What is vitamin D?
It is a fat-soluble vitamin synthesised primarily through the skin when exposed to ultraviolet B (UVB) radiation. It plays an important role in bone health, the immune system and calcium metabolism.
The relationship between UV rays and vitamin D
UVB rays are the main source of vitamin D synthesis.
Once the skin has been exposed to UVB, it can produce vitamin D3 (cholecalciferol) via cholesterol precursors.
1. UVB Rays Trigger Vitamin D Synthesis
The skin contains a cholesterol derivative called 7-dehydrocholesterol.
When exposed to UVB radiation (wavelength 290–315 nm), this compound is converted into previtamin D₃, which then thermally isomerizes into vitamin D₃ (cholecalciferol).
Vitamin D₃ is transported to the liver and kidneys, where it is converted into its active form, calcitriol (1,25-dihydroxyvitamin D).
2. Factors Affecting Vitamin D Production
Skin pigmentation: Darker skin (higher melanin) reduces UVB absorption, requiring longer sun exposure.
Latitude & season: UVB intensity is lower in winter and at higher latitudes, reducing vitamin D synthesis.
Time of day: Midday sun (10 AM–3 PM) provides the strongest UVB exposure.
Age: Older adults produce less vitamin D due to thinner skin.
Sunscreen use: SPF 30+ can block ~95% of UVB, reducing vitamin D synthesis.
3. Benefits of Vitamin D
Bone health: Promotes calcium absorption, preventing rickets (in children) and osteomalacia (in adults).
mmune function: Supports immune regulation and may reduce infection risks.
Mood regulation: Linked to serotonin production, potentially improving mental health.
4. Risks of Excessive UV Exposure
Skin damage: UV rays cause sunburn, premature aging, and DNA mutations.
Skin cancer: Prolonged UV exposure increases the risk of melanoma and non-melanoma skin cancers.
Vitamin D toxicity: Excess vitamin D (usually from supplements, not sunlight) can lead to hypercalcemia.
5. Balancing Sun Exposure & Safety
Short, frequent exposure: 10–30 minutes of midday sun (arms/legs exposed, no sunscreen) 2–3 times per week is often sufficient for vitamin D synthesis (varies by skin type and location).
Dietary sources: Fatty fish (salmon, mackerel), fortified dairy, and supplements help maintain levels.
Protection: Use sunscreen after initial exposure to prevent skin damage.
Do tanning machines have this function?
Most commercial tanning machines use UVA rather than UVB. UVA penetrates deeper, but plays almost no role in vitamin D synthesis.
Studies have found that, even when a small amount of UVB is present, the dose is much higher than that received from natural sunlight and the potential harm to the skin is greater.
Safer alternatives:
Sun exposure: 10–15 minutes of sun exposure to the face and arms every day.
Diet: fish, egg yolks and fortified milk.
Supplements: Vitamin D3 supplements should be taken under medical supervision.