☀️ Vitamin D is not a vitamin — it is a hormone.
It acts on over 2,000 genes, regulates testosterone, immune system, sleep and mood. Yet 80% of people are deficient, especially in winter months. Not through laziness — for structural reasons that conventional medicine continues to underestimate.
When people talk about vitamin D, most think of bones, calcium, children who need to "get some sun". This is a reductive view that is decades behind current research. Over the past 20 years science has completely overturned our understanding of this compound: vitamin D is not an accessory micronutrient — it is a steroid hormone that governs fundamental processes in every organ and tissue of the human body.
Vitamin D receptors (VDR) have been identified in almost every cell in the body — muscles, brain, heart, immune system, testicles, adrenal glands. This means that when vitamin D is lacking, virtually nothing functions at its full potential. And given that 80% of the population shows insufficient levels, we are talking about a silent epidemic affecting the energy, mood, immunity and hormones of millions of people.
Vitamin D: a hormone, not a vitamin
The distinction is not semantic. Vitamins must be obtained from outside because the body does not produce them. Vitamin D, on the contrary, is synthesised by the body itself from cholesterol, under the action of UVB ultraviolet radiation on the skin. The process is identical to that of other steroid hormones — testosterone, cortisol, oestrogens — which all derive from the same precursor: cholesterol.
Once synthesised in the skin (or obtained through diet), vitamin D is transported to the liver where it is converted into 25-hydroxyvitamin D — the measurable form in blood, the one tested in blood analyses. From there, it passes to the kidneys where it is further activated into the biologically active form: calcitriol (1,25-dihydroxyvitamin D3).
Calcitriol behaves exactly like a hormone: it binds to VDR receptors in target cells and regulates gene expression. It is not a simple "facilitator" — it is a direct regulator of over 2,000 genes in the human genome.
Scientific Evidence
A meta-analysis published in BMJ (Forouhi et al., 2012) analysing data from over 42,000 individuals across Europe found that 40% of the European population has 25-OH vitamin D levels below 50 nmol/L (frank deficiency) and that in Italy, especially in the over-50 age group and winter months, this percentage regularly exceeds 70–80%. The study emphasises that geographic latitude alone does not explain the spread of deficiency — modern behaviours (working indoors, sunscreen use, covering clothing) are the main factors. BMJ. 2012;345:e5439.
What vitamin D does: the 6 key functions
Far beyond bone health that everyone knows about, here are the functions that directly impact daily quality of life:
VDR receptors are present in mitochondria — the energy powerhouses of cells. Vitamin D regulates mitochondrial biogenesis and the efficiency of the Krebs cycle. Its deficiency directly compromises ATP production, explaining chronic fatigue that doesn't respond to rest.
VDR receptors are expressed in Leydig cells in the testicles — those that produce testosterone. Vitamin D stimulates steroidogenesis (production of steroid hormones) and reduces aromatase activity, the enzyme that converts testosterone into oestrogens. Low vitamin D = more conversion to oestrogens = less available testosterone.
Vitamin D doesn't simply "stimulate" the immune system — it regulates it precisely. It activates T lymphocytes and macrophages against infections, while simultaneously curbing excessive inflammatory responses. This explains why vitamin D deficiency is associated with both greater susceptibility to infections and autoimmune diseases.
VDR receptors are present in brain areas that regulate sleep, including the suprachiasmatic nucleus (the circadian clock). Vitamin D participates in serotonin synthesis — the precursor to melatonin — and directly regulates circadian rhythm genes. Its deficiency alters sleep architecture, reducing deep sleep phases (NREM3).
Vitamin D regulates the synthesis and release of dopamine and serotonin in the brain. Its deficiency correlates with higher rates of depression, anxiety, cognitive deficit and memory decline. Not coincidentally, the prevalence of seasonal depression (SAD) in Nordic countries perfectly corresponds to months of solar deficiency.
VDRs in muscle cells regulate protein synthesis and muscle contraction. Vitamin D deficiency is associated with muscle weakness, cramps and reduced recovery capacity after exercise — often erroneously attributed to "ageing" or "overtraining".
Why we're almost all deficient
The answer isn't "we don't get enough sun". The reality is more complex and structural:
- We work indoors. Most adults spend 90% of their time in enclosed environments. Glass filters 100% of UVB rays — so even sitting by a sunny window produces no vitamin D.
