How Sunlight Makes Vitamin D

What UV-B is

Sunlight carries many types of radiation. The warmth you feel is infrared. The light you see is visible. And then there is ultraviolet radiation — divided into UV-A and UV-B. UV-A penetrates deep into the skin, while UV-B hits the outer layer. That contact is what starts vitamin D production.

What happens in your skin

The outer layers of your skin contain a molecule called 7-dehydrocholesterol — a form of cholesterol that sits just under the surface, ready to react. When UV-B light reaches it, the energy splits the molecule open, creating a new substance called previtamin D3. The warmth of your skin then converts it into vitamin D3. Light does not inject vitamin D into you. It triggers your own skin to build it.

The journey to active vitamin D

Vitamin D3 made in your skin is not yet ready to use. It travels in your blood to the liver, which converts it into 25-hydroxyvitamin D — the form doctors measure in a blood test. From there it travels to the kidneys, where it becomes calcitriol: the active form your body can actually use. The whole process takes a few days, but the chain starts the moment UV-B touches your skin.

What vitamin D does

Calcitriol behaves more like a hormone than a vitamin — it carries signals to cells all over your body. Its most well-known role is bone health. Without enough vitamin D, your gut cannot absorb calcium from food, and calcium is what gives bones their hardness.

But vitamin D does much more than protect your skeleton. It helps regulate your immune system — the network of cells that fights infection — and it supports muscle strength and recovery. Research also links it to mood: scientists believe it influences serotonin, the chemical in the brain tied to how we feel. Vitamin D receptors exist in almost every tissue in the human body, which tells you this molecule matters far beyond bones.

What happens without enough

Over a billion people worldwide have low vitamin D levels, and most do not know it. In children, severe deficiency causes rickets — a condition where bones grow soft and bend under the body's weight. In adults, it leads to bone loss and raised fracture risk.

Beyond the skeleton, low vitamin D connects to higher rates of infection, autoimmune disease, and depression. Common early signs include fatigue, muscle weakness, and a persistent feeling of being unwell.

Why sunlight is the key

You can get some vitamin D from food — fatty fish, egg yolks, fortified products — but food alone rarely provides enough. Sunlight is the most efficient source by far. A short midday sun session can trigger more vitamin D production than food can provide in a week, and even 10 to 25 minutes is enough, depending on your skin tone.

The challenge is that UV-B is seasonal. In autumn and winter, especially at high latitudes, the sun does not rise high enough for meaningful UV-B to reach your skin. In Norway, where we are located, vitamin D production from sunlight effectively stops from October to March.

People who live far from the equator — or who spend most of their time indoors — face a real risk of deficiency. Sunlight is not just pleasant to be in. For vitamin D, it is the starting point of a process your body cannot easily replace.

Your body is designed to use sunlight. The sun is a source of light and warmth. But it is something more: the trigger for a process that keeps you well.

Go Deeper

If you want to explore this topic further:

• Vitamin D — Wikipedia: Vitamin D
• UV-B and skin synthesis — Linus Pauling Institute: Vitamin D and Skin Health (lpi.oregonstate.edu)
• Global deficiency and sunlight — PMC/NIH: Sunlight and Vitamin D: A global perspective for health (pmc.ncbi.nlm.nih.gov/articles/PMC3897598)
• Vitamin D and the immune system — PMC/NIH: Vitamin D and the Immune System (pmc.ncbi.nlm.nih.gov/articles/PMC3166406)
• Deficiency overview — NCBI Bookshelf: Vitamin D Deficiency / StatPearls (ncbi.nlm.nih.gov/books/NBK532266)