Vitamin D is technically a hormone produced when UVB rays hit skin. It regulates calcium absorption, supports immune cell function, and influences muscle strength. An estimated 1 billion people worldwide have insufficient levels, especially in northern latitudes during winter months.
Signs of Deficiency
Fatigue, bone pain, muscle weakness, frequent illness, and low mood can indicate deficiency — but symptoms are nonspecific. Blood testing (25-hydroxyvitamin D) is the only reliable diagnostic. Levels below 20 ng/mL are deficient; 30–50 ng/mL is generally adequate for most adults.
Getting Enough Naturally
10–30 minutes of midday sun on arms and legs several times weekly suffices for many light-skinned individuals. Darker skin requires longer exposure due to melanin's UV-blocking effect. Sunscreen reduces synthesis but prevents skin cancer — balance brief unprotected exposure with protection during extended outdoor time.
Food and Supplements
Fatty fish (salmon, mackerel), egg yolks, and fortified milk provide dietary vitamin D, but food alone rarely meets needs. D3 supplements (cholecalciferol) outperform D2 in raising blood levels. Typical maintenance doses range 1,000–2,000 IU daily; deficient individuals may need higher doses under medical supervision.
Who Should Test
People with limited sun exposure, darker skin, obesity (vitamin D sequesters in fat tissue), malabsorption conditions, or osteoporosis risk benefit from testing before supplementing blindly. Excess vitamin D causes hypercalcemia — more is not always better.