You get vitamin D three ways: sunlight on bare skin, certain foods, and supplements. Sunlight is the most powerful source, but where you live, the time of year, and your skin tone all determine whether you’re actually making enough. Most adults need 600 IU (15 mcg) per day, rising to 800 IU after age 70.
How Your Skin Makes Vitamin D From Sunlight
When UVB rays from the sun hit your skin, they trigger a chemical reaction in a cholesterol compound sitting in your skin cells. That compound converts into vitamin D3, which then travels to your liver and kidneys to be activated into the form your body actually uses. This process is remarkably efficient: a short session of midday sun exposure on bare arms and legs can produce thousands of IU, far more than any single meal provides.
The catch is that UVB rays only reach you under specific conditions. The sun needs to be high enough in the sky, which means the window for vitamin D production is roughly 10 a.m. to 3 p.m., even in summer. Outside those hours, the sun’s angle is too low and the atmosphere filters out most UVB before it reaches your skin.
How long you need depends heavily on your skin tone. People with darker skin (type V on the Fitzpatrick scale) need about 25 to 40 minutes of midday sun with at least 35% of their skin exposed during summer months, based on UK research. People with lighter skin produce the same amount in less time. The key is brief, regular exposure on bare skin rather than long sessions that risk sunburn.
Why Location and Season Matter
If you live above about 33° latitude (roughly Atlanta or Los Angeles in the US), your body cannot produce meaningful vitamin D from sunlight during winter months. The sun sits too low in the sky for enough UVB to penetrate the atmosphere. In Boston, at 42° north, the skin produces essentially no vitamin D from November through February. In cities like Edmonton, Bergen, or anywhere at 50° latitude or higher, that blackout period stretches to about six months of the year.
A few other things block vitamin D production that people don’t always realize. Glass absorbs all UVB radiation, so sitting by a sunny window, driving with the windows up, or working in a sunlit office does nothing for your vitamin D levels. The same applies to plexiglass and plastic.
What About Sunscreen?
Lab studies show that sunscreen applied thickly and evenly can dramatically reduce vitamin D production. But real-world trials tell a different story. When people used sunscreen normally in their daily routines (at moderate SPF levels around 16), researchers found no measurable drop in vitamin D levels. Most people don’t apply sunscreen as thickly or evenly as lab conditions assume, and they miss spots. In practice, enough UVB gets through.
Food Sources of Vitamin D
Very few foods naturally contain significant vitamin D. The richest natural sources are fatty fish (salmon, mackerel, sardines, and trout), cod liver oil, and egg yolks. Beyond that, most dietary vitamin D in Western countries comes from fortified foods: milk, orange juice, breakfast cereals, and some yogurts that have vitamin D added during manufacturing.
Even with a diet rich in these foods, it’s difficult to reach 600 IU per day from food alone. A serving of salmon provides roughly 400 to 600 IU depending on the type and preparation, but most other foods deliver far less per serving. This is why sunlight and supplements play such an important role.
Supplements: D3 vs. D2
Vitamin D supplements come in two forms: D3 (cholecalciferol, the same type your skin makes) and D2 (ergocalciferol, derived from plants and fungi). Both raise blood levels, but D3 is substantially more effective. Research comparing the two found that D3 raised blood levels roughly twice as much as D2 at equivalent doses, and in some analyses was nearly three times more effective when combined with oral tablets. Even when D2 was given at double the concentration, it still couldn’t match D3’s results.
For most people, a daily D3 supplement in the 600 to 1,000 IU range is enough to maintain healthy levels, especially during winter or if you have limited sun exposure. People with diagnosed deficiency may need higher doses temporarily, guided by blood testing.
How Much You Need by Age
The recommended daily intake set by the National Institutes of Health assumes minimal sun exposure:
- Infants (0 to 12 months): 400 IU
- Children and adults (1 to 70 years): 600 IU
- Adults over 70: 800 IU
- Pregnant and breastfeeding women: 600 IU
These are baseline amounts for bone health in healthy people. Some researchers and clinicians argue that optimal levels require more, particularly for people who are already deficient.
How to Know If You’re Getting Enough
A simple blood test measures your level of 25-hydroxyvitamin D, the circulating form that reflects your overall vitamin D status. The widely accepted thresholds are:
- Deficient: below 20 ng/mL
- Insufficient: 21 to 29 ng/mL
- Sufficient: 30 ng/mL or above
Most experts recommend maintaining levels above 30 ng/mL year-round for both children and adults. If you live at a northern latitude, spend most of your day indoors, have darker skin, or are over 65, you’re at higher risk for deficiency and testing is especially useful.
The Role of Magnesium
Your body can’t activate vitamin D without magnesium. Every enzyme involved in converting vitamin D into its usable form, both in the liver and kidneys, requires magnesium as a cofactor. If your magnesium intake is low, supplementing with vitamin D alone may not fully correct a deficiency. Good magnesium sources include nuts, seeds, leafy greens, and whole grains.
Upper Limits and Safety
The accepted safe upper limit for vitamin D supplementation is 4,000 IU per day for adults. You cannot overdose from sunlight because your skin self-regulates production, but supplements bypass that safeguard. In a three-year trial of healthy adults, hypercalcemia (dangerously high calcium in the blood) occurred in 0% of people taking 400 IU daily, 3% of those taking 4,000 IU, and 9% of those taking 10,000 IU. Vitamin D toxicity causes calcium buildup, which can damage the kidneys, heart, and other soft tissues. Older adults, particularly older women with a history of falls, may need to be more cautious even at doses near the upper limit.