Using National Academies thresholds, NHANES 2011-2014 found about 18% of people in the US aged 1 and older at risk of inadequacy (25(OH)D 12 to 19.6 ng/mL) and 5% at risk of deficiency (below 12 ng/mL), with higher rates in older adults, people with darker skin, and northern latitudes. Severe deficiency causes rickets in children (irreversible bone deformities if untreated) and osteomalacia in adults. Most cases are subclinical and detected by blood test.
Common symptoms
Bone pain or muscle aches
Muscle weakness, especially in the legs
Fatigue
Frequent illness or infections
Mood changes — depressive symptoms (in deficient populations)
Slow wound healing
Hair loss
Bone deformities in children (rickets) — bowed legs, delayed growth
Often asymptomatic until severe
At-risk groups
Adults aged 65+ (skin produces less vitamin D from sunlight)
People with darker skin (melanin reduces vitamin D synthesis)
People living above about 37° latitude (little UVB in winter)
Indoor lifestyle, office workers, night shift workers
People with obesity (vitamin D sequestered in fat tissue)
Exclusively breastfed infants without supplementation
People with malabsorption conditions (celiac, Crohn's, gastric bypass)
People taking corticosteroids, anticonvulsants, or weight-loss drugs
People who consistently use sunscreen or cover skin for cultural reasons
When to see a doctorPersistent bone pain or muscle weakness, or a condition that affects vitamin D absorption or metabolism, warrants asking a doctor about a 25-hydroxyvitamin D blood test; the Endocrine Society (2024) advises against routine testing of healthy people. The National Academies consider 20 ng/mL (50 nmol/L) or more sufficient for most people, with deficiency risk rising below 12 ng/mL (30 nmol/L). Important: do not take high-dose vitamin D (>4,000 IU/day) without lab confirmation — toxicity causes hypercalcemia.