# The Ultimate Guide to Vitamin D: Health Benefits, Deficiencies, and How to Supplement
Vitamin D regulates calcium absorption, supports bone health, and strengthens immune function. Most people don't get enough of it, despite its critical role in preventing fractures, respiratory infections, and autoimmune conditions.
The recommended dietary allowance for adults ages 19 to 70 is 600 to 800 International Units (IU) daily, though some researchers argue this target is too low. Deficiency develops when sunlight exposure drops or dietary intake falls short. People with darker skin tones in northern climates face higher deficiency risk because melanin reduces vitamin D synthesis from sun exposure.
Low vitamin D links to increased fracture risk, depression, and weakened immune response. A 2022 meta-analysis published in Nutrients found that deficient adults showed a 30 percent higher risk of respiratory infections compared to those with adequate levels.
Fatty fish like salmon and mackerel deliver substantial amounts per serving. Egg yolks, mushrooms exposed to sunlight, and fortified milk and plant-based drinks offer accessible options. A three-ounce serving of cooked salmon provides roughly 450 to 570 IU.
Sunlight exposure remains the most efficient source. Fifteen to thirty minutes of midday sun several times weekly helps most people maintain adequate levels, though geography, season, and skin tone affect production rates. Supplementation becomes necessary for those with limited sun access or documented deficiencies.
Vitamin D supplements come as D2 or D3. D3 proves more effective at raising blood levels and comes from animal sources or lichens. D2, derived from fungal sources, works but requires higher doses to match D3's potency.
Blood testing determines individual needs. A level below 20 nanograms