- We use sunscreen. SPF 15 reduces vitamin D synthesis by 99%. Necessary to prevent sun damage, but with the consequence of zeroing out hormone production.
- We live at critical latitudes. Between October and March, across central-northern Europe and much of the UK, the solar angle is too low to produce effective UVB. Six months a year when synthesis is practically zero.
- Diet alone is not enough. Food sources of vitamin D are few and concentrated: wild fatty fish, free-range eggs, mushrooms. An average diet provides 200–400 IU/day — a fraction of the real requirement.
- Obesity sequesters it. Vitamin D is fat-soluble — it accumulates in adipose tissue. In overweight individuals, a significant proportion gets "trapped" in fat and is not available for circulation and biological use.
- Stress consumes it. Chronic cortisol accelerates the catabolism of active vitamin D, reducing its bioavailability in target tissues.
Scientific Evidence
A randomised clinical trial published in Hormone and Metabolic Research (Pilz et al., 2011) randomly assigned 54 men with vitamin D deficiency either a vitamin D3 supplement (3,332 IU/day) or a placebo for 12 months. At the end of the study, the vitamin D group showed a 25.2% increase in total testosterone from baseline, versus no change in the placebo group. This is one of the most cited studies for the direct vitamin D–testosterone link. Horm Metab Res. 2011;43(3):223–225.
How to read your blood test results
The value to request from your doctor is serum 25-OH vitamin D (also written 25(OH)D or calcidiol). It is the only reliable indicator of vitamin D reserves in the body.
| Level (nmol/L) | Level (ng/mL) | Status | Implications |
|---|---|---|---|
| < 25 | < 10 | Severe deficiency | Risk of rickets, osteomalacia, immunosuppression |
| 25 – 50 | 10 – 20 | Deficiency | Fatigue, muscle pain, reduced immunity, low testosterone |
| 50 – 75 | 20 – 30 | Insufficiency | "Normal" by many labs, but suboptimal for hormonal functions |
| 75 – 150 | 30 – 60 | Optimal | Immune, hormonal and muscular function at maximum |
| > 250 | > 100 | Excess | Hypercalcaemia (rare with sun; possible with excessive supplementation) |
Important note: many laboratories indicate as "normal" any value above 50 nmol/L (20 ng/mL). This is the minimum level to prevent rickets in children — not the optimal level for hormonal, immune and metabolic health in adults. The most recent research indicates that for extra-skeletal functions (testosterone, immunity, sleep, neurology) levels between 75 and 150 nmol/L are needed.
Natural sources: sun and food
The sun — primary and irreplaceable source
Sun exposure remains the most efficient source of vitamin D. In summer, at Mediterranean latitudes, 15–20 minutes of sun with arms and legs exposed during central hours (between 10am and 3pm) can produce 10,000–20,000 IU — far more than any supplement. The key rules:
- Expose the largest possible surface areas (not just face and hands)
- Avoid sunscreen in the early phase of brief exposure
- Don't wash with soap immediately afterwards — synthesis requires a few hours in the skin
- Don't expect effective production from October to March in northern latitudes
Food sources
- Wild salmon — 600–1,000 IU per 100g (farmed salmon much less)
- Mackerel and herring — 400–800 IU per 100g
- Tinned sardines — 200–300 IU per 100g
- Cod liver oil — 400–1,000 IU per tablespoon
- Free-range egg yolk — 40–80 IU per yolk
- Sun-dried shiitake mushrooms — 100–1,600 IU per 100g (depends on UV exposure)
⚠️ Important warning: vitamin D supplementation should always be guided by a doctor based on blood test results. High and prolonged doses without monitoring can cause hypercalcaemia and kidney damage. Never start significant supplementation without first measuring your levels and consulting your doctor.
Vitamin D, Magnesium and the missing synergy
There is a detail almost nobody mentions: vitamin D does not work optimally without sufficient magnesium. Magnesium is a cofactor of all the enzymes that metabolise vitamin D — from liver conversion to renal activation. A person deficient in magnesium can take vitamin D and see poor results, simply because the enzymatic tools to activate it are missing.
This is why in the Romeo Method these two elements are considered together — not as isolated supplements, but as part of a biological internal ecosystem where each component depends on the others. Restoring magnesium and vitamin D simultaneously, through diet and conscious sun exposure, produces markedly superior results compared to the fragmented approach of "one problem, one solution".
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