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Answer · biomarkers

What causes vitamin D deficiency?

Short answer

Vitamin D deficiency is driven by inadequate sun exposure, higher latitude and winter season, darker skin pigmentation, higher body fat (which sequesters vitamin D), older age, malabsorption, and insufficient dietary or supplemental intake.

Reviewed by Dr. Laura PurdyMay 29, 2026

Full answer

Vitamin D is synthesized in skin from UVB exposure and then activated through the liver and kidneys. Deficiency reflects insufficient input, sequestration, or processing. The most common drivers are inadequate sun exposure (indoor lifestyles, sunscreen use, urban living, occupational schedules), higher latitude with limited winter UVB, darker skin pigmentation (which reduces cutaneous synthesis per unit UVB), higher body fat (which sequesters vitamin D in adipose tissue and lowers circulating 25-OH D), aging skin (which produces less vitamin D for the same UVB), malabsorption (celiac, IBD, post-bariatric anatomy), chronic kidney or liver disease, and simply inadequate dietary or supplemental intake. Repletion strategy is individualized in provider review.

Educational. Not a substitute for individualized provider review.

References & sources
  • Vitamin D — Primary source · oMeds
  • Reviewed by Dr. Laura Purdy, MD, MBA · Clinical Director, oMeds · Last reviewed 2026-05-29

Related answers

What is vitamin D?
The serum 25-hydroxyvitamin D test measures the storage form of vitamin D and is the standard marker of vitamin D status. Sufficiency is generally cited at ≥30 ng/mL; many optimization frameworks target 40–60 ng/mL, individualized to the patient.
Does vitamin D supplementation help?
Yes, when truly deficient. Repletion in confirmed vitamin D deficiency reliably raises serum 25-OH D into the sufficient range and improves musculoskeletal, immune, and fatigue markers in deficient patients. Supplementation in already-sufficient patients shows smaller and less consistent benefits.
Can low vitamin D cause fatigue?
Yes. Vitamin D deficiency is associated with fatigue, low mood, and reduced exercise tolerance, and randomized trials have shown improvement in fatigue scores with repletion. Vitamin D is rarely the only driver — it is read alongside iron, thyroid, and hormonal status.
Primary source
Vitamin D