormone than a typical vitamin. Once activated, it regulates the expression of over 200 genes involved in calcium absorption and bone metabolism, immune function, inflammation regulation, cell growth and differentiation, muscle function, and mood regulation. Severe deficiency causes rickets in children (softening and weakening of bones) and osteomalacia in adults (bone pain and muscle weakness), while chronic insufficient levels are associated with increased risk of [osteoporosis](/glossary/osteoporosis), fractures, autoimmune diseases, certain cancers, [cardiovascular](/glossary/cardiovascular) disease, depression, and respiratory infections.

Risk factors for vitamin D deficiency include living at northern latitudes (above 37°N), dark skin pigmentation (melanin reduces vitamin D synthesis), obesity (vitamin D is sequestered in fat tissue), limited sun exposure (sunscreen use, indoor lifestyle, covering clothing), aging (skin becomes less efficient at producing vitamin D), and certain medical conditions ([celiac disease](/conditions/celiac-disease), [Crohn's disease](/conditions/crohns-disease), kidney disease) that impair absorption or activation.

## Also Known As

Hypovitaminosis D Low vitamin D 

## Key Facts

-   • An estimated 42% of American adults are vitamin D deficient - making it the most common micronutrient deficiency in developed nations. 
-   • The body produces vitamin D when skin is exposed to UVB sunlight, but factors like latitude, season, skin color, and sunscreen dramatically affect production. 
-   • Vitamin D regulates over 200 genes, with receptors found in virtually every tissue in the body. 
-   • Food sources are limited - fatty fish, fortified milk and cereals, egg yolks, and mushrooms exposed to UV light are the primary dietary sources. 
-   • Current recommendations suggest 600-800 IU daily for most adults, though many experts advocate for 1,000-4,000 IU, particularly for those with deficiency. 

## How It Relates To Your Health

Vitamin D testing (25-hydroxyvitamin D blood test) is recommended for individuals at risk of deficiency, including those with limited sun exposure, dark skin, obesity, [osteoporosis](/glossary/osteoporosis), frequent infections, or unexplained fatigue and muscle weakness. If you're deficient, your doctor will recommend supplementation - typically vitamin D3 (cholecalciferol), which is more effective than D2 at raising blood levels.

Beyond supplementation, supporting vitamin D status involves sensible sun exposure (10-30 minutes of midday sun several times per week, depending on skin type and latitude), consuming vitamin D-rich foods, and maintaining a healthy weight (since obesity reduces vitamin D bioavailability). Regular monitoring of blood levels ensures supplementation is adequate without risking toxicity.

## Related Terms

[Micronutrients](/glossary/micronutrients)[Hormone](/glossary/hormone)[Immune Response](/glossary/immune-response)[Osteoporosis](/glossary/osteoporosis)[Collagen](/glossary/collagen)

## Sources

1.  [Vitamin D Deficiency](https://my.clevelandclinic.org/health/diseases/15050-vitamin-d-deficiency) - Cleveland Clinic
2.  [Vitamin D](https://ods.od.nih.gov/factsheets/VitaminD-Consumer/) - National Institutes of Health Office of Dietary Supplements
3.  [Vitamin D Deficiency](https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/vitamin-d-deficiency/faq-20058397) - Mayo Clinic

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