Vitamin D3
Fat-soluble hormone synthesized in skin from UVB exposure. Deficiency is the single most common nutrient deficiency in modern populations — and a candidate cause for cognitive, mood, and immune dysfunction.
What it is
Vitamin D3 (cholecalciferol) is the form the body synthesizes from 7-dehydrocholesterol under UVB sunlight. It is converted by the liver to 25-hydroxyvitamin D (the form measured in blood tests) and then by the kidneys to 1,25-dihydroxyvitamin D (the active hormone). Over 1 billion people globally have serum 25(OH)D below 20 ng/mL.
How it works
Functions as a steroid hormone. The vitamin D receptor is expressed in almost every cell type, including neurons in the hippocampus, prefrontal cortex, and substantia nigra. It regulates over 200 genes involved in neurotransmitter synthesis, immune modulation, and calcium handling. Deficiency reliably correlates with mood disorders, cognitive decline, and poor immune function.
What the research says
Evidence strength is ranked against the total body of peer-reviewed research, not marketing claims.
Who it helps
Anyone living above 40° latitude (most of North America, Europe). People with desk jobs. People with darker skin (higher melanin requires more UVB for the same D3 synthesis). Older adults (skin synthesis drops 50%+ after age 65). Anyone whose last blood test showed 25(OH)D below 30 ng/mL.
Who should skip it
People with sarcoidosis, primary hyperparathyroidism, or certain lymphomas (risk of hypercalcemia). People already above 60 ng/mL 25(OH)D. Always retest serum 25(OH)D after 3 months of supplementation — individual response varies 3-4x.
Typical dose
1000-4000 IU/day for most adults. The RDA of 600-800 IU is too low for deficiency correction — 2000 IU/day is a reasonable default without testing. Pair with 100-200 mcg of vitamin K2 (MK-7) to direct calcium into bones rather than arteries. Target 25(OH)D: 40-60 ng/mL.
How to take it
Take with the fattiest meal of the day (D3 is fat-soluble — absorption drops 30-50% on empty stomach). D3 (cholecalciferol) is superior to D2 (ergocalciferol) — more potent and longer-lasting in serum. Test serum 25(OH)D at baseline and again after 3 months.
Common misuses
People take 50,000 IU weekly 'megadoses' without testing — hypercalcemia is rare but real. People take D3 on empty stomach and absorb little. People take D3 without K2 and may shift calcium into soft tissue. People assume 'I go outside' means 'I make enough D' — through glass, with sunscreen, after 10 AM sun angle, you don't.
Where to buy
Browse Vitamin D3 on Amazon
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