The vitamin you get from the sun is vitamin D, often called the “sunshine vitamin.” Your skin produces it when ultraviolet B (UVB) rays hit a cholesterol compound naturally present in your skin cells, converting it into a precursor that your liver and kidneys then activate into usable vitamin D. It’s the only vitamin your body can manufacture on its own, and sunlight is the most efficient trigger.
How Your Body Makes Vitamin D From Sunlight
The process starts with a form of cholesterol sitting in your skin called 7-DHC. When UVB rays penetrate your skin, they transform 7-DHC into a precursor molecule, which travels to your liver for one round of processing, then to your kidneys for a second. The final product is the active hormone that your body actually uses. This activated form binds to receptors found in cells throughout your body, switching on genes that regulate everything from calcium absorption to immune function.
What Vitamin D Does in Your Body
Vitamin D’s primary job is managing calcium. It controls how much calcium your intestines absorb from food and how much your kidneys reclaim before it’s lost in urine. When those two sources aren’t enough to keep blood calcium at the right level, vitamin D signals your bones to release stored calcium into the bloodstream. This is a short-term fix that works fine occasionally, but if vitamin D stays low for months or years, your bones steadily lose density.
Beyond calcium, vitamin D directs the cells responsible for building and breaking down bone tissue. It helps generate the cells that remodel old bone and regulates key proteins produced by bone-building cells. Without adequate vitamin D, this entire system falls out of balance.
How Much Sun Exposure You Need
Exposing your bare arms and legs to midday sun (between 10 a.m. and 3 p.m.) for 5 to 30 minutes, twice a week, is generally enough to meet your vitamin D needs. The wide range exists because your skin tone matters enormously. Melanin, the pigment that darkens skin, competes with the cholesterol compound in your skin for UV absorption. People with darker skin may need up to ten times longer in the sun to produce the same amount of vitamin D as someone with fair skin.
Several other factors shrink or expand that window:
- Latitude: If you live above the 37th parallel (roughly a line from San Francisco to Richmond, Virginia), the sun’s UVB rays are too weak from fall through early spring to trigger meaningful vitamin D production, no matter how long you stay outside.
- Season and time of day: UVB intensity peaks at solar noon in summer. Early morning and late afternoon sun produces far less vitamin D.
- Sunscreen: A recent meta-analysis found that sunscreen use lowers blood vitamin D levels by about 2 ng/mL on average. Lab studies confirm sunscreen blocks the specific UVB wavelengths needed for production, though real-world results vary because most people don’t apply sunscreen thickly or evenly enough to block all UVB.
- Age: Older adults produce vitamin D less efficiently because the cholesterol precursor in skin decreases with age.
- Clothing and glass: UVB rays don’t pass through clothing or window glass, so sitting by a sunny window won’t help.
How Much Vitamin D You Need Daily
The NIH recommends these daily amounts:
- Babies (0–12 months): 400 IU (10 mcg)
- Children and teens (1–18 years): 600 IU (15 mcg)
- Adults (19–70 years): 600 IU (15 mcg)
- Adults over 71: 800 IU (20 mcg)
- Pregnant or breastfeeding women: 600 IU (15 mcg)
These numbers assume minimal sun exposure and represent the intake from food and supplements combined. If you’re getting regular midday sun on bare skin during summer months, your body may produce several thousand IU in a single session, well above the daily recommendation.
Signs of Vitamin D Deficiency
Mild deficiency often flies under the radar. The most common symptoms in adults are fatigue, bone pain, muscle weakness or cramps, and mood changes including depression. These are vague enough that many people chalk them up to stress or aging.
In children, deficiency can cause incorrect growth patterns, bowed or bent bones, joint deformities, and muscle weakness. A mild case might show up only as general soreness or weak muscles.
When deficiency persists, the consequences get more serious. Your intestines absorb less calcium and phosphorus, which forces your parathyroid glands to work overtime pulling calcium from your bones. Over time this leads to softened bones, a condition called osteomalacia in adults and rickets in children. Both significantly raise fracture risk. In severe, untreated cases of rickets, complications can include seizures and heart damage.
How Vitamin D Levels Are Measured
A simple blood test measures the circulating form of vitamin D. The results tell you where you fall on a clear scale:
- Below 12 ng/mL: Deficient. This level is associated with rickets in children and bone softening in adults.
- 12 to 19 ng/mL: Inadequate for bone and overall health.
- 20 ng/mL or above: Sufficient for most healthy people.
- Above 50 ng/mL: Potentially harmful, with risk increasing above 60 ng/mL.
That upper limit is worth noting. Vitamin D toxicity is essentially impossible from sun exposure alone because your skin self-regulates production. It comes from taking high-dose supplements over extended periods.
Food Sources and Supplements
Very few foods contain significant vitamin D naturally. Fatty fish like salmon, mackerel, and sardines are the best dietary sources. Egg yolks and beef liver contain small amounts. In many countries, milk, orange juice, and breakfast cereals are fortified with vitamin D to help close the gap.
For people who live at northern latitudes, have darker skin, work indoors, or cover most of their skin for cultural or medical reasons, supplements are often the most reliable option during months when the sun can’t do the job. Vitamin D supplements come in two forms: D2 (plant-derived) and D3 (the same type your skin produces). D3 is generally more effective at raising blood levels.