Why Vitamin D matters
Vitamin D functions more like a hormone than a vitamin. Every cell in your body has a Vitamin D receptor, and it regulates over 200 genes. Its best-known role is enabling calcium absorption in the gut — without adequate Vitamin D, you can consume plenty of calcium and still develop weak bones. But its functions extend much further:
- Immune regulation — Vitamin D modulates both innate and adaptive immune responses
- Muscle function — deficiency causes weakness and increases fall risk in older adults
- Mood and mental health — low levels are associated with increased depression risk
- Cardiovascular health — receptors are found throughout the heart and blood vessels
- Insulin sensitivity — deficiency is associated with increased type 2 diabetes risk
Symptoms of Vitamin D deficiency
Deficiency develops slowly and its symptoms are non-specific — meaning they overlap with dozens of other conditions. This makes it easy to miss without a blood test.
- Persistent fatigue that doesn't improve with sleep
- Bone and back pain — diffuse aching, especially in the lower back, hips, and legs
- Muscle weakness and difficulty climbing stairs or rising from a chair
- Frequent illness — impaired immune response leads to more colds and infections
- Low mood or depression, particularly in winter (seasonal pattern)
- Hair loss — in severe or prolonged deficiency
- Slow wound healing
🩸 The only way to know for sure
A blood test measuring 25-hydroxyvitamin D (25(OH)D) is the definitive test. Levels below 20 ng/mL (50 nmol/L) are deficient. 20–29 ng/mL is insufficient. Most experts consider 30–60 ng/mL optimal.
Who is at risk?
| Risk factor | Why it increases risk |
|---|---|
| Limited sun exposure | Indoor work, sunscreen use, climate |
| Dark skin tone | Higher melanin reduces UVB absorption |
| Living above 35° latitude | Less UVB radiation, especially in winter |
| Age 65+ | Skin produces less Vitamin D with age |
| Obesity | Fat tissue sequesters Vitamin D |
| Malabsorption conditions | Crohn's, celiac, gastric bypass reduce absorption |
| Breastfed infants | Breast milk is low in Vitamin D |
How much Vitamin D do you need?
The official RDA is 600 IU/day for adults under 70, and 800 IU/day for those over 70. However, many researchers argue these values are too low to achieve and maintain optimal blood levels — particularly for people with limited sun exposure.
For most adults at risk of deficiency, supplementing with 1,000–2,000 IU/day of Vitamin D3 is a safe and evidence-supported starting point. The safe upper limit set by health authorities is 4,000 IU/day for adults, though toxicity from supplementation is rare and generally only occurs at very high doses (above 10,000 IU/day for extended periods).
Vitamin D3 (cholecalciferol) is better absorbed and more effective at raising blood levels than Vitamin D2 (ergocalciferol). Always choose D3 when supplementing.
Can you get enough from sunlight?
In theory, yes. In practice, for most people living in temperate climates, no — especially in winter. UVB rays sufficient to produce Vitamin D only reach the earth's surface at certain latitudes and times of day. Above 35° latitude (roughly the level of Los Angeles, Madrid, or Beijing), there's effectively no Vitamin D production from sunlight between October and March.
On a summer day, fair-skinned people can produce 10,000–20,000 IU of Vitamin D from 15–20 minutes of midday sun on arms and legs. But sunscreen with SPF 30 reduces Vitamin D synthesis by about 95%, and glass blocks all UVB — so time spent near a sunny window doesn't count.
Find your personalised Vitamin D recommendation based on your sun exposure, location, and skin tone.
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