Not a vitamin — a hormone
Vitamin D is classified as a vitamin for historical reasons, but biochemically it is a steroid hormone. Unlike other vitamins, your body can produce it on its own — from cholesterol in your skin when exposed to UVB radiation from the sun. It is then converted in the liver to 25-hydroxyvitamin D (25-OH-D, the form measured in blood tests) and finally in the kidneys to its active form, calcitriol (1,25-dihydroxyvitamin D).
What makes vitamin D unique is its vitamin D receptor (VDR) — a nuclear receptor found in nearly every cell type in the body: immune cells, brain cells, muscle cells, intestinal cells, bone cells, and beta cells in the pancreas. When calcitriol binds to VDR, it regulates over 1,000 genes — roughly 5% of the entire human genome. Few substances have such broad biological impact.
Historically, we associated vitamin D only with bone health (rickets in children, osteomalacia in adults). But research over the past two decades has revealed a range of "extra-skeletal" functions: immune regulation, brain function, muscle performance, insulin sensitivity, cardiovascular protection, and anti-inflammatory activity. The role of vitamin D in the body is far broader than we previously understood.
💡 Did you know? Fifteen to twenty minutes of summer sun exposure (with arms and face exposed) produces 10,000 to 20,000 IU (International Units) of vitamin D — more than 10 to 20 times what your diet provides in a day. But at northern latitudes, the sun angle is only sufficient from April through September. The rest of the year, vitamin D production in the skin drops to nearly zero.
Vitamin D deficiency in the North — a silent epidemic
Population studies show that 70% of people in Scandinavian countries have suboptimal vitamin D levels during winter. Internationally, deficiency is defined as 25-hydroxyvitamin D below 50 nmol/L, but a growing number of researchers argue that optimal levels fall between 75 and 125 nmol/L.
At-risk groups for deficiency include: people with darker skin (melanin blocks UVB), older adults (the skin's vitamin D production drops by 75% after age 70), people with obesity (vitamin D gets stored in fat tissue and becomes unavailable), people who cover their skin for religious reasons, night-shift workers, and everyone living above the 37th parallel (all of Scandinavia) from October through March.
Diet contributes only 10 to 20% of your vitamin D needs. The best dietary sources are fatty fish (salmon provides 10 to 20 µg per serving, mackerel 8 to 12 µg, herring 5 to 8 µg), fortified milk and plant-based milk, egg yolks (1 to 2 µg per egg), and mushrooms exposed to UV light. The recommended daily intake in many countries is 10 µg (400 IU) — but many researchers consider this too low.
The immune system and vitamin D
The immunological role of vitamin D may be the most fascinating discovery of recent decades. Immune cells (T cells, B cells, macrophages, dendritic cells) all carry VDR receptors and enzymes that convert 25-hydroxyvitamin D into active calcitriol locally.
Studies have shown that T cells cannot activate without adequate vitamin D levels — they remain in a dormant state. Vitamin D also stimulates the production of antimicrobial peptides (cathelicidin, defensins) that serve as the body's "natural antibiotics" against bacteria and viruses.
A major meta-analysis (Martineau et al., BMJ, 2017 — 25 RCTs, 11,321 participants) found that vitamin D supplementation reduced the risk of acute respiratory infections by 12% overall, and by 70% in people with severe deficiency (below 25 nmol/L). The effect was greatest with daily supplementation (not monthly mega-doses) and in those with the lowest baseline levels.
The brain, mood, and depression
VDR receptors are densely distributed in the hippocampus, prefrontal cortex, and amygdala — brain regions central to memory, decision-making, and emotional regulation. Vitamin D directly influences serotonin synthesis by regulating the expression of tryptophan hydroxylase 2 (TPH2) — the enzyme that converts tryptophan into serotonin in the brain.
A meta-analysis (Anglin et al., 2013 — 31,424 participants) found that low vitamin D levels (below 50 nmol/L) increased the risk of depression by 75%. Intervention studies have yielded mixed results — vitamin D supplementation likely does not prevent depression in people with normal levels, but may have therapeutic benefits in cases of severe deficiency.
Seasonal affective disorder (SAD) in northern regions coincides with the period of lowest vitamin D levels. The link is not fully established (lack of light also directly affects the melatonin-serotonin balance), but vitamin D is likely a contributing factor. The combination of vitamin D supplementation and light therapy has been shown to be more effective than either alone.
🔬 The Martineau study (BMJ, 2017) is the strongest meta-analysis to date on vitamin D and immune function. Key finding: daily supplementation outperforms monthly mega-doses, and the effect is dramatically greatest in deficiency — a 70% reduction in respiratory infection risk at levels below 25 nmol/L.
Dosage, forms, and blood tests
Practical guidance based on current research:
- Form — Cholecalciferol (D3) is superior to ergocalciferol (D2). D3 raises and maintains blood levels 87% more effectively. Choose D3 in oil (better absorption) or as soft gel capsules.
- Dose — 2,000 to 4,000 IU (International Units) daily during October through March for most adults in northern climates. For documented deficiency, a physician may prescribe 4,000 to 10,000 IU during a loading phase (8 to 12 weeks) followed by a maintenance dose.
- Timing — Take it with a fat-containing meal (calcium absorption increases by 50%). Preferably in the morning — there is some evidence that evening doses may disrupt melatonin production.
- K2 combination — Vitamin K2 (menaquinone-7, 100 µg) directs calcium to the skeleton and away from blood vessels. D3 without K2 may theoretically increase vascular calcification at high doses — the combination is safer.
- Blood test — 25-hydroxyvitamin D is the correct marker. Interpretation: below 25 nmol/L = severe deficiency, 25–50 = deficient, 50–75 = suboptimal, 75–125 = optimal, above 250 = risk of toxicity.
- Sun exposure — April through September: 15 to 20 minutes of midday sun with arms and face exposed, without sunscreen. Do not burn — erythema (redness) does not increase vitamin D production, only skin damage risk.
Vitamin D myths
- "I can get enough from food" — At northern latitudes, this is practically impossible. You would need to eat 5 to 10 servings of fatty fish daily. Diet contributes 10 to 20% — the rest must come from sun or supplements.
- "Tanning beds give you vitamin D" — Modern tanning beds primarily use UVA radiation, not UVB. UVA burns and ages the skin but produces minimal vitamin D. The WHO classifies tanning beds as a Group 1 carcinogen.
- "Sunscreen blocks vitamin D" — In theory, yes, but studies show that most people apply too little and too unevenly to completely block UVB. Normal sunscreen use has minimal impact on vitamin D levels in practice.
- "Higher doses are always better" — Vitamin D is fat-soluble and stored in the body. Chronic intake above 10,000 IU per day without medical supervision can cause hypercalcemia (elevated calcium) leading to kidney damage. More is not always better.
- "Vitamin D cures everything" — Media reports have exaggerated the effects of vitamin D. It is not a cure-all. But in cases of documented deficiency — which 70% of Scandinavians experience in winter — supplementation produces measurable improvements in immune function, mood, and muscle performance.
Cipoli analysis
Group comparison and patternsCipoli group comparison coming soon
In this section, we will compare Cipoli users who take vitamin D supplements with those who do not — and explore how it correlates with energy, immune function, and mood.
The analysis will include:
Why isn't the analysis available yet? To create meaningful group comparisons, we need enough anonymized responses from our users. The more people who map their health, the better and more reliable the analyses become.
Help us get there faster
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