Vitamin D is the vitamin your body produces from sunlight. It’s the only vitamin humans can manufacture themselves, and the process starts the moment UVB rays hit bare skin. Your skin contains a cholesterol-based compound that absorbs UVB radiation and converts it into vitamin D3, which then travels to the liver and kidneys to become the active form your body uses.
How Your Skin Makes Vitamin D
The process works like a chemical chain reaction. A precursor molecule called 7-dehydrocholesterol sits in the outer layers of your skin. When UVB rays penetrate the skin, they break a chemical bond in that molecule, converting it into previtamin D3. Body heat then transforms previtamin D3 into vitamin D3, which enters the bloodstream and travels to the liver. There, it’s converted into a circulating form, and then the kidneys activate it into the hormone your cells actually use.
This active form of vitamin D influences the expression of more than 900 genes throughout the body, which is why its effects reach far beyond bone health. It plays a role in immune regulation, muscle function, and cell growth. People born without a functioning vitamin D receptor develop a condition called hereditary vitamin D resistant rickets, which illustrates just how essential this signaling pathway is from birth.
How Much Sun Exposure You Need
For most people, exposing bare arms and legs to midday sun (between 10 a.m. and 3 p.m.) for 5 to 30 minutes, twice a week, can produce enough vitamin D to meet your body’s needs. That range is wide because the amount you make depends heavily on your skin tone, age, latitude, and the time of year.
Skin pigmentation is the biggest individual variable. Melanin, the pigment that gives darker skin its color, acts as a natural sunscreen. People with dark skin may need up to ten times as long in the sun to produce the same amount of vitamin D as someone with fair skin. A person with light skin might need just 5 to 10 minutes of midday exposure, while someone with very dark skin could need 30 minutes or more under the same conditions.
Older adults also produce less vitamin D from the same amount of sun. The skin’s supply of the precursor molecule declines with age, so a 70-year-old generates significantly less vitamin D per minute of sun exposure than a 20-year-old.
When Sunlight Can’t Produce Vitamin D
Geography and season create hard limits on vitamin D production that no amount of outdoor time can overcome. In Boston (42°N latitude), sunlight from November through February doesn’t produce any vitamin D in human skin, even on cloudless days. In Edmonton (52°N), that dead zone stretches from October through March. The sun simply sits too low in the sky during winter months at these latitudes, and the atmosphere filters out the UVB wavelengths before they reach the ground.
This means that if you live in the northern half of the United States, most of Canada, the UK, or northern Europe, you physically cannot make vitamin D from sunlight for several months each year. During those months, your body relies entirely on stored vitamin D (which fat tissue holds onto) and dietary sources like fatty fish, fortified milk, and supplements.
What Your Vitamin D Levels Mean
A simple blood test measures your circulating vitamin D level. The results are reported in nanograms per milliliter (ng/mL), and the thresholds are straightforward:
- Below 12 ng/mL: Deficiency. At this level, children risk rickets (soft, weak bones) and adults risk osteomalacia, a painful condition where bones become soft.
- 12 to 20 ng/mL: Inadequate for bone and overall health in most people.
- 20 ng/mL or above: Sufficient for most people.
- Above 50 ng/mL: Potentially harmful, with risk increasing further above 60 ng/mL.
Toxicity from vitamin D is essentially impossible through sun exposure alone, because the skin self-regulates production. Once you’ve made enough previtamin D3, continued UV exposure breaks it down into inactive byproducts. Toxicity comes from taking too many supplements over a prolonged period.
Why Vitamin D Deficiency Is So Common
Despite being free and available from the sky, vitamin D deficiency is widespread. The reasons stack up quickly. Modern life keeps people indoors during peak UV hours. Sunscreen with SPF 30 blocks roughly 97% of UVB rays. Clothing covers most skin. People with darker skin need far more exposure. Older adults produce less. And anyone living above about 35°N latitude loses the ability to make vitamin D for part of the year.
The groups at highest risk include older adults, people with dark skin living in northern climates, people who are homebound or wear clothing that covers most of the body, and those with conditions affecting fat absorption (since vitamin D is fat-soluble and needs dietary fat to be absorbed from food or supplements). If any of these apply to you, a blood test is the only reliable way to know where you stand, and supplementation during winter months is often practical.
What Vitamin D Does in the Body
Vitamin D’s best-known role is helping the body absorb calcium and phosphorus, which are essential for building and maintaining bone density. Without enough vitamin D, your intestines absorb only about 10 to 15 percent of the calcium in your diet. With adequate levels, that jumps to 30 to 40 percent.
But the effects go well beyond bones. Vitamin D receptors are found in nearly every tissue in the body, including immune cells, the heart, the brain, and muscles. In the immune system, active vitamin D helps regulate the balance between fighting infections and preventing the kind of overreaction that drives autoimmune disease. Research has linked vitamin D deficiency to increased susceptibility to infections and a higher risk of inflammatory conditions, though the exact nature of these connections is still being refined.
Muscle function is another area where vitamin D plays a direct role. Low levels are associated with muscle weakness and an increased risk of falls in older adults, which is one reason maintaining adequate levels matters more as you age.