Complete Guide to Vitamin D: Benefits, Sources, Deficiency, and Supplementation
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What Is Vitamin D and Why Is It Unique
Vitamin D is technically a hormone precursor, not a traditional vitamin. Your skin synthesizes D3 (cholecalciferol) when UVB rays hit 7-dehydrocholesterol. Almost every cell in your body has vitamin D receptors โ it regulates over 1000 genes.
What Vitamin D Does in Your Body
Calcium absorption: Without adequate D, only 10โ15% of dietary calcium is absorbed (vs 30โ40% with D). Immune system: Vitamin D activates T-cells and reduces inflammatory cytokines. Muscle function: Deficiency causes muscle weakness and increased fall risk. Mood: D receptors are in brain areas regulating serotonin. Emerging research: may reduce risk of certain cancers, autoimmune conditions.
Signs and Symptoms of Deficiency
Fatigue and bone pain are the most common. Also: muscle weakness, frequent infections, hair loss, depression, impaired wound healing. Blood test: 25-OH-D level. Optimal: 40โ60 ng/mL (100โ150 nmol/L). Deficient: < 20 ng/mL. Insufficient: 20โ29 ng/mL.
Sun Exposure and Vitamin D
10โ30 minutes midday sun (arms + legs exposed) on fair skin = 10,000โ20,000 IU. Darker skin requires 3โ6ร longer. Glass blocks UVB. Sunscreen SPF 15 reduces synthesis by 99%. NovemberโMarch at latitudes above 35ยฐ N: virtually no UVB reaches the skin.
Food Sources of Vitamin D
Very few foods naturally contain D3. Best sources: fatty fish (salmon 11 mcg/100g), mackerel (16 mcg), sardines (12 mcg), egg yolks (1.1 mcg), UV-exposed mushrooms (up to 21 mcg). Most people cannot meet needs from food alone.
How to Supplement Vitamin D Correctly
D3 (cholecalciferol) is more effective than D2 at raising blood levels. Dose: 1000โ4000 IU/day for general population. Deficient individuals may need 5000โ10,000 IU/day short-term. Take with the largest meal (fat-soluble). Pair with Vitamin K2 (MK-7) for bone and arterial health. Retest 25-OH-D after 3 months.
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